abstract - wright laboratory · ultra-sensitive mass spectroscopy using a tandem van de graaff...
TRANSCRIPT
ULTRA-SENSITIVE MASS SPECTROSCOPY USING A TANDEM VAN DE GRAAFF
ACCELERATOR TO DETECT 10Be and 26Al
James Hayden Thomas
Yale University
1982
10Be and 26Al are cosmic ray produced radioactive isotopes with
half lives of 1.6xl06 and .72xl06 years, respectively. 10Be is produced
at a rate of 2.0xl0“2 atoms/cm2/sec and 26Al is produced at a rate of
about 2x10“4 atoms/cm2/sec. 10Be and 26A1 are incorporated into the
geologic sediments where they can be detected and used as
geochronometers. The very low concentrations of these isotopes can be
detected with- 8 decay counters or accelerator mass spectroscopy
techniques (AMS), but the AMS techniques are two to three orders of
magnitude more sensitive than the decay counting techniques.
The Yale tandem Van de Graaff accelerator was used as a mass
spectrometer to detect 10Be and 26A1. The ion source was modified so
that it could generate 500 nA beams of BeO“ from 1/2 mg samples of BeO.
The accelerator performance and stability was improved so that 30 nA of
Be4+ could be analyzed, and this made it possible to detect 10Be in
samples containing 10Be and 9Be in a ratio of 10“14 atoms/atom. Only 15
nA of Al“ could be generated in the source, and 5 nA of A l 5+ could be
transmitted through to the detector chamber. We were able to detect
26A1 in samples containing 26Al and 27Al in a ratio of 10“12 atoms/atom.
The background levels of 10Be and 26Al were undetectable. (1DBe/9Be <<
10“14 atoms/atom, 26Al/27Al << 10*12 atoms/atom.) Background counts due
ABSTRACT
to the isobar 10B were easily separated from the 10Be signal by range
energy loss techniques. A Mg isobaric background beam was not present
in the Al beam because we chose to accelerate the Al" ion, and Mg does
not form a negative ion. Other background counts (eg. 160, and 12C)
were removed from the Al beam by an electrostatic analyzer.
We have measured 10Be concentrations in many samples. The 10Be
profiles in manganese nodules exibit mm/My 'growth' rates, and the
growth rates may vary as a function of time. The 10Be/9Be ratios
measured at the surface of the nodules agree, within the observed
limits, with the measured 10Be/9Be ratios in the surrounding seawater.
The 10Be concentration in GEOSECS North Pacific deep water was
6100±1200 atoms/gram. This value indicates that 10Be must reside in the
oceans for long periods of time ( >1000 years) and is thoroughly
homogenized in the water. Therefore, a global average of the 10Be
deposition rate can be used to estimate the accumulation rate of the
deep sea sediments. We have measured accumulation rates of 1.7 and 4.7
mm/ky in two sediment cores taken from the Pacific ocean.
We have also measured 10Be in precipitation and in soils. The
observed 10Be deposition rate at the Naval research station in Greenland
was 3.3±.7 xlO-3 atoms/cm2/sec. While, the soil samples collected near
Mendocino California did not retain 10Be long enough to be able to
determine the age of the soils.
Vie have detected 26Al in the iron meteorite Mundrabilla. Our
results are very reliable and a factor of two lower than the activities
measured by Jf-y coincidence counting, and our analyses were made on
1 gram samples of the meteorite, whereas the %-% counter required
~700 grams of sample.
We also attempted to measure the 26Al concentration in a manganese
nodule, but the concentrations were too low. This rules out the
possibility of being able to 'double date1 manganese nodules or ocean
sediments with the present techniques. A new sputter ion source concept
and design will be required in order to incease the Al" output of the
source.
ULTRA-SENSITIVE HASS SPECTROSCOPY USING A TANDEM VAN DE GRAAFF
ACCELERATOR TO DETECT 10Be AND 26A1
A DISSERTATION
PRESENTED TO THE FACULTY OF THE GRADUATE SCHOOL
OF
YALE UNIVERSITY
IN CANDIDACY FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
by
James Hayden Thomas
December, 1982
ACKNOWLEDGEMENTS
I would like to thank my advisor and editor Professor Peter D.
Parker for his contributions to my education. This work would not have
been possible without the continuing efforts of Professor D. Allan
Bromley and the special efforts of Mr. Richard D'Alexander.
TABLE OF CONTENTS
Accelerator Mass Spectroscopy ...................................... 3
10Be and its A p p l i c a t i o n s ............................................. 4
26A1 and its A p p l i c a t i o n s ............................................. 9
10Be and 26A 1 .......................................................... 12
Chapter Two: Production and Deposition of 10Be and 26A l ............. 14
The Origin of Terrestial 10Be and 26A l ............................... 14
Deposition of 10Be and 26A l .......................................... 25
Chapter Three: Experimental Methods .................................... 32
Introduction: Decay Counting vs. the Accelerator Techniques . . 32
Accelerator Techniques Applied to Mass Spectroscopy ............ 33
A Brief Outline of the Techniques for Measuring 10Be and 26A1 . 38
Preparation of the S a m p l e ............................................ 42
The Ion S o u r c e .......................................................... 43
The Inflection Magnet ............................................... 50
The A c c e l e r a t o r ........................................................ 51
The Detection of 10B e ................................................ 55
The Detection of 26A 1 ................................................ 64
The Electrostatic Analyzer ........................................... 66
The Ionization Chamber Detector...................................... 73
Performance of the New Beam L i n e ................................... 75
Sample Data R e d u c t i o n ................................................. 86
Reliability of the D a t a ...............................................89
Detailed Operating Procedures for Detecting 10Be and 26A1 . . . 94
Chapter One: I n t r o d u c t i o n ................................................................................................. 1
i i i
The Manganese Nodule Problem ........................................ 95
The Dispersion of the Beryllium Isotopes in Sea Water . . . . 107
The Deep Ocean S e d i m e n t s ............................................. 110
Precipitation Samples .............................................. 114
10Be in the Earths S o i l s ............................................. 115
26Al in the Mundrabilla iron m e t e o r i t e ............................. 117
Chapter Five: Conclusion ................................................. 123
Appendix I ...................................................................126
Detailed Operating Procedures for Detecting 10Be ................. 126
Appendix I I ...................................................................182
Detailed Operating Procedures for Detecting 26A1 . . ............ 132
Bibliography ............................................................... 186
Chapter Four: R e s u l t s and D i s c u s s i o n ......................................................................... 95
i v
6
7
8
15
18
19
21
22
24
27
28
36
37
41
45
46
48
Manganese nodules in a sediment core ........................
10Be vs. depth in nodule RC16-D10 ..........................
The energy spectrum of solar and galactic cosmic rays . .
The Stormer cone ...............................................
The ionization losses of the low energy cosmic rays . . .
The cross section for the production of 10Be ..............
The- cross section for the formation of 26Al ..............
The differential yield of 10Be produced in the atmosphere
The expected fallout pattern of the long lived isotopes .
A schematic view of the earth's environment ..............
A plot of E/q vs. M/q ........................................
The Yale isotope analysis system for detecting 1DBe . . .
The Yale isotope analysis system for detecting 26A1 . . .
The UNIS ion s o u r c e ................... ........................
The reflected beam 'cone' geometry ........................
The new extraction electrode for the UNIS ion source . .
L i s t o f I l l u s t r a t i o n s
A manganese nodule i n c r o s s s e c t i o n .................................................
v
Figure 18: BeO beam current as a function of t i m e ................. 50
Figure 19: Inflection magnet scans ..................................... 52
Figure 20: Charge state distributions for 27Al and BeO ions . . . . 54
Figure 21: The 10Be absorber-counter telescope ....................... 57
Figure 22: The range of beryllium and boron in A l u m i n u m .......... 58
Figure 23: The gas-silicon AE-E d e t e c t o r .............................. 59
Figure 24: Typical AE-E spectra of a 44 MeV 9Be b e a m ................60
Figure 25: The Ortec chamber setup for 10B e ...........................61
Figure 26: Typical 10Be and 10B s p e c t r a ............................. 63
Figure 27: The effect of oil on the absorber foil................... 64
Figure 28: The mass to charge ratio for various i o n s ...............66
Figure 29: A schematic drawing of the electrostatic analyzer . . . . 68
Figure 30: A schematic drawing of the new b e a m l i n e ................. 71
Figure 31: The spark supression circuit .............................. 72
Figure 32: A large gas ionization chamber ............................ 74
Figure 33: A 228Th spectrum measured with the ionization chamber . . 76
Figure 34: A AE-E spectrum showing the ions in an 26Al beam . . . 79
Figure 35: A typical 26A1 s p e c t r u m ..................................81
v i
Figure 36: The 10Be e l e c t r o n i c s ......................................... 83
Figure 37: The 26A1 e l e c t r o n i c s ......................................... 84
Figure 38: The event r o u t i n e .............................................. 86
Figure 39: A form for recording the raw 10Be d a t a .......................88
Figure 40: A form for recording the reduced 10Be d a t a ................ 89
Figure 41: Transmission of 10Be and 26A l ................................91
Figure 42: A comparison of 8 counting and accelerator techniques . 92
Figure 43: Reproducability of the 10Be d a t a ........................... 94
Figure 44: 10Be vs. depth into a large manganese s l a b ................ 97
Figure 45: 10Be and 10Be/9Be in nodule T F - 5 ........................... 100
Figure 46: 10Be and 10Be/9Be in nodule A 4 7 - 1 6 ( 4 ) ...................... 101
Figure 47: 10Be and 10Be/9Be in nodule R/V V i t i a z ....................102
Figure 48: 230Th excess in nodules A47-16(4), TF-5, R/V Vitiaz . . 105
Figure 49: The ocean c u r r e n t s .......................................... 109
Figure 50: 10Be in core 1 0 1 7 6 ...........................................112
Figure 51: 10Be in core G P C - 3 ........... ............................. 113
Figure 52: Wave cut benches near Mendocino, Calif..................... 116
Figure 53: Cross sections of the meteorite Hundrabilla ........... 119
v i i
Table 2: A table of various techniques for particle identification . 35
Table 3: The origin of the 160 b a c k g r o u n d ...............................55
Table 4: A table of ions in the b e a m ..................................... 77
Table 5: A table of electrostatic analyzer settings.....................78
Table 6: Diffusion decay models ......................................... 98
Table 7: 26Al in the meteorite M u n d r a b i l l a ............................. 121
Table A l : The 10Be beam energies and NMR s e t t i n g s .................. 128
Table A 2 : The 26A1 beam energies and NMR s e t t i n g s .................. 134
L i s t o f Tab le s
Table 1: Cosmic ray i s o t o p e s and t h e i r h a l f l i v e s ............................................... 2
v i i i
CHAPTER ONE: INTRODUCTION
ray induced spallation of oxygen, nitrogen, and argon in the atmosphere
(La62). Soon after formation they oxidize and then precipitate with the
rain or with dry fallout and are deposited in the geologic sediments of
the earth. Because the half lives of 10Be and 26Al are sufficiently
long, (tt= 1.6xl06 years and .72xl06 years, respectively; see table"2
one) and yet short compared to the age of the earth, there is a
significant decrease in the concentrations of these isotopes in the
older strata of sedimentary geologic samples. The changes in the 10Be
or 26A1 concentrations, as a function of depth in the samples, can be
related to the relative age of the sample and to the sedimentation rate
at the time of its formation. Also, by double dating the samples with
both of these isotopes, we can determine the absolute ages of these
materials and hence determine the intensity of the cosmic rays at the
time these isotopes were created.
Detecting the small amounts of 10Be and 26Al found in geologic
samples is difficult, however. For example, 10Be is produced at a rate
of 2 .xlO-2 atoms/cm2/sec (So77, Re80), and so a sample that accumulates
at a rate of 1 cm per thousand years will have only about 108 atoms of
1DBe per gram of material. The 10Be can be detected with a p decay
counter, but due to the long half life of 10Be it takes 1012 atoms to
produce 1 decay per minute of activity. Therefore, a 1 kg sample of
material deposited at a rate of 1 cm/ky will yield only .1 decays per
minute.
The r a d i o a c t i v e i s o t o p e s 10Be and 26Al are produced by the cosmic
1
Table 1: The table lists several ofthe cosmic ray produced isotopes and their half lives (La62, La67, Ge72,Yi72). They have a wide range of half lives and are useful as tracers for studying a wide range of problems. 14C is particularly well known because its half life of 5,700 years has made it useful for dating anthropologic specimens.
Isotope Half Life
75 3 .3 d1. 6 x HU y5. 7 x 10 y 2 .6 0 y 7.2 x 10 y 1 . 1 x 1 0 y1 4 .3 d2 5 . 3 d 87.4d3. .1 x 10 y 3 5 .0 d 2 2 . 6 9 x 10 y1 . 3 x 105 y 2 . 1 x 1 0 y
A mass spectrometer would be a better instrument to use to detect
1DBe because the total number of atoms can be used for analysis rather
than those few which decay during the counting period. A conventional
mass spectrometer is not sufficient, though, since there are not enough
10Be atoms to form a measurable beam current and because the stable
isobar, 10B, is too close in mass to be differentiated from 10Be.
An accelerator mass spectrometer, on the other hand, can detect
individual atoms in a beam, and the isobars 10Be and 10B are easily
separated by standard nuclear physics techniques.
Accelerator Mass Spectroscopy
The idea of using an accelerator as a mass spectrometer is not a
new one. For their discovery of 3He, in 1939, Alvarez and Cornog (A139)
tuned the Berkeley cyclotron for a mass three beam. Although they ran
into the problem that their accelerator was neither suited for
quantitative beam intensity measurements nor properly shimmed for 3He,
they did manage to detect 3He by rapidly varying the cyclotron's
magnetic field. This resulted in a transiently tuned beam, and they
measured a 3He/4He ratio of ~10-9. The currently acccepted value is
1.4xl0*6 (Sh79). They also looked for 5He, but were unable to find it.
In 1977, Muller proposed using the Berkeley sector focussed
cyclotron as a quantitative mass spectrometer (Mu77b). He pointed out
that standard nuclear physics techniques could be used to count
individual nuclei in an accelerator beam and that the number of nuclei
could be related to the concentration of the isotope in the source.
Muller's first application of accelerator mass spectroscopy was to
search for an anomolously heavy isotope of hydrogen (Mu77a). The search
was based on the fact that a cyclotron accelerates particles with a
3
particular charge-to-mass ratio; this ratio need not be the quotient of
two integers. Muller was looking for the Han-Nambu quarks of charge +1
and an unknown mass. Instead of scanning the magnetic field of the
cyclotron, as Alvarez had done, he swept the cyclotron frequency over a
range that corresponded to quark masses of 1/3 to 8.2 times the mass of
the proton. He observed none, and he set a limit of 2 parts in 1019 for
the occurance of this kind of quark. Other groups have looked for
exotic particles, including the fractionally charged quarks, and so far
nothing unusual has been discovered (El70,Bo77,Mi79,Kl81).
Accelerator mass spectroscopy has also been used to measure the
concentrations of the cosmic ray produced isotopes found in terrestial
samples. 10Be and 14C are the easiest cosmic ray produced isotopes to
detect with accelerator techniques (Ne77). They are low in mass and can
be readily separated from their stable isotopes, and they are lower in
Z than their stable isobars, 10B and 14N, so they can be easily
separated from their isobars by range-energy-loss techniques. 10Be and
14C are especially important nuclides because they are two of the well
established chronometers of geologic and biologic processes.
1DBe and its Applications
10Be was suggested as a chronometer for geologic processes in the
1950's (Ar56), but because of its long half life and the difficulties
associated with detecting its low level of activity it has only been
used to study rather large (kg) samples. Muller recognized the
potential of accelerators for detecting 10Be in small (1 gram) samples,
but it was Raisbeck et al. (Ra78) who developed the cyclotron techniques
for detecting this isotope. At about the same time, Lanford et al.
(La80) and Kilius et al. (Ki80) developed the techniques for 10Be
4
In 1979 Lanford et al. (La80, Tu79a) demonstrated the mass
spectroscopy capabilities of the Yale tandem Van de Graaff accelerator
by measuring the 10Be content of a manganese nodule. These objects are
found on the ocean floor (See figure 1). They are composed primarily of
manganese and iron, although they incorporate other elements found in
the water. One of the mysteries associated with manganese nodules is
that they occur in regions where the ocean sediments accumulate at a
rate of several millimeters per thousand years (Tu78), and the nodules
remain at the ocean-sediment interface instead of being buried by this
rapid flux of sediments (see figure 2).
The Yale group measured the 10Be content of nodule RC16-D10 at
three different depths beneath the surface of the nodule. (See figure
3.) Although the depth intervals were large, ~1 cm, and the errors
were large, 20 to 40%, they demonstrated that the 10Be concentration
decreases exponentially with depth in agreement with previous results
measured with (J decay counters in similar, but larger nodules. If
this decrease is attributed to radioactive decay, then the nodule grew
at a rate of 4.5 millimeters per million years and was at least 5
million years old. We have recently extended and refined this work and
these data are presented in chapter four.
Other groups have also used accelerator mass spectrometry
techniques to study 10Be distributions in geologic samples. Brown et
al. (Br81, K182, and Mi81) used the tandem Van de Graaff at the
University of Pennsylvania to detect 10Be in river sediments deposited
in the Gulf of Mexico. They report that to within 13% all of the
samples had the same 10Be concentration and that the average
5
d e t e c t i o n a t tandem Van de Graaff f a c i l i t i e s .
Figure 1: A cross section of amanganese nodule. The nodule is about 8 cm long, and it is composed primarily of manganese and iron. This nodule is unusual in that it has two growth centers. Most nodules have only one.
Figure 2: A deep ocean sedimentsample with manganese nodules on the surface of the sediments. This box core was taken from the central Pacific. It shows some typical sizes and distributions of manganese nodules from the ocean floor.
Figure 3: 10Be vs. depth in noduleRC16-D10. The 10Be profile falls off exponentially with depth into the nodule. If this decrease is due to radioactive decay then the nodule grew at a rate of 4.5 mm/My. These data were collected by Lanford et al. (La80) using the Yale tandem Van de Graaff.
Be (xIO
at
oms/
g)
8
0 10 20 DEPTH (mm)
30
concentration was about an order of magnitude lower than that of top
layer of the deep ocean sediments. They also measured the 10Be content
of a single sample of topsoil and found a 10Be concentration similar to
the concentration in the river sediments. Their conclusion was that
the 10Be appears to be firmly attached to the soil and sediments,
because the transition from a fresh water to a marine environment does
not alter its concentration.
Monaghan et al, in the Geology department at Yale (Mo81a), are also
studying the geochemistry of 10Be in soils. Their preliminary data
indicate that 10Be has a complex interaction with soils and does not
remain in the soils for a long period of time. I will describe this
project in chapter four.
Finally, a group at Rutgers (Mo81b) has specialized in measuring
10Be in meteorites. They have ex;mined several stony meteorites and have
measured concentrations in the range from 1 to 2 xIO10 atoms/ gram.
Presumably, these data can be related to the exposure ages of these
meteorites because the 10Be is fcrmed by the interactions of the cosmic
rays with oxygen in the meteorite while it is in space.
26A1 and its Applications
26Al (t% = 7.2x10s years) is chemically very similar to 10Be. Lai
(La62), suggested that the chemical similarity of 10Be and 26A1 could
be exploited to produce an absolute chronometer for determining marine
sedimentation rates because the 26Al/ 10Be ratio has an effective half
life of 1.3 million years and should be independent of the isotope
production rates and sedimetation rates. I will discuss this more fully
in the next chapter.
26A1 occurs in much more dilute concentrations than 10Be. Lai has
9
estimated that the theoretical 26Al/10Be production ratio should be
about 1/100, and an experimental observation in deep sea sediments
(Am66) yields an 26Al/ 10Be ratio of .06 atoms/atom.
These low concentrations of 26Al, coupled with the large abundance
of 27Al in our environment, make it very difficult to measure 26A1 in
natural samples. Some progress has been made, though, with the
development of accelerator mass spectroscopy.
The Orsay group (Ra79a) has detected 26A1 with the linear-
accelerator-plus-cyclotron combination named ALICE. This high energy
accelerator was used to differentiate the isobars 26Mg and 26A1 by fully
stripping the ions; the charge state +13 was then unique to aluminum.
The drav/back to this system was that it was very inefficient. ( A
transmission factor of 6x10"5 .) But, they were able to measure
artificially enriched samples containing as few as 5x10s atoms of 26A1
in a ratio of 10-11 atoms of 26A1 to 27Al. 5x10s atoms yields a
activity of ~10 disintigrations per day, barely detectable by even the
most sophisticated p+ counting devices. Unfortunately, this level of
sensitivity is not sufficient for measuring 26A1 in natural samples,
because the natural 26A1/27A1 ratios are typically 10"13 atoms/ atom.
Raisbeck and Yiou (Ra81) have explored the possibility of
isotopically enriching natural samples. They have used the isotope
analyzer SIDONIE, at Orsay, to produce mA beams of Al+ . From their
results, they estimate that they can enrich natural samples by a factor
of 1000 with an efficiency of 5 to 10%. Whether this enrichment step
can be done quantitatively remains to be seen.
Other groups have considered using a tandem Van de Graaff to detect
26A1. The major advantage of the tandem is that it accelerates negative
ions, and while it is difficult to form an aluminum negative ion beam,
it is impossible to form a magnesium negative ion . (The electron
affinities are .46 and <0. respectively (Li76)). Therefore, the 26Mg
isobar will not be present in the beam and cannot interfere with the
detection of 26A1.
A group at the University of Rochester (Ki79) has detected 26Al in
a sample of Aluminum taken from the beam stop of an electron linac. The
sample was estimated to contain an 26A1/27A1 ratio of less than or equal
to 2 x 10"11 atoms/ atom. They measured a ratio of 4 x 10“12 with an
overall efficiency of about 10-4. Vie should note that the total
efficiency of the tandem isotope analyzer is not significantly greater
than the efficiency of the cyclotron-linac system, ALICE, due to the
difficulty of forming an Al“ beam.
Despite this difficulty, a group at Argonne National Laboratory
(Pa80) has used a tandem to investigate the cross sections for reactions
that involve 26Al. They were motivated by the observations of an
excessive amount of 26Mg in Ca-Al rich inclusions in the Allende
meteorite. Presumably, the excess 26Mg came from the decay of 26A1
which had been created by a super-novae.
In order to better understand this process, the Argonne group
studied the destruction of 26Al via the 26Al(n,p)26Mg reaction by
measuring the cross section for the inverse reaction, 26Mg(p,n)26Al.
They produced 26A1 by bombarding a stack of 26Mg foils with protons from
an FN tandem in the energy range from 5.3 to 7.0 MeV. The 26A1 produced
in the foils was chemically extracted and added to a known quantity of
27A10 powder. The 26A1/27A1 ratio in this mixture was then measured by
using the tandem as an ultrasensitive mass spectrometer. They were able
11
to analyze only .5 nA of 27A18+ but were able to detect 26A1/ 27Al
ratios as low as 10"11 atoms/ atom. This corresponded to having 1010
atoms of 26A1 in the Mg foils. The experimental cross sections
determined in this way agreed well with other nuclear physics
measurements and were somewhat lower than a statistical model
prediction.
10Be and 26Al
In the next chapter, I will show that information from a single
isotope , such as 10Be, cannot be used to uniquely determine the age of
a sample. Instead, two isotopes should be used to 'double date' these
samples.
26A1 and 10Be form a convenient pair for double dating. They are
produced by the cosmic rays in a fixed ratio, and they are similarly
deposited in geologic resevoirs due to their almost identical chemical
behavior (He81,La62). (Throughout this thesis, I will assume that the
observed chemical similarity of Be and Al is sufficient to prevent
biological and. geochemical fractionation of these elements while in the
ocean and in the deep sea sediments.)
The purpose of this thesis was to develop techniques to exploit
this fortuitous circumstance and to use these techniques to | study the
historic cosmic ray flux. In collaboration with the Department of
Geology and Geophysics at Yale, we have analyzed 10Be and 26A1jdistributions in a variety of samples. I present below the first set of
data, using accelerator techniques, which utilizes information from both
of these isotopes. I will also show how these kind of data can be used
to search for variations in the cosmic ray intensity, and I will
explain why we were not successful.
12
Chapter Two describes the production and deposition mechanisms for
the cosmic ray produced isotopes. This chapter will fully explain the
principles behind double dating. Chapter Three describes the techniques
used to measure 10Be and 26Al distributions. This chapter also includes
a description of the modifications made to the accelerator to turn it
into a quantitative mass spectrometer, and it describes the specialized
apparatus that was built and installed on a new beam line. In chapter
Four, I will present our data and discuss its consequences.
13
14
CHAPTER TWO: PRODUCTION AND DEPOSITION OF 10BE AND 26AL
The Origin of Terrestial 10Be and 26Al
The half lives of 10Be and 26A1 are too short (tH = 1.6xl06 years
and .72xl06 years respectively) to expect any residual concentrations of
these isotopes from the time of the condensation of the earth. The only
natural source of terrestial 10Be and 26Al, then, is from cosmic ray
interactions with atmospheric nuclei.
The cosmic rays which strike the earth's atmosphere come from two
sources, our sun, and the galaxy. The solar cosmic rays originate in
the photosphere of the sun and therefore consist primarily of protons
and ot particles, with only a small admixture of more complex nuclei.
The solar cosmic rays are accelerated by flares. Compared to the
galactic cosmic rays, the total flux of solar cosmic rays is high, but
their energies are low. The momentum spectrum of protons ejected by a
flare can be represented by an exponential of the form:
F=F0e"R/R°
where R 0 ranges from 50 to 300 ITeV/c, and F0=l to 10 particles/cm2-sec-
sr-MeV/c, depending on the flare (La67). The energy spectrum of a large
solar flare is shown in figure 4.
At energies above 1 GeV, the galactic cosmic rays are much more
abundant than the solar cosmic rays. (See figure 4.) The galactic
cosmic rays might be thought of as the contibution of the other 1011
stars in our galaxy, but the spectrum of energies generated by normal
Figure 4: The energy spectrum ofprotons in space due to a solar flare and from the galaxy. These data are a compilation of several sets of satellite, balloon, and earth based observations which are selfconsistent. (Ha69, La62a, Ba66,Br73).
Diffe
rent
ial F
lux
of Pr
otons
(p
artic
les/
cm2 •
sec •
sr • M
eV
)
15
stars falls off too rapidly to contribute to this flux. Instead, novae
and supernovae explosions can contribute a nonthermal component to the
spectrum at a rate as high as 1038 ergs/sec (Ha69), and this mechanism
probably accounts for the highest energy cosmic rays, some of which have
been observed with energies as high as 102° eV (more than a Joule per
particle). Another contribution to the high energy component of the
galactic cosmic ray flux comes from the acceleration of the low energy
cosmic rays by the magnetic and electric fields associated with the
collisions of large interstellar gas clouds.
The flux of galactic cosmic rays in space is expected to be
isotropic, to be constant in time, and to have a static energy spectrum.
This is due to the large number of sources of particles and the fact
that the particles are trapped by the galactic magnetic fields for times
that are comparable to the age of the galaxy (Me78). However, a
supernovae in the local solar vicinity could disrupt this steady state
condition for a short period of time (Hi73).
The flux of cosmic rays to the earth, however, is not independent
of time. Changes in the terrestial and the solar magnetic fields can
cause significant variations in the flux of low energy particles that
reach the earth's atmosphere. The effect of the earth's magnetic field
was first calculated by Stormer. He treated the earth's field in the
dipole approximation (Wo62), and his calculations of the trajectory of
incoming particles showed that there was a threshold momentum, below
which all particles were deflected away from the earth. The threshold
is dependent on the observer's geomagnetic latitude, X, as well as the
azimuth angle, <f>, and the zenith angle, e , of the particle. The •
threshold momentum, P, is given by:
16
P = M*cos4(X) / R 2*(l+{l-sin(c) cos (<f>) cos3 (X)}^)
where M is the magnetic moment of the dipole and R is the radius of the
earth. For protons arriving vertically, this equation reduces to
P = M*cos4(X) / 4.*R2 = 14.9 cos4(X) GeV/c.
This relationship is plotted in figure 5a. The essence of the figure is
that low energy cosmic rays enter the atmosphere only near the
geomagnetic poles, effectively reducing the total flux of low energy
particles to the earth. At an energy of 1 GeV, less than 18% of the
flux enters the earth's atmosphere. (See figure 5b.)
Many of the low energy cosmic rays do not cause nuclear
interactions because their energies are reduced by ionization losses in
the atmosphere. If R(E) is the range of a proton in air, then the net
loss of flux, or attenuation, as a function of energy is given by:
A(E) = e-{*00-R( E o » / L < E )
This is simply the exponential intensity-range formula. The threshold
energy, E 0, is the minimum energy required to produce an isotope such as
10Be, and L(E) is the nuclear interaction range ( or attenuation length,
expressed in g/cm2). This relationship is shown in figure 6. The
various parameters were chosen to be appropriate for 10Be; the incident
cosmic ray flux was assumed to be entirely protons, E 0 = 40 MeV, R(E) =
2.5x10 B e 1 '7^ g/cm2 , and L(E) = 200 g/cm2 (La62a). At energies
near the threshold energy for the production of 10Be, the effective
interaction rate drops rapidly to zero.
The high energy cosmic rays do not lose much energy due to the
ionization processes, and they can therefore travel to great depths in
17
Figure 5: The Stormer cone shieldsportions of the earth from the low energy cosmic rays, a.) This figure shows the cutoff energies, for protons arriving in the vertical direction, as a function of geomagnetic latitude (Wo62). b.)Figure b shows the effect of this shielding on the total incident flux of protons integrated over all latitudes.
Energy (G
eV)
E ffective Proton Interaction Rate( % of Total Flux)
_ r o o j . & o i o > - > j a > t oo o o o o o o o o
ro
o
CO
o
r\5
£
cn
C u to ff Energy for V ertica lly Incident Protons (GeV)
Figure 6: Ionization processesabsorb the energy of many low energy cosmic rays before they can produce a nuclear interaction. The effect of the ionization losses on the primary proton flux is shown as a percentage of the total flux.
200 400
600 800
1000 1200
1400 1600
ENERGY (M
eV)
E ffe c tiv e Proton Interaction Rate (% of Total F lux )
— | \ ) W A u i ^ > | 0 0 ^ Oo o o o o o o o o o
61
the atmosphere. These long range particles have enough energy to cause
secondary interactions, and the spallations induced in the atmosphere by
these secondary nucleons are the major contributors to isotope
production. 10Be is produced by the spallation of nitrogen and oxygen
while 26Al is produced by the spallation of argon. The cross section
for the formation of 10Be from nitrogen is shown in figure 7. The
reaction has a threshold energy of about 40 MeV and the cross section is
essentially flat above 1 GeV. The 26Al cross section, shown in figure
8, is similar, although the threshold is at about 130 MeV.
The rate of production of 10Be and 26Al is proportional to the
density of the target nuclei in the atmosphere, so that, for example,
the production rate of 26A1 is about 100 times lower than the production
rate of 10Be because argon makes up only 1% of the atmosphere.
The production rates are calculated by integrating the production
cross sections over the cosmic ray spectrum. The form of the integral
is given by (La62a) :
ifield = fdE f (E) (l-sinX(E)) L(E)/L*(E) (l-e_{R(E)"R(E° > >/L(E>) ,
where f(E) represents the energy spectrum of cosmic rays shown in figure
4. The 1 - sin(X) term represents the attenuation of the low energy
cosmic rays by the earth's magnetic field (as shown in figure 5b) and
L(E)/L (E) describes the probability with which the nucleus of
interest is produced per cosmic ray interaction. L(E) is the cosmic ray
k tinteraction length for all interactions, and L (E) is the 10Be or 26Al
kproduction interaction length. L (E) = A/No where N/A is the number
of target atoms per gram of target, and o is the cross section, (see
figure 7). The final term inside the integral represents the
20
Figure 7: The cross section for theproduction of 10Be via proton interactions with a nitrogen target. (The data points are from Re80 and Am7-2, the line through the data points is a semi-emperical model calculation, Si73.)
a (m
b)
21
E(MeV)
Figure 8: The cross section forformation of 26Al via proton interactions with an argon target. (The data points are from Re80, the line through the data is a semi- emperical model calculation, Si73.)
<t (m
b)
22
E ( M e V )
attenuation of the primary cosmic ray flux by ionization losses, (see
figure 6).
I have evaluated the terms of the integral for 10Be using the
information in figures 4,5,6 and 7. I have reduced the magnitude of the
solar cosmic ray spectrum to reflect the fact that only about 5 solar
flares occur per year and that they last for two days each (La62a). The
differential yield is shown in figure 9. The Stormer cone and the
ionization losses effectively eliminate the solar cosmic rays as a
source of 10Be. (The solar cosmic rays contribute even less to the
production of 26A1 because of its higher threshold for formation.) The
integrated yield for the solar cosmic rays is lxlCT5 atoms/cm2-sec-sr.
The yield from the galactic cosmic rays is 80 times larger,- 8xl0*4
atoms/cm2-sec-sr. (In this calculation, I have neglected the
contributions from the secondary nucleons produced by the high energy
cosmic rays, when these are included (La62, La62a, La67) the 10Be
production goes up by a factor of 4. The solar cosmic ray production
rate is then ~300 times smaller than the galactic production rate.)
The most recent theoretical estimate of the 10Be production rate is
2.1xl0-2 atoms/cm2-sec (Re80). The observed rate of deposition to the
ocean sediments is 1.0 to 3.0xl0-2 atoms/ cm2 sec (Kr82, Ta79b). The
global inventory of 10Be, based on these numbers, is over 100 tons. The
theoretical 26Al production rate is 1.1 x 10'4 atoms / cm2 sec, and so
the the predicted 26A1 / 10Be production ratio is .0052 This ratio is
not liable to change with time since the high energy component of the
primary cosmic ray spectrum is expected to depend on galactic parameters
which will not change on the time scale that is of interest to us ( a
few million years).
23
Figure 9: The differentialproduction rate of 10 Be in the atmosphere due to the primary cosmic ray protons. These data were caculated using the spectra in figure4. The production of 10Be by secondary neutrons was neglected. If the neutron yield had been included, the galactic yield would have been larger (La62a, Re80).
1000 10,000
100,000
ENERGY (M
eV)
Differential Production R ate ( particles/cm2 sec-sr-MeV)
O. o , o . O'qj oo <J>
Deposition of 10Be and 26Al
The beryllium and aluminum isotopes oxidize after they are created.
They attach themselves to sub-micron sized particles in the air and
follow the turbulent motion of the air masses. Those nuclei created in
the stratosphere can be exchanged by convection into the troposphere
where they are washed out of the air mass by wet precipitation (La62,
La67). The residence time of the cosmic ray isotopes in the
stratosphere can be a matter of years and so the strong lateral
currents here will cause an averaging of the concentrations of 10Be and
26Al over longitude.
The nuclei which are created in or exchanged into the troposphere
spend very little time in this zone. The turbulent motion of the air
masses in the troposphere effectively brings all nuclei down to cloud
height in a matter of a month or so. The 10Be and 26Al oxides are then
efficiently washed out of the air by rain storms. (This is only true for
the cosmic ray produced isotopes like 10Be and 26A1 that attach
themselves to particles. 14C can reside in the troposphere for long
periods of time in the form of C02 .)
The latitudinal pattern of deposition of the long lived isotopes is
a complex phenomenon. To first approximation, these isotopes are
deposited in a manner that is similar to the deposition pattern of soSr
from bomb fallout shown in figure 10. This deposition pattern has been
observed to be independent of the latitude at v/hich the isotope was
injected into the atmosphere; the increased flux at midlatitudes is due
to stratospheric mixing and a discontinuity in the tropopause at this
point. This figure should describe the long lived isotope deposition
pattern everywhere, except at very high latitudes where the cosmic ray
25
flux is enhanced due to a lack of the geomagnetic field. (For example,
compare figures 5 and 10.)
The latitude dependence of the deposition of the long lived
isotopes does not affect the deposition of these isotopes onto the ocean
floor because the concentrations are thoroughly homogenized by the ocean
waters. The layering of the ocean and the average mixing time of each
layer is depicted in figure 11. The surface layer of the ocean extends
to a depth of about 75 meters and is mixed by wind driven currents. A
thermocline appears in the ocean at the base of this layer. Here the
surface waters meet the cold, deep ocean waters. The deep waters are
driven by the Coriolis forces and thermal gradients, and therefore they
mix more slowly than the surface waters.
10Be and 26Al have been observed to remain in the ocean layers for
periods of time corresponding to the mixing time of each layer. (Ra79b,
Ra80, Th81b). Therefore, these isotopes are retained in the surface
waters for about 20 years, and in the deep ocean for approximately 1000
years, causing the homogenization of these isotopes.
The deep ocean water (average depth ~3500 meters) contains \
nanogram quantities of 9Be per kilogram of water (Me60). This would
lead us to expect a 10Be / 9Be ratio of:
2xl0-2 atoms/cm2-sec * 1000 years1°Be/9Be = ------------------------------------------------------------------
.5 ng/kg * 3500 meters * 6.7xl022 atoms/gram * 1 gram/cm3
~10-8 atoms/atom
which is easily measured. If the beryllium isotopes are deposited with
the accumulating ocean sediments in a ratio similar to this, then we can
use the 10Be/9Be ratio as a tool to study the sediments.
26
Figure 10: The expected falloutpattern of the long lived isotopes. The curve has been normalized to a global average of 1 atom cm'2 sec'1 (La62). The curve is a fit to the observed 90Sr fallout pattern of bomb debris.
GEOMAGNETIC LATITUDE, X 10° 20° 30° 40° 50° 60° 80°
Sin X
Figure 11: A schematic view of theearth's environment. The numbers represent very rough estimates of the size of each resevoir and its average circulation time (La67).
STRATOSPHERE(9 0 to 3IOg air/cm2)(Ito8 years)
TROPOPAUSE
ATMOSPHERE < TROPOSPHERE(720 to 9 4 0 g oir/cm*)(30to90days)
WATER VAPOUR( Ito 3g water/cm2 ) (4to 14 days)
TOPSOIL and BIOSPHERE
CLOUD LIMIT
« 7 5 M ETERS
OCEAN MIXED LAYER
WATERDEEP SEA
3500 METERS WATER
HUMUS and GROUNDWATER
OCEAN MIXED LAYER
20 YEARS)
SEA LEV EL
THERMOCLINEDEEP SEA(500 to3000 YEARS)
■ SE A BOTTOMSEDIMENTS
N i00
The sediments accumulate at a rate of several millimeters per
thousand years (Ta79a) so that 10Be should show a significant,
exponential, decrease in concentration as a function of depth in the
sediments. However, the sediments are diluted with detrital 9Be and
hence do not reflect the dissolved 10Be/9Be ratio in the oceans. Most
of the detrital 9Be originates from land sources which are relatively
free of 10Be yet contain the natural abundance of 9Be. The accumulation
of this material is not independent of time. So, the observed
variations in 18Be concentration in the sediments will reflect this
changing sedimentation rate.
One way to avoid the effects of detrital dilution would be to study
authigenic minerals - minerals that have been formed in situ by
scavanging their mineral constituents directly from the water. They
are formed on- the ocean floor and therefore they should reflect the
10Be/9Be ratios found in the water at the time of their formation.
For example, manganese nodules are accumulations of manganese and
iron that are scavenged directly from the sea water. One hypothesis to
explain the formation of the nodules is that they grow from a nucleus at
a rate of a few millimeters per million years, and if this is true, then
a measurement of 10Be concentration as a function of depth beneath the
nodule surface should reveal an exponential decrease of 10Be due to
radioactive decay (See figure 3). (It should be noted, however, that
some people do not accept this model of nodule formation. Rather, they
claim, it is possible that the nodules have been intact for millions of
years and that the Be isotopes have diffused into the nodules from
outside resulting in an exponential decrease of beryllium towards the
center of the nodule (eg. Ku79b).)
29
There are other unanswered questions about nodule formation. For
example, none of these hypotheses explain how manganese nodules can
remain in contact with the sea v/ater for million of years when there is
a high flux of sediments to the ocean floor. The sediments should bury
the nodules in a few thousand years, but in fact they do not.
Our studies of manganese nodules and deep ocean sediments are
hampered by our inability to determine the true age of the samples. The
'age' as determined by a single isotope, such as 10Be, is not reliable
since we do not know the time rate of change of the isotope's production
rate or it's deposition rate, but two isotopes could be used to uniquely
determine the age of a sample. The idea is simply that there are two
unknown terms in the equation for radioactive decay, the production
rate, P(t), and the sedimentation rate, S(t):
N = P(t0) e_X(t't»>/S(t0)
N is the observed number of atoms per unit volume, X the decay
constant , and t = time. As I have shown, however, the production rates
for 10Be and 26Al are proportional to one another so that we expect that
r(t)ioBe = a F (t )26^^
Furthermore, the chemistry and the sedimentation mechanisms for these
two isotopes are expected to be similar (La62,La67,Am66,Ra79a) and so
the ratio of 26A1 to 10Be should be independent of P(t) and S(t) and:
30
>6N ( t ) 2 6 p ( t o ) / 2 6 S ( t 0 ) e " X 2 6 ( t - t o )
10N (t ) 10P(t0)/10S(t0) e"Xlo(t to)
= ( 1 / a ) e - ( X2 6 - x i o ) ( t - t o)
The effective half life is 1.3xl06 years, and as long as a is
independent of time, then 10Be and 26A1 can be used as an absolute clock
with which to date geologic samples. We can then also turn the problem
around and look at 10Be and 26A1 concentrations as a function of time to
determine their true production and sedimentation rates.
Originally, my goal was to measure the 10Be and 26A1 concentrations
in ocean sediments cores to determine the absolute rate of deposition of
10Be on the ocean floor. I had hoped to relate this flux to the primary
cosmic ray intensity, but I was unable to detect 25A1 due to the large
amount of 27A1 found in the sediments. And so in collaboration with the
Department of Geology at Yale, I have concentrated my efforts on the
understanding of the growth history of manganese nodules. We also began
a program of studies to determine the deposition rate of 10Be and 26Al
as a function of latitude and longitude.
The methods of analysis will be described in the next chapter where
I will show • how the MP-tandem accelerator was converted to an
ultrasensitive mass spectrometer for quantitative anlaysis.
31
32
CHAPTER THREE: EXPERIMENTAL METHODS
Introduction: Decay Counting vs. the Accelerator Techniques
The techniques for detecting rare elements include:
Wet Chemistry
Ion Exchange Resins
Mass Spectroscopy
Decay Counting (for radioactive isotopes) and
Accelerator Mass Spectroscopy.
The decay counting and mass spectroscopic procedures are the most
sensitive and important techniques. The decay counting techniques are
very specific to an isotope since they detect the characteristic
radiation of the nucleus of interest. For example, 10Be decays by 3'
emission with an endpoint energy of 550 keV (Be76), and the 3 decay
can be detected with a proportional detector (Am66). Unfortunately,
most sedimentary geologic samples contain very little 10Be (La62, La67,
Ta79a, Ta79b, Ra79c). An activity of 1.0 decay per day per gram of
sample is not unusual. The sample size can be made very large, this
helps to overcome the background count rate of several counts per day,
but it often means that no detailed information has been collected from
the sampling site.
The accelerator mass spectroscopic techniques are much more
sensitive. Using standard nuclear physics techniques, individual atoms
can be detected and counted rather than having to wait for the
improbable decay of its nucleus to generate a count. But, the overall
efficiency of todays accelerators is only one part in 105 (Th81b), and
so accelerator mass spectroscopy is most useful for detecting isotopes
with very long half lives (>1000 years).
The advantages of the accelerator mass spectrometer are that many
samples can be analyzed per day, and the sample sizes are small (grams
instead of kilograms). Present day accelerators can detect resevoirs
containing as few as 106 atoms of 10Be in a 10Be/9Be ratio of lO-14
atoms/atom. New accelerators are being designed especially for mass
spectroscopy and perhaps they will further improve the efficiency of the
techniques (Pu81).
Accelerator Techniques Applied to Mass Spectroscopy
Several excellent review articles on accelerator mass spectroscopy
have been written recently. The general principles of operation are
covered in a paper by Purser (Pu79). The application of particular
machines to mass spectroscopy is reviewed by Henning and Litherland
(He80, Li81), and the General Ionex design for a small accelerator, to
be dedicated to mass spectroscopic measurements, is described by Purser
(Pu81) .
I will describe, briefly, the general techniques used in
accelerator mass spectroscopy, and then I will proceed with a
description of the specific techniques used at Yale.
The behavior of a charged particle beam in electromagnetic fields
is governed by a few simple relations (see table two). For example, an
electrostatic field causes a beam deflection that is proportional to the
ratio of charge state to energy, (E/q)"1, and the radius of curvature of
an ion beam in a uniform magnetic field is proportional to the square
root of the product Mass x Energy/ (charge state)2 , ME/q2 . The
33
Table 2: A table of varioustechniques for particleidentification.
(1) Magnetic:
(2) E lectrosta tic:
(3) Energy Loss;
(4) Tim e of Flight:
(5) Cyclotron:
PARTICLE IDENTIFICATION
2Radius o: ( Mass * Energy / Charge state )
A.x oc ( Energy / charge state ) *
2 2 AE < x Z / (velocity)
t « velocity *
Frequency “ velocity /rad ius
combination of these two devices defines a point in a plot of M/q vs.
E/q (see figure 12).
The mass of a beam can be defined with other techniques. For
example, if the beam energy is >1 MeV per amu, then the beam energy can
be measured, independently of the charge state, with a solid state or
gas proportional detector. Also, the atomic number of the beam
particles can be estimated by measuring the beam energy loss in a thin
detector, because the energy loss is proportional to z2/(velocity)2
(En74). And, the beam velocity can be estimated by measuring the time
of flight between two detectors (Be79). Any two of these techniques
will uniquely identify the mass of the beam.
At Yale, magnetic analysis and energy loss information is used to
uniquely identify 10Be. Magnetic and electrostatic anlysis is used to
identify 26Al.- These techniques will be described more fully in the
remainder of this chapter.
The Yale isotope analysis system is shown in figure 13. The heart
of the system is a tandem Van de Graaff accelerator; it serves two
purposes. It raises the beam energy so that individual particles can be
detected with nuclear physics techniques, and it serves as a molecular
dissociator.
The sensitivity of conventional mass spectrometers is limited by
how well the device can eliminate molecular interferences (eg. 12C 14N
when detecting 26Al). The tandem Van de Graaff mass spectrometer
eliminates this problem by dissociating the molecules in the stripper
gas in the high voltage terminal. Only atoms, in various positive
charge states, then, are present in the beam after the stripper canal,
so there are no longer any molecular interferences.
35
Figure 12: A plot of E/q vs. M/q. Aconstant magnetic field combined with a constant electrostatic field defines a unique mass/charge ratio.
MA
SS/C
HA
RG
E
36
Figure 13: The Yale isotope analysissystem for detecting 10Be. The sample is introduced into the Cs ion source, at the left. A BeO' beam is extracted and accelerated towards the high voltage terminal. The beam is stripped to charge state 3 and then 4, and then further accelerated and analyzed before passing into a detector chamber. The 9Be beam current is measured with the removable faraday cup after the analyzing magnet. The 10Be nuclei are counted in the detector chamber.
YALE MP TANDEM ISOTOPE ANALYSIS SYSTEM FOR D E T E C T IN G l0 Be
Gridded Lens
Ion Source Magnet
High Voltage ~ IO M V
Be°" i Be3+ B®Quadrupole Lens
Cs Ionizer
/ / Sam ple (chang eab le )
Gas ^ Carbon Stripper poj|
Analyzing M agnet
R em ovable Faraday Cup
S w itc h M a g n e t
A bsorber
E a n d A E Detectors
Another feature of the tandem Van de Graaff mass analyzer that is
useful is that it requires the injection of negative ions (Be77).
Negative ions simplify the detection of 26Al because the isobar, 26Mg,
does not form a negative ion, and so the 26A1 beam can be detected
without this interference.
A Brief Outline of the Techniques for Measuring 10Be and 26Al
The MP tandem isotope analysis system for measuring 10Be/^Be ratios
is shown in figure 13. The samples are chemically extracted from the
material to be studied and a known amount of 9BeO , about one
milligram, is added to give them sufficient bulk for easy handling.
The sample is then inserted in the Cs sputter ion source.
BeO forms a more intense negative ion beam than does Be (Mi77) and
so the ion BeO" is extracted from the source and is sent through a 35°
bending magnet on its way to the accelerator. (The magnet performs a
crude mass separation with a resolution of about 1 part in 25.) The
BeO" molecule is then accelerated to the high voltage terminal ('.10
MV) where the molecule is dissociated and the Be atom is stripped to a
3+ charge state in a stripper gas canal. The beam is further stripped,
to the 4+ charge state, by a thin carbon foil located 1/4 of the way
down the high energy tube. The 9Be4 + and 10Be4+ beams emerge from the
accelerator with a total energy of 39.76 and 40 MeV, respectively. The
beam is then momentum analyzed with a high resolution , 90°, bending
magnet. The intensity of the 9Be beam is measured with a faraday cup
located after the analyzing magnet, but the intensity of the 1DBe beam
is too low to be measured in this way. Instead, the 10Be beam is
allowed to pass into a detector chamber. Here, the beam passes through
38
an absorber, to eliminate the isobar 10B, before going into a AE-E
telescope for particle identification and counting.
The 9Be beam is sufficiently intense (~20 nA) so that the the
accelerator can be tuned directly on this beam. Hov/ever, the 10Be beam
cannot be tuned directly, and so the 10Be tuning is determined by using
a 9Be guide beam with the same rigidity (HE/q2) as the 10Be beam.
A measurement of the 10Be/ 9Be ratio is made by alternately tuning
the 9Be beam into the faraday cup and then the 10Be beam into the
detector chamber. Only three magnets must be changed between these
measurements: the inflection magnet, the high energy quadrupole, and the
analyzing magnet. Each cycle is kept to a few minutes duration in order
to eliminate the effects due to slow drifts in the accelerator tuning.
The measured ratio is then compared to the 10Be / 9Be ratio measured in
a standard sample. This step eliminates mass fractionation effects due
to different charge state probabilities at the terminal and due to
different transmission efficiencies for each isotope. By combining this
measurement with a knowlege of how much 9Be was added to the sample, we
can determine the number of 10Be atoms in the sample.
The techniques for detecting 26A1 are somewhat different because
the isobar, 26Mg, has a lower atomic number than aluminum. As a result
the 26Mg cannot be ranged out of the accelerator beam with a thin
absorber foil. This problem is solved by injecting negative ions into
the accelerator. As was mentioned previously, Mg does not form a
negative ion. However, MgO does form a negative ion and so the Al“ ion
is accelerated rather than the more abundant A10" ion (Pa80, Ki79).
Carbon and oxygen also have a greater range in an absorber foil
than does aluminum and so these ions will contaminate the 26A1 beam.
39
But, they can be removed from the beam by electrostatic analysis. (See
table 2 and figure 12.) So, a new beam line was constructed which
included an electrostatic separator (~ 10° bend). The accelerator
configuration used for analyzing samples containing 26Al is shown in
figure 14. ■ The samples that are introduced into the ion source are
typically a few milligrams of A10, with a known 27Al content. An Al"
beam is extracted from the source and injected into the accelerator.
The beam is stripped only once, to charge state 5+ , to avoid 13C beams
with the same magnetic rigidity (ME/q2) and electrostatic rigidity (E/q)
as the 26A1 beam. The 27A1 beam is measured in the faraday cup while
the 26A1 beam is transported through the electrostatic analyzer and
counted with an ionization chamber telescope.
An 26A1/27A1 ratio measurement is made by alternately tuning the
27A1 beam into the faraday cup and then the 26A1 beam into the
ionization chamber. Only the terminal voltage of the accelerator is
changed between these measurements. None of the magnets are readjusted.
(We found that cycling the terminal voltage betv/een 7.992 and 8.300 MV
was a quick and reliable way to alternate between the aluminum beams.
The Be beams were more easily tuned by having a fixed terminal voltage
and adjusting the accelerator magnets because the Be beams would have
required a large voltage change at high voltage, 9.655 to 10.790 MV, if
the magnets had not been readjusted.) This style of beam tuning causes
a change in velocity of the beam in the stripper canal and may cause
some additional mass fractionation effects. However, all measurements
are compared to a standard sample with a known 26Al / 27Al ratio so that
all such fractionation effects are factored out.
40
Figure 14: The Yale isotope analysissystem for detecting 26Al. An Al" beam is extracted from a sample in the Cs sputter ion source. The beam is then accelerated towards the high voltage terminal. The beam is stripped to charge state 5, and then further accelerated and analyzed before passing into a detector chamber. The 27Al beam intensity is measured with the removable faraday cup after the analyzing magnet. The 26A1 beam is electrostatically analyzed and the 26A1 nuclei are counted in the detector chamber.
2 6 Al A NA LYSIS S Y S T E M
GRIDDED
The geologic samples containing 10Be and 26A1 are of diverse
origin. They range from meteorites to water to ocean sediments, and
each type of sample requires special treatment in order to prepare it
for an accelerator analysis. The purpose of the sample preparation is
to extract the Be and Al from the sample and deliver it in concentrated
form to the cesium sputter ion source. This concentration step can
increase the atoms/ gram ratio of the sample by two to seven orders of
magnitude.
The natural beryllium content of most samples is only a few parts
per million and so a 9BeO 'carrier' is added to the sample in order to
have a visible precipitate during the chemical procedure. For the
aluminum analyses, the samples already contain large fractions (~1%)
of 27A1 and so no Al 'carrier' is required.
The chemistry of beryllium and aluminum is quite similar. The
similarity is a result of the empirical rule of diagonals, whereby
elements lying diagonally adjacent to one another, on the periodic table
of the elements, are observed to have similar chemical properties
(He81). In fact, separating Be and Al is quite difficult. This fact
ensures that the sedimentation rates of Be and Al are similar and makes
'double dating' possible. (See chapter 2.)
The manganese nodule samples were prepared by the Physical Research
Laboratories in Ahmedabad, India, according to the prescription of Amin,
Kharker, and Lai (Am66). Their procedures yield radiochemically pure
samples of BeO and A10. This extreme purity is required for 8
counting applications in order to eliminate other short lived 8
emitters.
42
P r e p a r a t io n o f the Sample
Our application does not require such extreme purity since the
accelerator can easily discriminate between the various isotopes.
Therefore, a simplified procedure was developed, in the department of
Geology at Yale, for extracting Be and Al from the ocean water samples
and the deep sea sediment samples (Ma80a).
The Ion Source
A practical source of heavy negative ions was not possible until
the pioneering efforts of Krohn (Kr62). He observed the emission of
negative ions from surfaces that were bombarded by positive cesium ions,
and he interpreted the result as being due to the presence of an excess
of Cs on the target material.
Middleton and Adams (Mi74) exploited these results in the design of
their Universal Negative Ion Source (UNIS)which is shown schematically
in figure 15. A cesium resevoir is used to generate a neutral cesium
vapor. The vapor bleeds through a porous tungsten ionizer, held at high
temperature, which removes an electron from the cesium atoms. The
resulting ion beam is then focussed on an inverted cone shaped sample
held in a rotating sample wheel. The currently accepted model for
negative ion formation suggests that the cesium beam serves two
functions. First, it provides a thin layer of neutral cesium on the
surface of the sample; this monolayer reduces the work function of the
surface (Mi76). Secondly, the cesium beam sputters some of the material
in the sample. The high energy (~20 keV) cesium beam creates a
collision cascade in the target material, and some of the target atoms
are ejected in the backwards direction with sufficient energy to leave
the surface (-5 eV ). In doing so, they gain an electron from the
cesium monolayer (An76). The resulting negative ions are extracted
43
Figure 15: The Unis ion source. Thesamples are loaded in the rotatable 'cone' wheel and bombarded with cesium. Negative ions are generated and then extracted towards the right.
S O L E N O IDP IE R C E ELECTRODE
T U N G S T E NIO N IZ E R
C Y L IN D R IC A LE X T R A C T IO N
L E N S
C
%
S A M P L E M A TE R IA L TO BE
S P U T T E R E D
\ Xl \
ROTATING SA M PLE M C O N E W H E E L
T- 2 0 K V
The UNIS source generates intense beams of most elements (Mi77) and
it allows the user to run 12 different samples without breaking the
source vacuum. This was the source configuration used by Lanford et al.
for the original 10Be work done at Yale (La80). The UNIS source has
several drawbacks, though. It requires large amounts of material (>20
mg) to be packed into the cone shaped sample holder, it is frequently
difficult to get powders to stick to the cone, and the emittence from
such a large area sample is not very good. Several authors have
proposed new source designs that would circumvent these problems (Ch75,
Ch76, An75, Ty76, Mi80). Unfortunately, these new designs allow for the
mounting of only one sample in the source at a time. Our work requires
that we be able to choose from among many samples without undue delay.
Therefore, I undertook to modify the UNIS source at Yale.
I was primarily interested in handling small samples and so I had
to develop a new sample holder. It is shown schematically in figure 16
(Th81a). The new 'cone' consists of a jacket which fits into the space
designed for the old cone and it contains a vertical insert. (Later a
new cone wheel was built which accepted the vertical insert directly,
without the jacket.) The sample is packed into a dimple on the front
side of the insert. The dimple is approximately 1 mm deep and it holds
0.5 mg to 10 mg of the sample material. The cesium beam passes to the
side of the insert and is reflected back onto the sample by a bias
electrode held at +100 volts.
I also redesigned the negative ion extraction optics. The original
design used an approximately cylindrical lens design to extract the
negative ions. This lens geometry caused the beam to diverge because
45
electrostatically and injected into the accelerator.
Figure 16: The reflected beam 'cone1 geometry. The sample is mounted inthe dimple on the front side of the flat strip. The Cs beam is allowed to pass to one side of the strip and is reflected back onto the sample.
INVERTED CONE SAMPLE HOLDER
Cylindrical L e n s w ith Positive B ia s
C
BeO
Ao>
the field lines are terminated too rapidly by the apertures in the ends
of the cylinders. Therefore, the cylindrical lens was replaced by a
Pierce electrode (See figure 17). The improvement due to the new lens
v/as substantial; the Al' output increased by a factor of 2, from a
typical value of less than 1 nA at the image faraday cup to 2 nA. The
BeO“ beam current has been as high as 500 nA at the source and as much
as 30 nA of this can be transported through to the target chamber.
In summary, the sensitivity of the accelerator-mass-spectrometer
was improved by more than an order of magnitude just by reducing the
required size of the sample from 20 mg to <1 mg due to the change in the
sample holder geometry. The better emittance of the reflected beam
geometry contributed another factor of two improvement and the Pierce
electrode contibuted a factor of two. Overall, the improvement was
almost 100 fold over what was possible with the initial source
configuration. We have recently demonstrated that the 10Be detection
limit is now about 1 part in 1014.
The overall efficiency of the source is not very high. One sample
was sputtered at a rate of about 1 mg per hour of operation. With a 20
nA beam on target, the total efficiency was only - 10~5 . The loss of
beam in the machine only accounts for an order of magnitude of this,
and so the efficiency of the source was ~ 10'4 .
There were other problems with the UNIS source. We had observed
that there was a characteristic 'warm-up' time for each of the BeO
cones. The sample output would start out very low, increase over the
period of an hour, and finally level off (see figure 18). (The AlO
cones did not 'warm-up'. Their output was constant but very low.) We
wanted to understand the cause of this behavior, since waiting for the
47
Figure 17: The new extractionelectrode for the UNIS ion source. The cylindrical lens was replaced by a Pierce electrode. (Pi49, LiOO)
P I E R C E E L E C T R O D E S
W H E E L W I T H 16 P O S IT IO N S
Figure 18: BeO beam current as afunction of time. The sample was mounted in stainless steel and aluminum 'cones'. The BeO beam took about 1 hour to reach maximum output. If the source is not properly tuned, the conditioning can take much longer.
Sour
ce
Curr
ent
(nA
of B
eO
49
T i m e ( m in )
BeO samples to reach a usable level of output consumed a considerable
amount of beam time. We also might have been able to increase the
negative ion yield for the AlO samples. We investigated many possible
explanations, but in the end, we did not find a satisfactory answer as
to why the samples respond so slowly to the Cs bombardment.
In the course of this investigation , we tried mounting the samples
in pure copper holders and we discovered that the inflection magnet
could not adequately resolve A l ” from the much more intense Si' beam
produced from the silicon (~1 %) in the stainless steel sample
holders. Thereafter, v/e mounted our AlO samples in very pure (>.999)
copper holders (see figure 19).
The Inflection Magnet
In order to select and identify the mass of the beam coming out of
the source, a computer controlled power supply was used to power the
inflection magnet. The computer could sweep the inflection magnet
slowly, and produce a plot of the magnet setting vs. the beam
intensity. Some typical plots are shown in figure 19. The new power
supply was set by a Camac crate controller from v/ithin the source cage.
An optical fiber link provided the connection between the Camac crate
(at ~ -300 kV) and an Intelligent Systems Corporation microprocessor
based computer. This system increased the sensitivity of the magnet
setting, as compared to the old electromechanical system, by at least an
order of magnitude. It reduced the time required to change the
inflection magnet, and the reliability of the settings was also
improved.
50
Figure 19: These plots were producedby a microprocessor based system that could automatically adjust the inflection magnet current and record the intensity of the transmitted ion beam. The upper figure shows how poorly the 27A1 and 28Si peaks are resolved even though the sample was A10 mounted in a high purity copper holder. The BeO" beam, shown in the lower part of the figure, was sufficiently intense so that there was no problem with identifying the beam.
INTE
NSIT
Y (A
RB.
SCAL
E)
INTE
NSIT
Y (A
RB.
SCAL
E)
51
The tandem acclerator itself required very few changes in order to
be used as a mass spectrometer. But, we did upgrade several components
in order to increase their stability, so that, for example, the
generating voltmeter could be used to stabilize the accelerator voltage
to ±1 kV.
The high voltage terminal contains two systems for stripping the
beam, carbon foils and nitrogen gas. The carbon foils were used for the
initial 10Be experiments, but this was a mistake because the BeO'
molecule was being accelerated and the coulomb explosion of the molecule
in the foil caused the 10Be beam to diverge. A simple calculation based
on the potential energy stored in the molecule yields:
V = ZjZ2 e 2/ (47T 6Q R 0)
Equating this to the kinetic energy and calculating the momentum of each
particle reveals that the beam divergence angle is:
sin2 (0) = m 1m 2;itT/(E*(m1+m2 )2)
T is the kinetic energy of the molecular fragments. For Z 1=Z2=3, R 0= 1
angstrom, and E= 10.0 MeV, yields 0 ~ 2xl0'3 radians. Assuming a 10
meter drift from the terminal to the high energy quad results in a
maximimum beam divergence of 2 cm. The entrance aperture to the high
energy quad is 4 cm wide, but it v/ould be unrealistic to expect that a
beam so large could be focussed onto a 1 mm wide slit in front of the
object cup. Most of the beam will be lost.
The later 10Be experiments and all of the 26Al experiments were
performed using a nitrogen gas stripper. The advantage of the gas
stripper is that the stripping occurs in several steps. The first
collision between the molecule and a gas atom will produce a neutral
52
The Accelerator
molecule, the second collision should dissociate the molecule, and
subsequent collisions will strip the atoms of their electrons.
Therefore, the effect of the coulomb explosion will be diminished.
Experimental observation confirmed these ideas. The gas stripper
produced twice as much of the Be beam as the foil stripper had.
Typical charge state distributions for Be and Al emerging from a
gas stripper are shown in figure 20. The choice of charge states was
not determined by the abundance of each ion. The Be ion was doubly
stripped to the 3+ and then 4+ charge state so as to eliminate an
intense oxygen background beam (see table 3). The aluminum experiments
were run in the 5+ charge state in order to avoid a C 13 background beam.
(See figure 28.)
There are other backgound beams. These beams are created by ions
that pick up or lose electrons (ie. strip) by colliding with the
residual gas in the accelerator tubes. This can happen anywhere in the
accelerator, and so there can be residual beams that will come through
at any magnet setting. 160 and 12C, as well as 10Be and 13C were
particularly troublesome. Therefore, an additional stage of mass
selection was needed. We chose two different techniques for the two
isotopes 10Be and 26Al.
The Detection of 10Be
The unique identification of 10Be is relatively simple because the
strongest contaminant beams have ranges which are shorter than the range
of a 40 MeV 10Be beam. This means that a thin foil can be used to
absorb the contaminant ions. Any residual ions that have emerged from
the foil can be positively identified by using a AE-E particle
telescope. A schematic diagram of this setup is shown in figure 21.
53
Figure 20: The charge statedistributions for 27Al and 9BeO ions emerging from a nitrogen gas stripper located in the terminal of the accelerator. The charge statedistribution for a 3+/4+ beam is included on the BeO plot. This beam emerges from the gas stripper in the 3+ charge state and is stripped again to the 4+ charge state in a carbon foil located 1/4 of the way down the high energy tubes (Wa81).
CHAR
GE
STAT
E PR
OBA
BILI
TIE
S (%
)
54
TERMINAL VOLTAGE (MV)
Table 3: The origin of the 160background in the 10Be experiments. Row one lists the final charge of the oxygen beam. Row two lists the number of charges that are exchangedwith the residual gas in theaccelerator vacuum pipes. Row three lists the final beam energy. Theveritcal line indicates the maximum 160 energy that can be stopped in a foil without also stopping the 10Be beam. In other words, for a B e 10 beam at 32 MeV, the 80 MeV 160 beam cannot be stopped.
3+/4 + 40 MeV ^ B e beam
Final Charge 1 2 3 4 5 6 7Charge Change -2 -3 -2 -2 -1 1Beam Energy 6 14 25 39 56 76
+ 10 3 32 MeV Be beam
Final Charge 1 2 3 4 5 6 7 8Charge Change -2 -2 -1 1 2Beam Energy 9 20 36 55 80
Figure 21: The 10Be absorber-countertelescope identification system. The accelerator beam will contain boron and beryllium. The boron is stopped in the absorber. The beryllium is transmitted through to the AE-E telescope for positive particle identification.
A b so rb er/C o un ter-Te lesco pe Identification System
B e4 + 1
t/
L/<
A A A
Absorber A E E
(17.25 m g /cm 2 Silicon Detectors Aluminum)
The most important contaminant beams were 40 MeV 1DB which passes
through all of the magnetic analysing elements with the 10Be beam and a
76 MeV 160 beam originating in the terminal from the break up of the BeO
molecule and a subsequent charge exchange in the accelerator tube. The
absorber foil was chosen to be thick enough to stop the 76 MeV 160 beam
but thin enough to pass just enough of the 10B beam to allow its use in
tuning and maintaining the 10Be beam. (See figure 22 and the discussion
below.)
The AE-E telescope for the 10Be experiments was originally a pair
of thin solid state Si(SB) detectors. In order to eliminate the risk of
damaging the AE detector by rotating it into 0° with an intense beam
in the target chamber, I designed a new detector telescope. It
consisted of a thick Si(SB) detector (the E detector) and a gas
proportional counter (the AE detector). The new detector is shown in
figure 23. This detector was very rugged and had satisfactory energy
resolution. The detector telescope was calibrated by observing the
reaction products produced at 30° by a 9Be beam on a 9Be target. (See
figure 24.) The elements He, Li, Be, 3, and C are all well separated on
the plot.
The detectors and absorbers were mounted in a standard 76 cm
diameter Ortec scattering chamber. (See figure 25.) The detector
telescope is mounted 5 cm behind the absorber foils. The detectors can
be rotated between 0° and 60°. When the detectors are at 60°, a 1/4"
aperture is automatically aligned for beam transmission measurements.
As mentioned above, the absorber foils are not actually thick
enough to stop all of the 10B . Instead, some of the 10B is allowed to
enter the detector telescope (see figure 26). 10Be and 10B are isobars
57
Figure 22: The range of berylliumand boron in Aluminum (No70).
RANG
E (m
g/c
m2
)
E N ER G Y (MeV)U100
Figure 23: The gas-silicon AE-Edetector. The beam energy was measured with a silicon detector that had been immersed in isobutane. The AE signal was picked up by a wire loop placed in front of the silicon detector.
H AVAR Entrance (
W indow •d
(
Gas - AE Output
S i (S B )-E Output
Gas Feedthroughs
Figure 24: Typical AE-E spectra ofa 44 MeV 9Be beam on carbon and beryllium targets. The detector was positioned at 30° with respect to the beam.
e totals e +
9 Be Beam on 9 Be Target E = 4 4 M eV 6 - 3 0 °
^ TOTAL = E + A E
9Be Beamon ,2C Target E= 44 MeV 0 = 30°
Figure 25: The Ortec chamber setupfor 10Be. When the detector is rotated to 60°, the second aperture is aligned with the beam axis (0°) for beam transmission measurements.
1/4" APERTURE
\-------C|o o|
y
SA E -E a DETECTOR J
10BeREAM
XABSORBER
1 /4" APERTURE F0,LS
3 /1 6 " A PER TU R E
l / l6 " o r 1 /8" APERTURES
a - ) ~ ~ -
\"ORTEC" CHAMBER
Figure 26: The left hand figureshows a typical multichannel scaler spectrum of the 10B count rate. The right hand figure shows how well the 10Be is separated from the other events in the AE-E spectrum.
BO
RO
N-10
COUN
T RA
TE
T IM E
l°B .. . . , , IOBe...■fls 9 Be
E TOTAL = E + A E
10Be Beam with 15 mg Absorber E = 4 0 MeV 0 = 0 °
and they have not been separated in any way, other than with the
absorber foil, so when the 10B beam is maximized the 10Be beam is also.
In figure 26, the 10Be count rate was only a few counts per minute, but
the 10B count rate was several hundred per second. This was enough to be
able to easily tune the accelerator for maximum transmission. The 10B
count rate is recorded as a function of time by the computer. If the
count rate drops off significantly during a run, the data are discarded.
A typical 10B count rate plot is displayed on the left hand side of
figure 26.
The 10B beam can cause problems if there are any hydrocarbons on
the absorber foils because 7Be can be produced via the reaction H ( 10B,
a)7Be. (See figure 27.) While the 7Be can be differentiated from the
10Be in the AE-E plot, the separation is not completely clean and
could be a severe limitation at very low 10Be intensities. The solution
to this problem is to use a clean set of absorber foils.
In summary, the combination of electrostatic acceleration, magnetic
analysis and range-energy-loss information is sufficient to uniquely
identify 40 MeV 10Be. A measurement of the 10Be / 9Be ratio in a sample
is done in two steps. First, the 9Be beam current is measured in a
faraday cup at the image position of the analyzing magnet. Then, by
adjusting the inflection magnet, the high energy quadrupole, and the
analyzing magnet, a 10Be beam is tuned into the Ortec scattering
chamber. The 10Be nuclei are individually counted for several minutes
and the ratio of 10Be counts / 9Be current is compared to the ratio
measured when a standard sample is in the source.
The Detection of 26A1
26A1 is more difficult to identify than is 10Be due to the
63
Figure 27: The effect of oil on theabsorber foils. 7Be was produced via the H ( 10B,a)7Be reaction. The 7Be counts lie below and to the left of the 10Be peak and are difficult to separate from the 10Be counts.
E TOTALBe Beam with 16 mg Absorber
E = 4 0 MeV 0= 0°
interference of intense oxygen and carbon background beams v/hich are
produced by charge exchange processes in the high energy tubes. It is
impossible to discriminate against these beams with range-energy-loss
techniques because they have a lower atomic number than aluminum and
hence have a longer range in an absorber foil. But these beams can be
separated with an electrostatic analyzer. This device allowed us to
uniquely identify the beam's mass since the beams emerging from the
analyzer have a known E/q ratio which when combined with magnetic
analysis (which defines HE/q2) then determines M/q. An M/q value of
26/5 v/as chosen (50 MeV 26A l 5+) because the 5+ beam is very intense and
this value of M/q is reasonably well separated from the other
contaminant groups. The nearest contaminant would be 1603+ v/hich is 3%
away in terms of M/q. (See figure 28 for a comprehensive list of M/q
values. Note- that the charge states of 13C have the same values of M/q
as the even charge states of 26Al, to within .1%)
The Electrostatic Analyzer
The design of the electrostatic analyzer v/as controlled by three
factors. It must be able to separate beams that differ in M/q by only
1%, it must be simple to construct, and it must be inexpensive. The
traditional design for such a device would be to have a pair of curved
plates separated by a small gap, and bent through 90°. The optics of
this design have been thoroughly studied, and many analyzers of this
kind have been built (eg. Wa47). Unfortunately, for 50 MeV Al5+, these
are impractical devices. The radius of curvature would have to be 13
feet or more and that is not a simple thing to construct. A simpler
design would be a parallel-plate electrostatic deflector (eg. Wa47). In
this case, the deflection of a charged particle in a uniform field is:
65
Figure 28: The mass to charge ratiofor various ion species. The even charge states of 26A1 have the same M/q ratio as the charge states of 13G.
MA
SS/CH
AR
GE
ION SPECIES
26A| t I2C ̂ I3C | I6q [ 27A|
p o
CM
cn
CD
0— CO -
+
° ?
—-si + -
CD+
—
00
04CO +
+•
CD____________ 00_ t CD-----------------±Ol + N+ ---- Ol+
A+
no+
"X* FT+ +
+CD+
Ol+
04 K+ +
04+
Ol -----------------+ Ol
Q .CO
+00
+
^1+
CD+
+ cn+
po+
+
04+
99
X(cm) = q * e (kV/cm) * Y2(cm)/(4000. * E(MeV) )
where Y is the length of the device, X is the deflection and q is the
net charge on the ion. In order to resolve a 1% variation in E/q, or
M/q, the beams should be separated by AX > 1 mm and so X must be of
the order of 10 cm. For a 50 MeV beam in the 5+ charge state, this
requires a one meter long device and an electric field of 40 kV/cm. It
would be prohibitively expensive to provide so large a field gradient
over a 10 cm gap.
Therefore, a modified parallel plate electrostatic analyzer was
designed and built. Its design is shown schematically in figure 29.
The beam is injected at an angle with respect to the long axis of the
analyzer and the beam follows a curved path through the device. As far
as I know, this kind of electrostatic analyzer has never been studied
before. Due • to the bends in one of the plates, the optics are not
simple enough to solve analytically and therefore I attempted to model
this geometry on the computer with a program to solve Laplaces equation
on a grid (Ac70) and a ray tracing program to integrate the equations
of motion of a charged particle travelling throught the calculated
field. The Laplace program turned out to be very inefficient; the ratio
of length to width of the problem, 100:1, meant that a huge number of
grid points had to be used in order to compute the electrostatic field
with reasonable resolution. However, the computer simulations did show
that to a good approximation the electric field between the plates was
merely the voltage, on the plates divided by the distance between the
plates. With this simple approximation, the equations of motion could
be integrated by hand.
The result of this calculation is summarized below for the
67
Figure 29: A schematic drawing ofthe electrostatic anlayzer. The ends of the aluminum cathode are bevelled at an angle of 4.2° so that the beam can be deflected through a larger angle than would be possible in a parallel plate device.
£
I— H--1--1--10 1 2 3 4
S C A L E ( i n c h e s )
S T A IN L E S S S T E E L
FIELD CLAMF
/ "
1
BORON NITRIDE INSULATOR
ELECTROSTATIC SEPARATOR
TALUM INUM BASE
o00
following conditions: the smallest gap between the electrodes in the
parallel section is 1 cm, the ends of the analyzer are 4 cm apart, and
the beam starts from a point that is .5 cm away from the cathode at one
end. Then, the deflection of the beam after traversing the analyzer is
approximately given by:
X(cm)=3.4 8 - 0*Y(cm)+q*E(kV/cm)*Y2(cm)*.51/( 4 0 0 0 .* E (MeV))
The beam is injected at an angle 0=.O977 radians with respect to the
long axis of the device, and it is 101.6 cm (40.0 inches) long. This
equation demonstrates that the analyzer produces about one half of the
deflection that a parallel plate device would produce, but a large
inter-electrode gap is not required. For the condition that the beam
leaves the analyzer on a path that is .5 cm away from the cathode at the
exit slit, this equation can be rearranged to show the voltage required
to bend the beam:
V(kV) = 7.585 * E(MeV)/q
A 1% change in E/q corresponds to a deflection of approximately .95 mm.
The electrostatic analyzer was built in the WNSL machine shop. The
anode was constructed of stainless steel and was polished with alumina.
The cathode was machined out of a piece of aluminum and ground and
polished with alumina. The cathode was then anodized in a 15% sulfuric
acid solution (Ma54, Ha49) at 25° C by passing 20 amperes of current
through the piece for 45 minutes. The action of the solvent produced a
soft oxide layer. This surface was then 'sealed1 (hardened) by boiling
the cathode in distilled water for 45 minutes. The resulting finish was
about 5 microns thick, very smooth, and extremely hard.
The choice of aluminum for the cathode material was made on the
69
basis of studies made at Cern (Ge63, Ro64) and in the Soviet Union
(Er71) which have shown dissimilar electrodes will hold a much larger
electric field gradient than will electrodes composed of the same
material. In particular, these studies have shown that an aluminum
oxide coated cathode and a stainless steel anode can hold fields as high
as 150 kV/cm. (Two stainless steel electrodes can hold only about 75
kV/cm, and the reverse situation, a stainless cathode and an aluminum
anode, is worse yet.)
The electrostatic analyzer was mounted inside a 9" diameter beam
pipe. The assembly includes field clamps and adjustable slit apertures
at the entrance and exit to the analyzer as well as provisions for
moveable target ladders that can intercept the beam before and after
passing through the device.
The electrostatic analyzer was installed on a new beam line, R 45°.
The new beam line includes a large oil diffusion pump for the analyzer,
two Spellman 40 kV (±) power supplies, a focussing quadrupole, an
ionization chamber detector, and appropriate beam defining slits and
vacuum valves. (See figure 30.)
The high voltage feedthroughs and spark suppression circuit for the
analyzer are enclosed in plexiglass above the device. The circuit is
shown in figure 31. The resisitors were designed to prevent large
current surges that occur when the analyzer sparks. Also, large,
transient voltages are shunted to ground by the RF chokes and
capacitors.
The initial voltage conditioning of the analyzer took about one
hour, and as long a good vacuum is maintained in the chamber further
voltage conditioning is not necessary. The analyzer is routinely
70
Figure 30: A schematic drawingthe new beamline for detecting 26Al
ELECTROSTATIC CHARGE STATE SEPA R A TO R
C H A M B E R S E P A R A T O R L E N S M A G N ET
Figure 31: The spark supressioncircuit for the electrostatic analyzer.
6 0 M _n_ RES ISTO RS .25/1F CAPACITORS
CHOKES A R E 20 TURNS OF COPPER W IRE ON DIA.
FERR ITE DONUT.
operated with 80 kV/cm between the electrodes. It sparks only
occasionally, once every few hours, and it resets itself to the correct
voltage in a matter of a fraction of a second.
The Ionization Chamber Detector
A large ionization chamber detector was built for the new beam
line. Its width and length were chosen so that the alignment tolerances
would be minimal, and so that low gas pressures and thin windows could
be used for the detection of heavy ions. The detector is shown in
figure 32. It is similar in design to the detector used at the focal
plane of an Enge split pole at the Univ. of Rochester (Ur78), but is
only about 1/2 the size. The electrical design is similar to that
chosen by Erskine for the Argonne focal plane detector (Er76). The beam
enters the detector through a thin window and deposits its energy in a
large volume of gas. The electron-ion pairs separate and the e"'s are
drawn up through a Frisch grid to the anodes. The anodes are three
plates maintained at ground potential. They are 13 cm wide and 7.5 cm
long. The anode plates are arranged so that they collect energy loss
information from three separate regions of the detector, front, middle,
and back. The cathode is not segmented and it collects the total energy
signal. It is 24 cm long and 13 cm wide, and it is capacitively coupled
to the grid so that it collects the total charge of the ions in the
beam. The cathode is biased to -600 volts, the grid is biased to 40% of
the cathode potential, and the cathode-grid spacing is 4 cm. The anode
to grid spacing is 1.5 cm. The grid was made of .004" diameter gold
plated tungsten wire and the wires were spaced 1 mm apart. The detector
is filled with isobutane and the gas flows continuously through the
detector in order to prevent the build up oxygen and other contaminants.
73
Figure 32: A large gas ionizationchamber used for detecting heavy ions. The anode is segmented in order to collect energy loss information in three regions of the detector. The energy signal isderived from the ions collected at the cathode.
74
BOTTOMVIEW
TOPVIEW
^z!
AE 2:
AE,|
TO ANODES TO GRID-----
-10 VOLTSPRE AMP
TO CATHODE
rf i x.0 0 5
lO OM -A.
-T200
300M j v .PRE AMP
FRONT! VIEW
P■600 VOLTS
The detector performs very well. Figure 33 shows a series of
spectra collected , at various gas pressures, when a 228Th source was in
front of the detector. The alpha energy resolution is about 250 keV.
For heavier ions, the detector performance is demonstrated by it's
ability to distinguish 52 MeV 25Mg from 54 MeV 24Mg. At the present
time, the limiting factor in terms of energy resolution is probably the
energy loss in the entrance window, 2 mg/cm2 of Havar. The entrance
window can be made much thinner since the 26A1 data are recorded with
only 20 torr of isobutane in the detector. (The 2 mg/cm2 Havar window
is convenient, though, since it will support 1 atmosphere of pressure
across the 1 cm x 5 cm entrance window.)
Performance of the New Beam Line
With the aperture at the entrance of the analyzer opened up to 3 mm
in width, the transmission through the analyzer was 100% and the beam
was 1.5 mm in diameter at the exit aperture. Using the ionization
chamber installed at the exit of the analyzer, many different heavy ion
beams can be detected which are the result of charge exchange in the
high energy tubes, as we have discussed, and their energies and charge
states are easily calculated. Tables four and five (Wa81) summarize
these calculations. The example shown in the tables is for a 50 MeV,
26A15+ beam (ME/q2 = 52.) The entries in the tables give the ion beam
energies and the electrostatic analyzer settings. By scanning the
electrostatic analyzer, we have observed isotopes of B, C, 0, Mg, Al,
Si, and P. Figure 34 shows all of the beams observed on a single AE-E
plot. The counting rate ranged from a few counts per second for
phosphorus to several thousand counts per second for oxygen to several
hundred thousand counts per second for the 27 Al ions.
75
Figure 33: A 228Th o decayspectrum observed with the ionization chamber. The a energies are 8.78, 6.78, 6.28, 6.07, 5.68, and 5.43 MeV. The 8.78 MeV a stops in the detector when it is filled with 90 torr of isobutane. At lower pressures, the 8.78 MeV as do not stop.
ENERGY
ENERGY
• j •
228Th SOURCE ENERGY SPECTRUM
90 Torr
ENERGY SPECTRUM 70 Torr
ENERGY SPECTRUM 50 Torr
AE 2 SPECTRUM 50 Torr
77
Table 4: The analyzing magnetselects ion beams with a very specific rigidity ,"VilE/q2 . Table 4 lists the energies of various beams that have HE/q2=52. Mo entries are shown for beams that can come through the analyzing magnet but cannot be fully deflected by our electrostatic analyzer.
C H A R G E
coco<
1 2 3 4 5 6 7 86 8 . 77 7 . 48 6 . 59 5 . 8
1 0 5 . 2 2 0 . 811 4 . 7 1 8 . 91 2 4 . 3 1 7 . 31 3 4 . 0 1 6 . 01 4 3 . 7 1 4 . 91 5 3 . 5 1 3 . 9 3 1 . 21 6 3 . 3 1 3 . 0 2 9 . 31 7 3 . 1 1 2 . 2 2 7 . 51 8 2 . 9 1 1 . 6 2 6 . 01 9 2 . 7 1 0 . 9 2 4 . 62 0 2 . 6 1 0 . 4 2 3 . 4 4 1 . 62 1 2 . 5 9 . 9 2 2 . 3 3 9 . 62 2 2 . 4 9 . 5 2 1 . 3 3 7 . 82 3 2 . 3 9 . 0 2 0 . 3 3 6 . 22 4 2 . 2 8 . 7 1 9 . 5 3 4 . 72 5 2 . 1 8 . 3 1 8 . 7 3 3 . 3 5 2 . 02 6 2 . 0 8 . 0 1 8 . 0 3 2 . 0 5 0 . 02 7 1 . 9 7 . 7 1 7 . 3 3 0 . 8 4 8 . 12 8 1 . 9 7 . 4 1 6 . 7 2 9 . 7 4 6 . 42 9 1 . 8 7 . 2 1 6 . 1 2 8 . 7 4 4 . 83 0 1 . 7 6 . 9 1 5 . 6 2 7 . 7 4 3 . 3 6 2 . 43 1 1 . 7 6 . 7 1 5 . 1 2 6 . 8 4 1 . 9 6 0 . 43 2 1 . 6 6 . 5 1 4 . 6 2 6 . 0 4 0 . 6 5 8 . 53 3 1 . 6 6 . 3 1 4 . 2 2 5 . 2 3 9 . 4 5 6 . 73 4 1 . 5 6 . 1 X 3 . 8 2 4 . 5 3 8 . 2 5 5 . 1 7 4 . 93 5 1 . 5 5 . 9 1 3 . 4 2 3 . 8 3 7 . 1 5 3 . 5 7 2 . 83 6 1 . 4 5 . 8 1 3 . 0 2 3 . 1 3 6 . 1 5 2 . 0 7 0 i 83 7 1 . 4 5 . 6 1 2 . 6 2 2 . 5 3 5 . 1 5 0 . 6 6 8 i 93 8 1 . 4 5 . 5 1 2 . 3 2 1 . 9 3 4 . 2 4 9 . 3 6 7 J 13 9 1 . 3 5 . 3 1 2 . 0 2 1 . 3 3 3 . 3 4 8 . 0 6 5 i 3 8 5 . 34 0 1 . 3 5 . 2 1 1 . 7 20.8 3 2 . 5 4 6 . 8 6 3 J 7 8 3 . 24 1 1 . 3 5 . 1 1 1 . 4 2 0 . 3 3 1 . 7 4 5 . 7 6 2 i 1 8 1 . 24 2 1 . 2 5 . 0 1 1 . 1 1 9 . 8 3 1 . 0 4 4 . 6 6 0 J 7 7 9 . 24 3 1 . 2 4 . 8 1 0 . 9 1 9 . 3 3 0 . 2 4 3 . S 5 9 J 3 7 7 . 44 4 1 . 2 4 . 7 1 0 . 6 1 8 . 9 2 9 . 5 4 2 . 5 5 7 J d 7 5 . 64 5 1 . 2 4 . 6 1 0 . 4 1 8 . 5 2 8 . 9 4 1 . 6 5 b i 6 7 4 . 04 6 1 . 1 4 . 5 1 0 . 2 1 8 . 1 2 8 . 3 4 0 . 7 5 5 J 4 7 2 . 34 7 1 . 1 4 . 4 1 0 . 0 1 7 . 7 2 7 . 7 3 9 . 8 54 J 2 7 0 . 84 8 1 . 1 4 . 3 9 . 8 1 7 . 3 2 7 . 1 3 9 . 0 5 3 . . 1 6 9 . 3
9 5 . 79 3 . 69 1 . 68 9 . 68 7 . 8
MA
SS
78
Table 5: The electrostatic analyzer(ESA) selects beams with a specific energy to charge ratio, E/q. Table 5 lists the ESA setting required to transmit one of the beams listed in table 4. (The settings are in arbitrary units, =9760*E/q.) Since the analyzing magnet defines*^HE/q2 and the ESA defines E/q, a beam emerging from both devices has a unique mass/charge ratio (m/q). Beams that cannot be analyzed by the ESA are not listed (Wa81).
C H A R G E
1 2 3 4 5 6 7 8 £6 1 6 1 2 .7 1 3 8 2 .8 1 2 0 9 .9 1 0 7 5 .
1 0 9 6 7 . 1 9 3 4 .11 8 7 9 . 1 7 5 9 .1 2 8 0 6 . 1 6 1 2 .1 3 7 4 4 . 1 4 8 8 .1 4 6 9 1 . 1 3 8 2 .1 5 6 4 5 . 1 2 9 0 . 1 9 3 4 .1 6 6 0 5 . 1 2 0 9 . 1 8 1 4 .1 7 5 6 9 . 1 1 3 8 . 1 7 0 7 .1 8 5 3 7 . 1 0 7 5 . 1 6 1 2 .1 9 5 0 9 . 1 0 1 8 . 1 5 2 7 .2 0 4 8 4 . 9 6 7 . 1 4 5 1 . 1 9 3 4 .2 1 4 6 1 . 9 2 1 . 1 3 8 2 . 1 8 4 2 .2 2 4 4 0 . 8 7 9 . 1 3 1 5 . 1 7 5 9 .2 3 4 2 1 . 8 4 1 . 1 2 6 2 . 1 6 8 2 .2 4 4 0 3 . 8 0 6 . 1 2 0 9 . 1 6 1 2 .2 5 3 8 7 . 7 7 4 . 1 1 6 1 . 1 5 4 8 . 1 9 3 4 .2 6 3 7 2 . 7 4 4 . 1 1 ) 6 . 1 4 8 8 . 1 8 6 0 .2 7 3 5 8 . 7 1 6 . 1 0 7 5 . 1 4 3 3 . 1 7 9 1 .2 8 3 4 5 . 6 9 1 . 1 0 3 6 . 1 3 8 2 . 1 7 2 7 .
2 9 3 3 4 . 6 6 7 . 1 0 0 1 . 1 3 3 4 . 1 6 6 8 .3 0 3 2 2 . 6 4 5 . 9 t > 7 . 1 2 9 0 . 1 6 1 2 . 1 9 3 4 .3 1 3 1 2 . 6 2 4 . 9 3 6 . 1 2 4 8 . 1 5 6 3 . 1 8 7 2 .3 2 3 0 2 . 6 0 5 . 9 0 7 . 1 2 0 9 . 1 5 1 1 . 1 8 1 4 .3 3 2 9 3 . 5 8 6 . 8 7 0 . 1 1 7 2 . 1 4 6 S . 1 7 5 9 .3 4 2 8 4 . 5 6 9 . 8 5 3 . 1 1 3 S . 1 4 2 2 . 1 7 0 7 . 1 9 9 1 .
3 5 2 7 6 . 5 5 3 . 8 2 0 . 1 1 0 5 . 1 3 8 2 . 1 6 5 8 . 1 9 3 4 .
3 6 2 6 9 . 5 3 7 . 8 0 6 • 1 0 7 5 . 1 J 4 3 . 1 6 1 2 . 1 8 8 1 .3 7 2 6 1 . 5 2 3 . 7 8 4 . 1 0 4 6 . 1 3 0 7 . 1 5 6 8 . 1 8 3 0 .
3 8 2 5 5 . 5 0 9 . 7 6 4 . 1 0 1 8 . 1 2 7 3 . 1 5 2 7 . 1 7 8 2 .
3 9 2 4 8 . 4 9 6 . 7 4 4 . 9 9 2 . 1 2 4 0 . 1 4 8 8 . 1 7 3 6 . 1 9 8 4 .4 0 2 4 2 . 4 8 4 . 7 2 5 . 9 6 7 . 1 2 0 9 . 1 4 5 1 . 1 6 9 3 . 1 9 3 4 .
4 1 2 3 6 . 4 7 2 . 7 0 8 4 9 4 4 . 1 1 8 0 . 1 4 1 5 . 1 6 5 1 . 1 8 8 7 .
4 2 2 3 0 . 4 6 1 . 6 9 1 . 9 2 1 . 1 1 5 1 . 1 3 8 2 . 1 6 1 2 . 1 8 4 2 .
4 3 2 2 5 . 4 5 0 . 6 7 6 • 9 0 0 . 1 1 2 5 . 1 3 5 0 . 1 5 7 5 . 1 7 9 9 .
4 4 2 2 0 . 4 4 0 . 6 5 9 . 8 7 9 . 1 0 9 3 . 1 3 1 9 . 1 5 3 9 . 1 7 5 9 . 1 9 7 8
4 5 2 1 5 . 4 3 0 . 6 4 5 . 8 6 0 . 1 0 7 5 . 1 2 9 0 . 1 5 0 5 . 1 7 1 9 . 1 9 3 4 .
4 6 2 1 0 . 4 2 1 . 6 3 1 . 8 4 1 . 1 0 5 1 • 1 2 6 2 . 1 4 7 2 . 1 6 8 2 . 1 8 9 2
4 7 2 0 6 . 4 1 2 . 6 1 7 . 8 2 3 . 1 0 2 9 . 1 2 3 5 . 1 4 4 1 . 1 6 4 6 . 1 8 5 2
4 8 2 0 1 . 4 0 3 . 6 0 5 . 8 0 6 . 1 0 0 7 . 1 2 0 9 . 1 4 1 0 . 16 1 2 . 1 8 1 4
Figure 34: A AE-E spectrum showingthe various heavy ions that are present in a 50 MeV 26A1S+ beam. The electrostatic analyzer selects only one of these beams at a time.
E TOTA L
Figure 35 shows a AE-E spectrum collected with the electrostatic
analyzer set for 26A1. The oxygen peak, in this figure, results from
the tail of the 1603+ beam; it has an electrostatic analyzer setting
that is only 3% lower than the 26A1 setting. The oxygen count rate, of
a few per second, is at least 1000 times smaller than when the
electrostatic analyzer was tuned for the isq3 + beam.
The procedure for making quantitative 26A1 measurements starts with
tuning an 27A1 guide beam through the accelerator. This beam determines
all of the accelerator magnet settings, except for the ion source
inflection magnet. The 27A1 beam intensity is then measured with a
faraday cup after the analyzing magnet, and then the inflection magnet,
the gridded lens (at the entrance to the accelerator), the terminal
voltage, and the electrostatic analyzer settings are shifted to allow a
50 MeV 26Al beam to pass through to the ionization-chamber detector
where the 26A1 count rate is measured. The 26A1 and 27A1 measurements
are repeated several times and the ratio of 26A1 counts to 27A1 current
* is compared to the ratio obtained from a sample with a known 26A1 / 27A1
content.
The accelerator settings for 26A1 cannot be fine tuned on an
isobaric background beam because there isn't one; 26Mg does not form a
negative ion. The magnetic settings for 26A1 are determined with the
27A1 guide beam and the electrostatic settings are determined by
focussing residual beams into the high energy faraday cup. These
settings are fine tuned using the 26A1 count rate from an enriched
sample, but when a new sample is shifted into position there is no
practical way to fine tune the system for the nev; sample. As a result,
the 26A1 measurements are not as reliable as the 10Be measurements.
80
Figure 35: A typical 26A1 spectrum.The accelerator and the electrostatic analyzer were tuned for 26A1. The 26A1 events are well separated from all other events and they are easily counted.
E TOTAL
Al Standard Sample
The electronics for both the I0Be and the 26A1 experiments were
very simple and were set up using standard NIM modules.
The detector arrangement in the 10Be experiments divided the beam
energy between two detectors. Therefore, the electronics was designed
to send the AE signal and the sum of the two signals (the total energy
signal) to the ADCs (see figure 36). It was important that Ortec 572
spectroscopy amplifiers were used with the AE detector. The 572s have
variable time constant signal shaping, and since the ionization chamber
signals were very slow, we had to use 2 microsecond shaping to collect
all of the charge. The Si(SB) detector amplifier, then, also had to be
operated with 2 microsecond shaping so that a distorted signal would not
be produced by the sum amplifier. The AE and Etota ̂ signals were
then sent to the ADCs along with a logic pulse.
A timing single channel analyzer (TSCA) was used to differentiate
the 1°B signal from other signals. The logic signals from the TSCA were
sent to a rate meter at the operators console to enable them to tune the
accelerator on the 10B beam.
For the 26A1 experiments, a large gas AE-E ionization-chamber was
used. The energy signal from this detector is a total energy signal and
so no sum amplifier is needed. The electronics for the 26A1 experiments
is shown in figure 37.
For both the 26A1 and the 10Be measurements, four signals are sent
to the frontend of the computer. They are the AE and E signals, the
logic pulse and a pulse from a 10 Hz pulser used for measuring time.
(The 'frontend1 is a special IBM-Yale interface containing several
scalers and ADCs. It provides a flexible way to arrange these
82
The E l e c t r o n i c s a n d C o m p u te r I n t e r f a c e
Figure 36: The 10Be electronics.
Be ELECTRON ICS
Figure 37: The 26Al electronics.
TO COMPUTER AE SIGNAL
TO COMPUTER EVENT I
TO COMPUTER ETOTAL SIGNAL
TO COMPUTER BCI INPUT
26ai ele c t r o n ic s00
components so that the digitized experimental information can be
transferred to an IBM 4341 computer.) The AE and E signals are each
input into an ADC. The logic pulse (labelled Event 1) defines the
occurance of an event and causes the ADCs to be read by the computer.
This logic pulse is also used as an input to a scaler to count the total
number of events. The 10 Hz pulser is input into a second scaler to
provide a time reference for the event analysis program.
The computer program to collect and store data was written with the
data aquisition language DAL. A flow chart describing the program is
shown in figure 38. The AE and E signals are stored in a two
dimensional analyzer, and up to three gates may be drawn around regions
of interest in this analyzer. Events that fall inside the gates are
counted and the total number of counts in each of these gates is typed
on the experimenter's console at the end of a run. Events that fall
inside the gates are also sent to one of three multichannel scalers
(MCSs). (The MCSs record the 10B count rate as a function of time, for
example.) A typical display of the AE-E spectrum and of a
multichannel scaler spectrum is shown in figure 26.
Sample Data Reduction
The data reduction techniques for the 10Be and 26Al experiments are
similar. I will describe only the 10Be case.
The raw data is recorded on a form shown in figure 39. (During the
analysis of the data, the experimenter should check all of the NMR and
inflection magnet settings to see if any of them were unreasonable.) It
is necessary to determine the transmission efficiency, the 9Be current
before and after each run, and the number of 10Be events observed during
each run. This information is recorded on another form, shown in figure
85
Figure 38: A flow chart of the eventroutine used to collect the 10Be and 26A1 data.
EVENT ROUTINE FLOW CHART
Figure 39: A form for recording theraw 10Be data.
SAMPLE # ________ DATE_________ TIME EXPERIMENTER ____________________BEAM CURRENT INTEGRATOR SCALE__________________
NMR Inflection 9Be Current or Counts/Time_______________ ION Dial Setting Mognet High Energy Quads l0Be Counts/Time ______9Be guide beam
n \ / k i 9 BeI M A G E . TARGETCiV M
NMR = 23.8138 TRANSM ISSIO N
l0Be beam GVM
N MR = 23.8138('°Be) and
2 2 .5 2 6 5 ^
9 Be >°Be
9 Be ,0Be
9 Be
l0Be
9 Be
10 Be
9 Be l0Be
Figure 40: The table gives theresults of a typical series of 10Be measurements. The image cup currents are digitized so that they appear here as the number of digitized counts per 30 seconds (nA= scale setting * counts / (100*time) ). The 10Be counts were collected in 400 second long intervals and were reduced to a Q value and finally to the number of 10Be atoms/gram sample by using the formulas given in the text.__________________________________
Sample ImageBefore
ImageAfter
Transmission%
3ecounts
Dead Time%
Q 9 SpectrumNumber
Wt(g)sample
Be atoms per gram
Wt(mg)carrier
M5 2412 2715 72 76 1 12.5 none 496. 1 .5 x l0 8 52.2715 3154 109 0 15.5 1-8 ± .43154 3087 75 0 10.0 14-183087 1862 65 0 11.0 19-23
12±2 1.3±.2
0000
40, together with information about how much dead time there was during
each run, the original weight of each sample (in grams), how much 9BeO
carrier was added to each sample (in milligrams), and the spectrum
number under which the raw data was recorded on magnetic tape.
The data is reduced, one run at a time, using intermediate
quantities which I call Q and Q. The numerical scale factors are
arbitrary and I always work with beam currents normalized to the 10 nA
full scale settings. The reduction formulas are as follows:
1°Be(counts/400 seconds) * 600.Q = --------------------------------------------------------------------------------------------------------------------------------------
(l-%dead time)(Image counts before + after)(% transmission)
Q = Q * weight of carrier(mg)/weight of sample (g)
Then the number of 10Be atoms per gram of sample is given by
Q(sample) 6.02xl02°10Be atoms/gram = ____________ * (standard ratio) * ______________
Q(standard) 25. grams/mole
All measurements, on a particular sample, are averaged together. The
quoted error is the standard deviation of the mean.
Reliability of the Data
We have conducted an extensive series of tests in order to
determine the reliability of our data. The sources of uncertainty can
be broken down into two groups, transmission effects and mass
fractionation effects.
The transmission of the beams through the accelerator is very
reliable. We have measured the 10Be and 26A1 count rates in the
detectors, as a function of the inflection magnet and terminal voltage
settings, many times. Typical results are shown in figure 41. The
inflection magnet plot shows that field excursions of ±10 units (arb.)
89
Figure 41: The transmission of lcBeand 26A1 as a function of theinflection magnet and terminal voltage settings.
COUN
TS
/MIN
UTE
co
un
ts
/ m
inu
te
90
M A G N ETIC F IE L D (A rb . Units) TER M IN A L V O L TS (M V )
26 Al
MAGNETIC FIELD (Arb.Units) TERMINAL VOLTS(MV)
are insignificant. The magnet is usually stable to within ± 10 units.
The terminal voltage plot demonstrates that the 10Be beam is stable
against voltage excursions of ± 5 KV and the 26A1 beam is stable against
voltage excursions of ± 2 kV. The terminal voltage usually remains
stable to ± 2 kV. If either the inflection magnet or the terminal
voltage drift by more than these amounts, then the data are discarded.
Figure 42 shows that the isotope transmission is essentially
independent of the concentration of the isotope in the sample, for equal
data collection times, and so a single standard sample can be used to
compare the results of measurements taken from many different samples
with widely different concentrations. The nonlinearity of the 26A1 plot
is the result of increasing the collection time as the 26Al
concentration decreases. The collection times ranged between 100
seconds and 1000 seconds. The accelerator is only stable for about 400
seconds at a time and so the accelerator was begining to detune itself
during the longer runs.
Errors caused by mass and charge fractionation effects are
eliminated by making all measurements relative to a known standard. The
mass fractionation effects will be the same for both samples and the
effects should cancel out.
A typical measurement, at Yale, is reproducable to within 25%.
Occaisionally, the error can be as large as a factor of two. The size
of the error is independent of the number of counts observed, except for
when the observed number of events is very small (ie. < 10). Figure 43
summarizes a series of 40 measurements on a standard 10Be / 9Be sample.
The measurements were made over a period of 2 years, under varying
conditions. The poisson errors were <10% for each measurement and so
91
Figure 42: A comparison of 10Be and26Al standard concentrations measured with the accelerator and with a 0 decay counter.
[26A|/2
7A
|] (£
Decay)
[26A I/27Al] (Accelerator) [l0Be/9Be] (Accelerator)
toN)
Figure 43: Results of a test of thereproducability of our 10Be analyses by multiple measurements on the same sample. About 40 differentmeasurements were made on this sample over the course of 2 years.
10 Be Measurements on Standard Samples
□ S i J S s s c < a i i )
I £ = 7 .5 S S C <recenl)
— I— I— I--------10 20 30 40 50 60 70 80 90
Concentration (arbit. units)
the overall errors must be due to systematic effects. (We do not have
sufficient data to construct a similar plot for 26A1.)
These errors are not caused by the contamination of the samples
while in the source-. Many times, a 10Be standard or an 26A1 standard
was placed next to a blank sample in the source. The standard sample
was sputtered for a long period of time and then the blank sample was
analyzed. The result has always been that the cross contamination is
down by at least three orders of magnitude.
Detailed Operating Procedures for Detecting 10Be and 26A1
I have included two appendices with this thesis. They provide a
list of step by step procedures for collecting the 10Be and the 26A1
data. These procedures were used to collect the data to be presented in
the next chapter.
94
95
CHAPTER FOUR: RESULTS AND DISCUSSION
The Manganese Nodule Problem
Manganese nodules are found on the ocean floors, all over the
world. They occur in fields which can extend for many square kilometers
and the distance between nodules can be as small as a few centimeters.
Manganese nodules come in many sizes and shapes, but the object of
our studies were the small (< 10 cm diameter) spheroidal nodules. A
cross sectional view of one of these nodules (see figure 1) indicates a
distinct center and a ring like structure that extends to the outer
edges of the nodule. (Figure 1 is somewhat unusual in that this nodule
has two centers. Most nodules have only one.) The nodules are composed
primarily of iron and manganese. The nodules also contain relatively
large amounts of Co, Cu, and Ni (.1 to 1%) which accounts for their
potential value as a mineral resource.
The nodules are found at the ocean sediment interface, with the top
surface of the nodules exposed directly to the sea water. (See figure
2) The interface region betwen the ocean and sediments is a complex
region (So78). The boundary layer is several centimeters thick and
contains mucoproteins, other organic material, and mixed sediments. The
nodules apparently sink into this boundary layer but they do not sink
into the sediments as few are found buried in the sediments.
The occurance of the nodules in the boundary layer is something of
a mystery, since the nodules occur in regions where the bottom sediments
accumulate at a rate of several millimeters per thousand years. We
would expect, then, that in order to avoid burial by the sediments the
nodules must float in the bounday layer. This suggestion is not very
satisfying since there is no known reason why this statement should be
true. The problem is made worse by evidence that suggests that the
nodules are very old. Figures 3 and 44 illustrate typical 10Be vs.
depth profiles for manganese nodules. Although the sample size and
depth intevals are large, the figures illustrate that the 10Be
concentration falls off exponentially with depth in the nodules. One
interpretation of these profiles is that the nodules grow at a rate of
several millimeters per million years, and so these nodules are more
than 5 millions years old.
One way out of this dilemma is to assume that these nodule's 10Be
profiles are due to the diffusion of 10Be in from the surface of the
nodules. Table six illustrates two simple models that may describe the
10Be profiles: radioactive decay and diffusion-decay (Ku79b). Both
hypotheses lead to an exponential decrease of 10Be in the nodules.
We tested these hypotheses by measuring the slopes of the 10Be and
23°Th profiles in several nodules. If the profiles were due to
radioactive decay then the slopes, expressed as a growth rate, would
agree. But if diffusion plays a role, then the apparent growth rates
will be different (Gu78).
We chose three small, spheroidal nodules to assay for their 10Be
concentration. Nodule A47-16(4) came from the north equatorial Pacific
ocean at a location of 9° 2.3' north, 151° 11.4' west and at a total
depth of 5040 meters (Domes area A). Nodule TF-5 was collected from the
south Pacific, 13° 53' south, 150° 35' west and at a depth of 3623
meters. And, nodule R/V Vitiaz was collected from the southern Indian
96
Figure 44: 10Be vs. depth into alarge manganese slab. These data were collected with an ultrastable fj decay counter. The integrated count rate for these samples was, typically, 5 to 10 counts per hour. The background rate was 2 to 3 counts per hour. The size of the samples ranged from 50 to 200 grams of material (Sh78).
,0Be
at
oms/
gram
(x
IO9
Table 6: The table presents twomodels to explain the 10Be profiles found in manganese nodules. The decay and diffusion-decay models yield simple exponential 10Be profiles. We have tested these two models and the results are discussed in the text.
DIFFUSION AND DECAY MODELS
Decay: -dN _ ^ =dt
-A tsoln: N.= NQ e
2Diffusion-Decay: d N _ dND -r-s- - S — - AN = o dx* dx
ocean, off the west coast of Australia, at a location of 26° 48' south,
108° 15' east and at a total depth of 5258 meters.
The nodules were sampled in successive layers by scraping the
surfaces with a dental drill. The entire surface area of nodules TF-5
and R/V Vitiaz was removed, but only the upper half, with respect to the
sediments, of nodule A47-16(4) was removed for analysis. The thickness
of each layer was estimated from the weight of material collected and
from direct measurements of the nodule diameter after grinding. A
typical sample represented a shell of material 2 mm thick and weighed 5
grams. The beryllium in the samples was chemically extracted and
converted to BeO and the 10Be content of the samples was then measured
with the Yale tandem Van de Graaff. About 10% of each sample was saved
so the the 9Be content of each layer could also be determined.
The measured 10Be / 9Be ratios are shown in figures 45, 46 and 47.
The data are presented in terms of 10Be atoms per microgram of 9Be. The
natural abundance of 9Be in these nodules was measured by flameless
atomic absorbtion spectrophotometry (Kr81). We prefer to refer to the
10Be data relative to the 9Be content rather than the number of 10Be
atoms per gram of nodule material because an increased deposition rate
of 9Be should lead to a corresponding increase in the 10Be deposition
rate and therefore the 10Be/9Be ratio should be independent of the
deposition conditions. The horizontal bars in the figures represent the
depth intervals sampled and the vertical bars indicate the error
associated with each measurement. The typical errors are ± 25 %. The
background levels of 10Be are negligible.
The 10Be data exibit simple exponential behavior, with some
variations, for each nodule. Nodule TF-5 has the simplest profile. If
99
Figure 45: 10Be atoms/gram innodule TF-5.
(6t//SUJ0|D)
39g /3
9
100
Figure 46: 10Be atoms/gram in noduleA47-16(4).
101
O'
<*•tfi
Eo
a>CDO
a>CD
O
DEPTH (mm)
Figure 47: 10Be atoms/gram innodule R/V Vitiaz.
,0Be
/ 9
Be
(ato
ms/
/ig)
102
its decrease is interpreted in terms of a growth rate, the rate of
growth was 1.3 mm/My and the nodule is over 10 million years old. This
rate is similar to the growth rates determined, on other nodules, by
conventional geologic methods such as fossil diatom dating (Ka80).
Nodule A47-16(4) shows a 10Be/9Be decrease that corresponds to a growth
rate of 2.4 mm/My. Nodule R/V Vitiaz shows an apparent change in growth
rate during the last few million years. The depth interval 0-7 mm
indicates a growth rate of 1.9 mm/My, while the 7-14 mm interval shows a
more rapid apparent growth rate of 6.7 mm/My. It is interesting to note
that the R/V vitiaz sample is the only one of the nodules that is close
to a spreading mid-ocean ridge. The nodule is on the Indian plate which
is moving northward, and the nodule is now about 1000 miles from the
ridge (see figure 49). Since the Mn concentration of seawater increases
near these ridges, it is not unreasonable to speculate that the nodule
was growing more rapidly in the past due to its proximity to the ridge.
The thorium isotope data for these nodules was collected by
Krishnaswami et al. (Kr82). They measured the 227Th, (a measure of
231Pa), 230Th, and 232Th concentrations ( tH = 3.4xl04 years, 8.0 x 104
years, and 1.41 x 1010 years respectively) in the outer most mm of each
of the three nodules. The data is summarized in figure 48. The data
shows evidence for changing growth rates in the first mm of each nodule.
The nodules TF-5 and R/V Vitiaz show satisfactory agreement between the
average Th and Be growth rates. TF-5 indicates an average growth rate
of 1.3±.l mm/My by the Be method and a growth rate of 1.1 mm/My by the
thorium method. The nodule R/V Vitiaz has a 10Be growth rate of 1.9+.3
mm/My in the outermost 7 mm of the nodule. The Th data indicates a
growth rate of 1.2 mm/My for the outermost millimeter. The data for
103
Figure 48: 230Th excess in theoutermost mm of nodules A47-16(4), TF-5, and R/V Vitiaz (Kr82).
23
0Th
exc/
23
2Th
SP
ECIF
IC
AC
TIV
ITY
I03
5
2
I02
5
2
10'
5
2
,0 0.0 0.2 0.4 0.6 0.80.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4
D E P TH (mm)
nodule A47-16(4) shows the poorest agreement. The Th data indicates an
average growth rate of 5 mm/My in the outermost mm of the nodule and the
Be data indicates an average growth rate of 2.4±.6 mm/My. It should be
pointed out that the Th data indicates three separate growth rates for
the first mm of this nodule, indicating that the nodule has undergone
considerable changes in its environment recently. (eg. see Kr78b)
The Th data for the nodule RC16-D10 also shows satisfactory
agreement with the 10Be data. This is the nodule shown in figure 3.
The 10Be data for this nodule was collected by Lanford et al. (La80).
The thorium data yields a grov/th rate of 1.5 mm/My in the 0-600 micron
depth interval and a growth rate of 6 mm/My for depths greater than 600
microns, while the 10Be data indicates an average growth rate of 4.5
mm/My.
We tested the diffusion-decay hypothesis, listed in table 6, with
the Be and Th data from nodules TF-5, R/V Vitiaz, and A-47-16(4). The
slope of the isotope profiles, in this model, is:
A=(S2/4D2 + )./D)% - S/2r
which can be rearranged to appear as A2D+AS=X, where \ is the decay
constant, S= growth rate, and D is the diffusion constant. Assuming
that the Be diffusion constants are the same for all three nodules (and
similarly for the Th diffusion constants) then there 5 unknown
quantities. They are Dge , D^, S(TF-5), S(Vitiaz), and S(47-16(4)).
A least squares fit to the data yields:
D = 1.0±3.0 mm2/My Be
D ^ = .02±.01 mm2/My
S(TF-5)= .95±.06 mm/My
105
106
S(Vitiaz)= l.li.l mm/My
S(A47-16(4))= 5.0±.3 mm/My
X2= -32
We cannot rule out the possibility of diffusion based on this model.
But, the Be diffusion constant is consistent with zero and the Th
diffusion constant is too small to contribute significantly to the
isotope profiles. We can conclude that the nodules exibit real growth
at a rate of several mm/My and therefore the nodules are very old.
We have also attempted to detect 26Al in a manganese nodule from
Domes Area C (15.0° N, 126.° W) in the central Pacific. We have
examined a 1 gram sample taken from the top side of the nodule. We
found that the surface of the nodule contained 2.0±2.xl08 26A1
atoms/gram. (No aluminum carrier was added during the chemical
preparation of this sample and yet ~20 mg of Al was recovered after
the separation. This meant that the observed 26Al/27Al ratio in the
nodule was l.Ofl. xlO-12 atoms/atom, which is the limit of sensitivity
of our apparatus. ) We were unable to detect 26Al in a 1 gram sample
taken from beneath the surface of the nodule. These results rule out
the possibility of double dating the manganese nodules with our present
techniques, but they do help to resolve the question of whether cosmic
dust can contribute to the 26A1 inventory on the earth.
It has been reported in the literature (Am66) that the 26Al/10Be
ratio found in the sediments is an order of magnitude higher than is
expected from the cosmic ray production of these isotopes in the
atmosphere. (The 26Al/10Be ratio is predicted to be .01; La62, La67.)
The 10Be concentrations near the surface of north Pacific manganese
nodules is about 2x1010 atoms/gram and this leads to an upper limit on
the 26Al/10Be ratio of .011100%. Therefore, as long as Be and Al are
not chemically differentiated in the oceans, it seems that all of the
26A1 can be attributed to cosmic ray interactions in the atmosphere,
and it is not necessary to invoke other mechanisms for introducing 26Al
into the environment.
The Dispersion of the Beryllium Isotopes in Sea Water
We have also measured the 10Be content of deep sea water to
determine how much beryllium is available to the nodules. Our sample
was collected from the main nodule field of the Pacific at 29° 38'
north, 121° 29' west. This location was the GEOSECS station 500 and the
sample was taken between the depths of 4031 and 4188 meters. A total of
300 kg of water was sent to Yale and processed by the techniques
described in chapter III. We measured a total 10Be concentration of
6100 ± 1200 atoms/cm3 of sea water.
The average 10Be/9Be ratio on the surface of the north pacific
nodules A47-16(4), Aries-13D, and Mn-139 (Ku79b) is l.OxlO10 atoms per
microgram. Using this value, and our 10Be deep water value, we would
predict a 9Be concentration of .6 ± .1 nanograms per kilogram of water.
This number is in good agreement with the 9Be concentrations measured in
the north Pacific by C. Measures and J. Edmond (personal communication
to S. Krishnaswami). They have measured concentrations ranging from .25
to .4 ng/kg.
These data indicate that the 10Be/9Be ratio on the surface of the
nodules is similar to the 10Be/9Be ratio found in the surrounding
waters, reinforcing the idea that the nodules scavenge their minerals
from the water and not from the sediments. (The 10Be/9Be ratio in the
sediments is lower than the ratio observed in the water due to an excess
107
The flux of 10Be to the nodule A47-16(4) can be calculated with the
expression
F = A 0 * G * p
where F is the flux in atoms/ cm2 sec, A 0 is the surface concentration
of 10Be in atoms/gram, G is the growth rate, and p is the density of
the nodule. Using an average density of 2 g/cm3, the Th growth rate,
and the 10Be concentration of the nodule extrapolated to the surface of
the nodule, the flux of 10Be to the nodule was 8xl0"4 atoms/cm2 sec.
This is only 4% of the estimated global production rate of 1.8x10"2
atoms /cm2 sec. Krishnaswami et al. (Kr82) have determined the 10Be flux
to the sediments associated with nodule A47-16(4), they found a flux of
l.OxlO"2 atoms/ cm2 sec. The nodules, therefore, incorporate only a few
percent of the beryllium available in the ocean water. The rest goes
into the sediments.
Our value of 6100 ± 1200 10Be atoms/cm3 in deep Pacific water is
three times as large as the 10Be concentration found in the deep Indian
ocean water by Raisbeck (Ra80, Ra79a). He measured the 10Be
concentration in the Atlantic and Indian ocean surface layers and in the
deep Indian ocean waters. He found 740 ± 250 atoms/cm3, 765 ± 265
atoms/cm3, and 2200 ± 400 atoms/cm3, respectively. If these data
represent the average 10Be concentrations in the bodies of water from
which they were taken, then the Pacific water contains the most 10Be.
This is possible since the deep ocean currents circulate around the
Antartic in such a way that the Pacific water is the oldest (see figure
49).
A simple box model calculation (La62, Yo78) can determine the
108
o f 9Be i n th e s e d im e n t s c o n t r i b u t e d b y c o n t i n e n t a l d e b r i s . )
Figure 49: The ocean currents. Thesurface currents are driven by the winds. The deep water currents are driven by Coriolus forces and thermal gradients and these forces cause the deep waters to circulate around the antarctic in an easterly direction.
The approximate locations of the samples collected from the oceans are indicated on the deep water map. The nodule R/V Vitiaz was found at the location indicated by the symbol n . The solid line, nearby, indicates the location of the spreading mid-ocean ridge that occurs along the edge of the Indian plate. R/V Vitiaz was closer to this ridge in the past. Nodule TF-5 was found at x and nodule A47-16(4) at o . The location of the Domes area C nodule is indicated by a + symbol. The water sample was taken from GEOSECS station 500 at = , the core 10176 wascollected at + , and the core GPC-3 was collected at • .
1 0 9
S u r fa c e Currents
residence time of a given isotope in the ocean. The residence time, T,
is given by
T = C * h / Q
Where C is the observed concentration of the isotope, h is the height of
the water column, and Q is the rate of injection of the isotope. These
calculations yield a residence time of 10 years in the ocean surface
layer, and 1400 and 3700 years in the deep waters of the Indian and
Pacific oceans. These values are equal to or longer than the accepted
values for the mixing time of the surface waters and the waters of the
deep ocean (see figure 11), suggesting that the Be isotopes are
completely homogenized before they are deposited in the ocean sediments.
The Deep Ocean Sediments
Due to the fact that 10Be is produced by the galactic cosmic rays
and due to the long residence time of 10Be in the oceans, we expect that
1DBe will be laid down in the ocean sediments at a very uniform rate.
Therefore, 10Be profiles, as a function of depth in the sediments,
should be a sensitive indicator of the rate of accumulation of the
sediments.
We have analyzed two deep ocean sediment cores taken from the
Pacific. Core 10176 was located in the central Pacific at 9° 31' north
and 145° 15' west. Core GPC-3 was taken from the central North Pacific
at 30° 20' north and 157° 49' west and from a depth of 5705 meters. Core
10176 shows large 10Be fluctuations with depth (see figure 50). These
fluctuations probably represent dilution of the .sediments by a variable
input of continental debris. The average sedimentation rate for the
first 400 cm of this core was 4.7 mm per thousand years. The data point
at 800 cm is very interesting. Tanaka (Ta79a, Ta79b, TaBO) has reported
110
Figure 50: 10Be in core 10176.
10Be
ATOM
S/GR
AM
(xIO
10)
11 1
1 0
86
4
2
I
.8
.6
.4
.2
0 2 0 0 4 0 0 6 0 0 8 0 0DEPTH (C M )
DEEP SEA SEDIMENTS - CORE 10176
CENTRAL PACIFIC —9° 3 I.5 1 N.Lattitude _ 145° 59.4 ' W. Longitude
i
o
a similar sharp decrease in 10Be concentration in several sediment cores
taken from the Pacific. He has observed an order of magnitude drop in
10Be concentration in sediments deposited ~2 million years ago. He
estimates that if the sedimentation rate was the same before and after
the decrease, then the change in 10Be concentration represents an
erosional era that lasted for ~5 million years.
Core GPC-3 promises to yield much more information. (See figure
51.) The paleomagnetics of this core have been studied extensively, and
so our 1DBe data v/ill eventually be wedded to a larger amount of
conventionally gathered knowlege about this core. The average
sedimentation rate for the first 400 cm of this core v/as 1.7 mm/ky.
Once again, we see a significant decrease in the 10Be concentration at
depths greater than 400 cm. These data are in fair agreement with the
results based • on paleomagnetic data. Corliss (Co79) has measured a
sedimentation rate of 2.5 mm/kY that lasted for ~2 million years.
Before that, the sedimentation rate was 1.1 mm/kY.
We had also hoped to detect 26A1 in this sediment core and thereby
to double date the samples in order to determine an absolute date for
each layer of the sediments. (The ages, quoted above, are based on
geologic estimates.) In this way, we could have measured the absolute
10Be flux to the earth's surface and we could have made quantitative
statements about time variations in the primary cosmic ray intensity.
It does not appear that we will be able to accomplish this goal because
the concentrations of 26A1 in the sediments cannot be detected with our
present techniques. The 26Al production rate is two orders of magnitude
less than the 10Be production rate and the 26Al is diluted by a high
abundance of 27Al already present in the sediments (~10%, Am66). The
112
Figure 51: 10Be in co re GPC-3.
113
1008 06 0
4 0ooO
x 20Eo
io £ 8
O 6o<1) 4
QQO
2
I
—4 1 I I —
— nn o n ~z.
—
—from the
—
— i t iIn * H
North Pacific —
— —— M —
— -1- —
— —
i .... I i . 1100 3 0 0 5 0 0 7 00 9 0 0
DEPTH ( c m )
expected 26Al/27Al ratio is then ~5xl0'14 atoms/atom. This is much
lower than our present detection limit for 26A1 of 10'12 atoms/atom.
Precipitation Samples
We have recently started to measure the direct flux of 10Be onto
the earth's surface by collecting wet and dry precipitation at several
locations from around the world. So far, we have analyzed three
samples; a sample of ice and snow from Greenland and two rain water
samples from India. The ice was collected at the U.S. Naval research
station in Greenland. The sample represents 1 years precipitation onto
one square meter of the earth's surface. The sample weighed 382.5
pounds and contained l.OxlO9 atoms of 10Be. This corresponds to 6000 ±
1400 atoms/gram and to a depostion rate of 3.3±.7 xlO-3 atoms/cm2 sec.
The rainwater samples were collected by the Physical Research
Laboratories in Ahmedabad, India. The samples were taken with small
collectors at several sites. They represent one years precipitation,
although they were collected in 3 months since there was no rain at
these locations for the other 9 months of the year. These samples
contained 25 liters of water or less. We were able to detect 10Be in
two samples. We measured a 10Be concentration of 4000 ± 2500 atoms/cm3
at Madras, India. The other sample was collected at Khandale, India. It
yielded 9000 ± 3000 atoms/cm3. A blank sample processed and analyzed
along with these rain water samples yielded a 10Be/9Be atom/atom ratio
of less than 6x10"14, two to three order of magnitude smaller than the
ratios measured in the samples. (We will be able to calculate the flux
of 10Be to these localities as soon as we know the size of the rain
water samplers.)
We are continuing to investigate this problem. Large area (1000
114
cm2) rain collectors have been placed at several locations in the United
States, and should help us resolve the question of how 10Be deposition
depends on geomagnetic latitude.
10Be in the Earths Soils
If 10Be attaches itself to soils for a long period of time, then we
can use the 10Be concentration as a dating tool to determine the age of
various geologic formations since the soils should integrate the total
1DBe flux. In other words
dN/dt = D - X * N ,
where D represents the average delivery rate of 10Be to the soil. The
solution to this equation is :
N = D/X * (l-e-Xt)
which relates the 10Be concentration directly to time.
We chose to investigate soil samples from the wave cut benches near
Mendocino, Calif. These soils are known to be very old, although the
ages are uncertain, and they sit atop a series of historic coast lines
(See figure 52). Near Mendocino, 5 terraces are distinctly visible.
They stretch inland for about 10 miles and rise to an elevation of 600
feet. Our samples were taken from the 1st and 5th terraces. The first
terrace has been estimated to be 100,000 years old. The fifth terrace
is ~500,000 years old.
Our data will be presented in the thesis of M. Monaghan (Yale
Geology). Instead of 10Be remaining in these soils, it is apparently
washed out of the soil columns and has a total residence time of about
14000 years.
115
Figure 52: Wave cut benches nearMendocino, Calif.
WAVE CUT BENCHES
elevation 5 9 0 1/ F>th T p rrn rp
elevation /8 0 'Ist Terrace
~ 10 MILES
26Al in the Mundrabilla iron meteorite
The cosmic ray bombardment of meteorites produces many isotopes
from the target elements in the meteorite. A complete knowlege of the
isotope distributions in a meteorite should, in principle, allow us to
calculate the irradiation 'history of the object v/hile in space including
the spectrum and time variation of the cosmic rays. The questions that
can be answered in practice are usually: how long had the meteorite been
in space, and when did it enter the earth's atmosphere.
The exposure age and the terrestial age of a meteorite are most
easily determined by comparing the abundances of pairs of isotopes.
Usually a radioactive isotope (which comes into equilibrium with its
production rate fairly rapidly) and a stable isotope (which integrates
the total flux of the cosmic rays) are compared.
26Al and -21 Ne form such a pair. 21Ne is stable, and yet it makes
up only ~50 ppb of the atmosphere so that meteoritic quantities of
21Ne (~10-8 cm3 STP/gram) can be detected in the laboratory without
serious problems due to terrestial contamination. 21Ne is detected by
highly sensitive, but conventional, mass spectroscopic techniques
(Vo79). The 26A1 concentrations are more difficult to detect. 26A1 is
a positron emitter so the decay is usually detected via a 2f-2f
coincidence technique; due to the annihilation of the positron, 26A1
gives off a unique signal in the form of two back-to-back 511 keV gamma
rays plus a gamma ray from the subsequent decay of the excited 26Mg
nucleus. Unfortunately, the 26Al count rate is often so low that only
the 511 keV % rays can be counted and this signal is not unique.
Accelerator mass spectroscopy offers an alternative way to measure
26A1 in meteorites. The accelerator techniques are particularly useful
117
in studying the iron meteorites because the Al content of these
meteorites is very low, and this serves to increase the 26Al/27Al ratio.
Using the techniques described in chapter 3, it is possible to detect
26Al/27Al ratios as low as 10'12 atoms/atom, and this is sufficient
sensitivity to be able to measure the 26A1 content of 1 gram samples of
the iron meteorites such as Mundrabilla.
The Mundrabilla iron meteorite is a medium octahedrite and is
classified as an anomalous member of the group 1A meteorites. The
meteorite is unusual because it contains only about 75% iron-nickel.
(Medium grained octahedrites typically contain 90% iron and 8% nickel
(MaOO).) The remaining portion of the meteorite is composed of the
mineral troilite (FeS). (Ha77). Two fragments of Mundrabilla (weighing
6 tons and 16 tons) were recovered by the Max Planck Institute fur
Kernphysik. Because of its large size, Mundrabilla offers the unique
opportunity of studying a well shielded meteorite. Schlotz (Sc77) has
measured the noble gas concentrations in several samples of Mundrabilla.
He has also measured the 26Al activity of several of the troilite
inclusions. Four of these samples were provided to us via G. Herzog and
D. Pal at Rutgers who also did the chemical preparation of the samples
for us. The samples were taken from two slabs cut through the meteorite
(see figure 53). Two of the samples came from the same location (II-5
and troilite-I, II-5 is a metal sample (Fe-Ni) and troilite I is the FeS
fraction of that sample). The other samples came from different
locations and were composed of Fe-Ni.
The results of our analysis are presented in table 7. The 21Ne,
38Ar, and 26A1 data of Schlotz are also presented in the table. We
measured an 26A1 concentration in troilite-I that is equivalent to
1 1 8
Figure 53: 26A1 was measured in foursamples taken from Mundrabilla. The meteorite was cut into several slabs; samples II-5, 11-20 and Troilite Icame from slab II. Sample IX-13-1 came from slab IX.
119
\\t\
y(cm) Slab - IE \
7.4±1.8 dpm/kg. Using the 2f-2f coincidence technique, Schlotz
measured an 26A1 activity of 15 dpm/kg in this sample. (He also
measured .7 dpm/kg in Fe-Ni II, but it is difficult to compare this to
any of our Fe-Ni data since the samples come from different locations in
the meteorite). The FeS data show a factor of two discrepency between
the two techniques. We feel that the accelerator data is correct
because the 26Al/27Al ratio in this sample was very large, 10"9
atoms/atom, and was easily measured by the AMS techniques. (We have
recently remeasured the 26A1 contents of troilite-I and 11-20 and this
reexamination agrees with the original results.) It is possible that
the FeS samples were contaminated with 22Na or 44Ti and these isotopes
can interfere with the interpretation of a %-X spectrum.
The larger amount of 26A1 in the troilite is to be expected since
the cross section for formation of 26A1 in sulfur is higher than the
cross section in iron. (The Fe-Ni phase of iron meteorites contains
<<1% sulfur (MaOO) whereas the troilite contains almost 50% sulfur.)
Since the 38Ar production rate is not affected by the sulfur content of
the samples, the increase in the 26A1 production rate in troilite is
shown best by the 26Al/38Ar ratios shown in table 7. The table shows
that the 21Ne/38Ar ratio is also larger in the troilite than in the Fe-
Ni of II-5.
We can use the 21Ne/26Al ratio to determine the exposure age of
Mundrabilla. To first order, this ratio would be expected to be
independent of the depth of the sample beneath the meteorites surface,
and this is in agreement with our data. (The 26Al/27Al ratio measured
in IX-13-1 was at the lower limit of our detectability and so the poor
quality of that datum is not unreasonable.) The weighted average of the
120
Table 7: Our measurements of 26A1 inthe iron meteorite Mundrabilla are presented along side of the 21Ne and 38Ar data of Schlotz (Sc77) and the 53Mn data of Hampel et al. (Ha77). The mass of our samples is given in grams, and the 26A1 and 53Hn data are presented in terms of dpm/kg. The noble gas data are presented in units of lO-8 cm3 STP/g.
Sample Mass 2 6 a i 5 3 i v ,Al Mn 21Ne
38Ar 26A l/38Ar 21N e/38Ar 21 / 2 6 A TNe/ Al 2 8 Al( Schlotz)11-20 1.68 1.0±.4 165. . 8 4. 76 .21 . j y . 8±. 3II—5 1.97 .8± .4 140. . 74 4.45 .18 . 17 . 9 ± 4IX -13-1 .90 . 1±. 07 65. .25 1.48 .07 .17 2 . 5±2.
T ro ilite -I . 82 7. 4±1. 8 6.03 3.08 2.5 1.95 .8±.2 15. (717. grams)Fe-Ni II 1333. . 7
21Ne/26Al ratios in Mundrabilla is .9±.2 (10_8cm3STP/g) / (dpm/kg).
This number can be used in the model of Lipshutz et al (Li65). They
have measured the 21Ne/26Al ratio in several iron meteorites v/ith known
exposure ages and have determined that
T=511. * R * 21Ne/26Al My
where R is a numerical factor set equal to .38 . The data in table 7
yields an exposure age of 175 ±30 My. This value agrees fairly well
with the exposure age estimated by Hampel et al. (Ha77); they measured
the 53Mn/38Ar ratio in 10 samples of Mundrabilla and calculated an
exposure age of 230±25 My. Since 26A1 has a shorter half life than 53Mn
(L = 3.7 My ), we v/ould expect that the 21Ne/26Al method would over"2
estimate the exposure age due to the decay of the 26A1 for the duration
of the meteorites terrestial age. But the 26Al age is equal to or less
than the 53Mn age, so the terrestial age of Mundrabilla must be short
compared to the half life of 26A1 (t, = .72 My).*2
122
123
CHAPTER FIVE: CONCLUSION
The Yale HP tandem accelerator has been successfully used to detect
10Be and 26A-1 at extremely low levels. The 10Be detection limit is as
low as lxlO-14 atoms/atom (10Be/9Be). This corresponds to detecting the
10Be in a resevoir containing fewer than 106 atoms of 10Be. For
samples with 10Be/9Be ratios >10‘13, we can measure that ratio to a
precision of ±25%. The 26A1 detection limit is 10"12 atoms/atom
(26A1/27A1). This limit corresponds to being able to detect the 26A1 in
a resevoir containing as few as 107 atoms of 26A1. Unfortunately, this
level is still too high to be able to detect 26Al in small (~1 gram)
geologic samples, but it is useful for meteorites.
These limits represent a factor of 100 improvement over the
previous 10Be detection limit at Yale, and a factor of 100 improvement
over conventional techniques for detecting 26A1. Most of this
improvement was acheived by optimizing the geometry and the operating
conditions of the UNIS heavy ion source. The accelerator can now
generate 30 nA of 40 MeV Be4+ and 5 nA of 50 MeV Al5+ beam measured at
the image point of the 90° analyzing magnet.
In collaboration with the Yale Geochemistry group, we measured the
10Be concentration as a function of depth beneath the surface of three
manganese nodules. Two of the nodules showed a simple exponential
decrease of 10Be as a function of depth. The nodule R/V Vitiaz had a
more complex 10Be profile,- about 3 million years ago, the slope of the
profile changed from an effective growth rate of 6.7 mm/My to an
effective rate of 1.9 mm/My. This change may be related to the location
of R/V Vitiaz which has been moving northward on the Indian plate, av/ay
from a spreading mid-ocean ridge.
The 10Be data for these nodules has been compared v/ith the
Th data of Krishnaswami et al. (Kr82). The two sets of dataC a L c S S
give similar growth rates for two of the nodules. For the third nodule,
both data sets show a great deal of variability. The correlation
between the 10Be and the Th growth rates for the nodules hasexcess ^
shown that the nodules grow at mm/My rates, they are very old (1-10
million years), and that diffusion of Th and Be into the nodules does
not significantly alter the shape of the isotope profiles. We have also
measured the 26Al concentration at the surface of a nodule. The
estimated 26Al/10Be ratio for this nodule agrees with the expected
atmospheric production ratio and this rules out the possibility of
significant contributions to the 26A1 flux from cosmic dust.
Our investigation of 10Be profiles in two sediment cores from the
Pacific ocean show that these cores accumulated at a rate of 4.7 and
1.7mm/ky, and these profiles are consistent with the data of Tanaka
(Ta80) that show that about 2 million years ago there was a major
disruption of the sedimentation process in the Pacific.
Our results also show that the deep ocean water which is in contact
with the surfaces of the nodules and the sediments contains 10Be/9Be in
approximately the same ratio as is found on the surface of the nodules.
This ratio is altered in the sediments due to the contribution of 9Be
from detrital material. The concentration of ,10Be in the deep ocean
water indicates that the residence time of 10Be in the oceans is very
long (>1000 years). This means that the 10Be will be thoroughly
homogenized in the ocean before being deposited in the sediments.
124
10Be does not reside in the atmosphere long enough to be thoroughly
mixed. Therefore, a pronounced lattitude dependence of the deposition
of 10Be with the rain is expected. We have begun a program to measure
the 10Be flux which will be completed soon. We have already analyzed
precipitation samples from Greenland and India.
Measurements of the 10Be profiles in very old and stable soils
from Mendocino, California indicate that 1DBe does not remain in these
soil columns for a long period of time. Instead, the 10Be has a mean
residence time of about 14000 years. These results are being
investigated further.
Finally, we have detected 26Al in 1 gram samples of the Mundrabilla
Iron meteorite. We observed a distinct increase in the 26A1 production
rate in the FeS inclusions of troilite compared to the Fe-Ni which makes
up the majori-ty of this meteorite. Combining our 26A1 data with the
21Ne data of Schlotz (Sc77), we have calculated an exposure age of 175
million years for this meteorite.
The present limitation to our 26Al/10Be measurements is the Al"
output of the ion source. The Al" intensity could be increased by at
least two orders of magnitude before background beam intesities would
become important. A source output of 1 yA of 27Al" would make it
possible to double date manganese nodules and ocean sediments in order
to determine the constancy of the galactic cosmic ray flux. In order to
acheive an Al" beam as large as that will require an entirely new source
concept and design (eg. Mi80).
125
126
APPENDIX I
Detailed Operating Procedures for Detecting 10Be
This appendix gives a detailed list of things to do in order to
measure 10Be and 26A1 with the Yale tandem. It is intended to serve as
a reference for our collaborators. The 10Be procedures listed here are
to be considered as an update to WNSL internal report 70 (Th80).
This procedure has been developed in order to minimize the time
required to measure the 10Be content of a sample and to prevent certain
catastrophic errors such as destroying the AE-E detector telescope.
The procedure should be followed carefully and in the order given.
The experimentor is responsible for selecting a set of BeO samples,
up to 16, for analysis. The set should include a 10Be/ 9Be standard and
a pure 9BeO sample as a blank.
The collimator, apertures. absorbers, and detectors should be set
up in the Ortec chamber as shown in figure 25. The electronics should
be set up as shown in figure 36. There are two beam current integrators
that must be set up. One integrates the 9Be current in the image
faraday cup after the analyzing magnet. The other integrator measures
the amount of beam that has been transported to the target chamber.
Table Al lists the energies and terminal voltages required for the
9Be guide beam, the 10Be beam , and the 9Be beam. The 9Be guide beam
should be set up at this time, using a gas stripper in the terminal.
The slits in front of, and behind, the analyzing magnet should be opened
up to .150" total width.
Table Al: The 10Be beam energies andNMR settings.
Be Guide Beam;
Energy: 44.422 MeV Charge State: 3+/4 +NMR Frequency: 23. 8138 MHz Term inal Voltage:
10„ „Be Beam;
Energy: 40.000 MeV Charge State: 3+/4 +NMR Frequency: 23. 8138 MHz Term inal Voltage:
9Be Beam :
Energy: 39.760 MeV Charge State: 3+/4 +NMR Frequency: 22.5265 MHz Term inal Voltage:
10. 790 MV
9. 655 MV
9.655 MV
1.) Make sure that the detector telescope is not in the beams path. A
9Be beam directly into the telescope will destroy the detectors.
2.) Rotate a new sample into position in the source.
3.) Wait H to 1 hour for the BeO~ beam intensity to grow.
3.5) Make sure that the detector telescope is not in the beams path.
4.) Raise the terminal voltage to a true value of 10.790 MV (on 3/4/80
we had to subtract 20 kV from this number to get the GVM reading), and
tune the accelerator for the 44.422 MeV 9Be guide beam with the NMR set
for 23.8138 MHz. ( The NMR is the primary reference for beam energy,-
this step calibrates the GVM. The NMR has an automatic stabilization
circuit which is activated by setting it in the 'run' mode.) Tune the
beam through to the target chamber. First, install the 1/16"
collimators and then rotate into place the 1/8" collimators. The beam
may be positively identified by replacing the absorber foils by a 12C or
9Be target foil. The spectrum of particles energies observed at 30
degrees should agree with the spectra shown in figure 24.
5.) Measure the beam current in the image cup. Several 30 second
integrations are required for each measurement. (Moving the image cup
will upset the zero level of the image cup beam current integrator.
Therefore, put the image and source cups 'in' and then zero the meter,
then remove the source cup in order to make a reading.)
Measure the beam current at the target. The absorber foils should
still be in the beam path and the detectors rotated to 60 degrees. The
image to target transmission efficiency, just measured, will be used to
scale the 1DBe concentration of this sample for comparison with other
samples. The transmission efficiency is usually 70% or better.
Record the inflection magnet gauss meter reading (eg. ~6000 for
128
mass = 25). Record the analyzing magnet dial setting (eg. 6741), and
record the high energy quadrupole settings (eg. 31.2, 32.3). These
settings will be used for the 10Be beam.
6.) Lower the terminal voltage to a true value of 9.655 MV. (Use the
calibration in step 4 as a guide.) This voltage corresponds to 9Be3+/4+
at 39.76 MeV and to 10Be3+/4+ at 40.00 MeV.Set the NMR to 22.5265 MHz. Set the analyzing magnet and record
the dial setting.
Optimize the inflection magnet seting for 9BeO by peaking the
brightness of the source. Record the gauss meter reading.
Adjust the low energy steerers and the gridded lens.
Optimize the high energy Quads for 39.76 MeV 9Be and record the
settings. These settings should be proportional to the settings for 40 MeV 10Be (and 44.4 MeV 9Be) by a factor of V9/10 .
Adjust the zero level of the image cup beam current integrator and
measure the integrated 9Be beam current.
7.) Rotate the detector telescope to 0 degrees and install the absorber
foils.
8.) Put the source cup in and set the inflection magnet for 1DBeO. An
initial estimate of this setting can be found by scaling the 9BeO
setting b y V 26/25 . The ultimate setting is found by optimizing the
10Be or 1°B counts vs. the inflection magnet setting. While optimizing
the 10Be counts, be careful to locate the 9BeO peak before each cycle.
This is because the source conditions can drift so as to require a new
inflection magnet setting. You need to measure the difference. A,
between the magnet settings for 9BeO and 10BeO as a function of 10Be
counts. The 10BeO peak can be found, in the future, by adding A to
129
the 9BeO setting. The 10Be gauss meter reading is usually 120 divisions
higher.
Record the inflection magnet setting.
Reset the high energy quad to the values recorded in step 5.
Reset the NMR to 23.8138 MHz and readjust the analyzing magnet.
Record the dial setting.
9.) Put the beam on target and tune the inflection magnet and the
terminal voltage in order to maximize the 10B count rate. Count 10Be
for 400 seconds. You can expect about 1 count/ 100 seconds. (The total
integration time is set using BCIMAX at the keyboard. 400 seconds seems
to be a reasonable upper limit for the collection time compared to the
time constants associated with the accelerator's stability.)
Occaisionally, check the 10B count rate in the multichannel scaler. If
it has changed significantly, then discard the data.
10.) Put the image and source cups 'in'.
11.) Optimize the inflection magnet setting for 9BeO.
Set the analyzing magnet to the dial setting recorded in step 6.
Set the high energy quad to the values recorded in step 6.
Optmize the 9Be in the image cup and record the values of the
magnet and the integrator scale setting. Measure the integrated 9Be
current in the image cup.
12.) Go to step 8.
Cycle through steps 8 to 12 for about one hour. Check the
'aperture in' vs. the 'aperture out' readings of .the source to see if it
is properly tuned. It it becomes necessary to retune on the same
sample, go to step 3.5.
At the end of the measurements for a sample, do steps 3.5, 4, and
130
131
5.
To load a new sample, go to step 1.
132
APPENDIX II
Detailed Operating Procedures for Detecting 26Al
The experimentor is responsible for selecting a set of AlO samples,
up to 16, for analysis. The set should include an 26A1/27A1 standard
and a pure 27A10 blank.
The electrastatic analyzer, ionization chamber and collimators
should be set up as in figure 30. The electronics should be set up as
in figure 37.
There are two beam current integrators that must be set up. One
integrates the 27Al current in the faraday cup at the image position of
the analyzing magnet. The other integrator measures the amount of beam
that has been transported to the target chamber.
Table A2 lists the energies and terminal voltages for the 26Al and
the 27A1 guide beam. The 27Al guide beam should be set up at this time,
using a gas stripper. The slit apertures in front of, and behind, the
analyzing magnet should be opened up to .100" total width.i
1.) Rotate a new sample into position in the source.
2.) Wait about h hour for the surface of the sample to be sputtered
away. It may be contaminated. The sample will also outgas for awhile.
3.) Lower the terminal voltage to a true value of 7.992 MV (often the
GVM reading is about 100 kV lower than this number), and tune the
accelerator for the 48.152 MeV guide beam with the NMR set for 34.2925
MHz. (The NMR is the primary reference for beam energy; this step
133
Table A2: The 26Al beam energies andNMR settings.
27Al Guide Beam:
Energy: 48.152 MeV Charge State: 5+NMR Frequency: 34.2925 MHz Term inal Voltage: 7.992 MVEnergy/charge state: 9.630
Al Beam;
Energy: 50.000 MeV Charge State: 5+NMR Frequency: 34 .2925 MHz Term inal Voltage: 8.300 MVEnergy/charge state: 10. 000
Si Beam;
Energy: 46.442 MeV Charge State: 5+NMR Frequency: 34 .2925 MHz Term inal Voltage: 7. 707 MVEnergy/charge state: 9.288
calibrates the GVM. The NMR has an automatic stabilization circuit
which is activated by setting it in the 'run' mode.) The beam may be
positively identified by lowering the terminal voltage by 285 kV. A
small 28Si beam should occur here. If not, then raise the terminal
voltage by 285 kV. If you find a beam here, then you were on the wrong
beam.
4.) Set the electrostatic analyzer for 27Al (E/q = 9.630), and insert
the target faraday cup, with high voltage electron suppression, into the
beams path. Measure the target beam current. Record the beam current
integrator scale setting. Several 30 second integrations are required
for each measurement.
Measure the beam current in the image cup. (Moving the image cup
will upset the zero level of the beam current integrator. Therefore,
put the image and source cups 'in' and then zero the integrator. Then
remove the source cup in order to make a reading.) The image to target
transmission efficiency, just measured, will be used to scale the 26A1
concentration of this sample for comparison with other samples.
5.) Record the inflection magnet gauss meter reading.
Raise the terminal voltage to a true value of 8.300 MV. (Use the
calibration in step 3 as a guide.) This voltage corresponds to 27Al5+
at 50 MeV. Before you change the inflection magnet, adjust the low
energy steerers and the gridded lens to optimize the 27Al beam measured
in the high energy cup.
6.) Put the source and image cups 'in'. Set the electrostatic analyzer
for 26A1 (E/q = 10.000), and remove the target faraday cup. Set the
inflection magnet for 26A1. An initial estimate of this setting can be
found by scaling the 27A1 setting by ~]/ l o j l l . The best setting is
134
found by optimizing the 26A1 counts measured in the ionization chamber.
While optimizing the 26A1 counts, be careful to locate the 27A1 peak
before each cycle. This is because the source conditions may drift so
as to require a new inflection magnet setting. You need to measure the
difference. A, between the magnet settings for 27Al and 26A1 as a
function of 26A1 counts. The 26Al peak may be found, in the future, by
subtracting A from the 27A1 setting.
The 26A1 gauss meter reading is usually about 120 divisions lower
than the 27A1 setting. Record the inflection magnet setting.
7.) Put the beam on target and count 26A1 events for 400 seconds. You
can expect only a few counts per run. (The total integration time is
set using BCIMAX at the keyboard. 400 seconds seems to be a reasonable
upper limit for the collection time compared to the time constants
associated with the accelerators instability.)
8.) Put the image and source cups 'in'. Optimize the inflection magnet
for 27A1. Lower the terminal voltage to a true value of 7.992 MV.
Optimize the low energy steerers and the gridded lens settings with the
27A1 beam in the high energy cup. Measure the integrated 27A1 beam
current in the image cup. Record the integrator scale setting.
9.) Go to step 5.
Cycle through steps 5-9 for about 1 hour. Check the aperture 'in'
vs. the 'aperture out' readings of the source to see if it is properly
tuned. If it becomes necessary to retune on the same sample, go to step
3. After the last measurement on a sample, do steps 3 and 4.
To load a new sample, go to step 1.
135
BIBLIOGRAPHY
136
Ac70 F.S. Acton, Numerical Methods That Work, Harper and Row, 1970.
A139 L.W. Alvarez and R. Cornog, Phys. Rev. 56(1939)379. "3He in Helium".
Am66 B.S. Amin, D.P. Kharkar and D. Lai, Deep Sea Research 13(1966)805. "Cosmogenic 10Be and 26Al in Marine Sediments".
Am72 B.S. Amin, S. Biswas, D. Lai and B.L.K. Somayajulu, Nuc. Physics A195(1972)311. "Radiochemical Measurements of 10Be and 7Be Formation Cross Sections in Oxygen by 135 and 550 Mev Protons".
An75 H.H. Anderson and P. Tykesson, IEEE Trans. on Nuc. Sci., NS-22(1975)1632. "A Pig Sputter Ion Source for Negative Ions".
An76 H.H. Anderson, IEEE Trans. on Nuc. Sci. NS-23(1976)959. "Sputtering and Ion Source Technology".
An81 John Mark Anthony, private communication.
Ar56 J.R. Arnold, Science, 124(1956)584. "10Be Produced by CosmicRays".
Ba66 V.K. Balasubrahmanyan, D.E. Hagge, F.B. McDonald, in Recent Advances in Cosmic Ray Research J. Gauger and A.J. Masley eds. Western Periodicals, North Hollywood, 1966. "Low Energy Cosmic Rays"
Be76 Behrens and Szybisz, Physics Data 6-1(1976)1. "Shapes of Beta Spectra"
Be77 C.L. Bennet et al., Science 193(1977)508. "Radio-carbon Dating Using Electrostatic Accelerators: Negative Ions Provide the Key".
Be79 R.R. Betts, NIM 162(1979)531.
Bo77 R.N. Boyd, D. Elmore, A.C. Melissinos and E. Sugarbaker, PRL 40(1978)216. "Search for Fractionally Charged Tungsten Ions".
Br73 G. Brooke in Cosmic Rays at Ground Level A.W. Wolfendale ed.,Institute of Physics, London. "Protons and Pions"
Br81 L. Brown, I.S. Sacks, F. Tera, J. Klein and R. Middleton, submitted to Earth and Planetary Science Letters. "lOBe in Continental Sediments".
Ch75 K.R. Chapman, NIM 124(1975)299. "The Inverted Sputter Source".
Ch76 K.R. Chapman, IEEE Trans. on Nuc. Sci. NS-23(1976)1109. "Development of The Inverted Sputter Source".
Co79 B.H. Corliss and C.D. Hollister, Nature 282(1979). "Cenozoic
137
E170 J.W. Elbert, A.R. Erwin, R.G. Herb, K.E. Nielsen, M. Petrilak and A. Weinberg, Nuclear Physics B20(1970)217. "A Quark Search in Ordinary Matter Using Simultanious Measurement of Mass and Charge".
En74 J.B.A. England, Techniques in Nuclear Structure Physics Part Two, New York: John Wiley.
Er71 V.A. Ergakov, I.I. Kryuchkov and Y.V. Trebukhovskii, Pribory i Tekhnika Esperimenta 6(1971)20. "Electrical Insulation in Electrostatic Analyzers: A Review". English translation available.
Er76 J.R. Erskine, T.H. Braid and J.C. Stoltzfus, NIM 135(1976)67. "An Ionization Chamber Type of Focal Plane Detector".
Fi77 R. Finkell, S. Krishnaswami and D.L. Clark, Earth and Planetary Science Letters 35(1977)199. " 10Be in Arctic Sediments".
Ge63 C. Germain and R. Tinguely, NIM 20(1963)21. "Electrostatic Separator Techniques at CERN".
Ge72 Chart of the Nuclides, eleventh edition, 1972.
Sedimentation in the Central North Pacific".
Ge82 Donald S. Gemmell, private communication. See also "Collisional Effects in the Passage of Fast Molecular Ions Through Foils", 9 th ICACS, Lyon-, France, 1981.
Gu78 F. Guichard, J.L. Reyss and Y. Yokoyama, Nature 272(1978)155. "Growth Rate of Manganese Nodules Measured with 10Be and 26A1".
Ha49 G. Hass, Journal of the Optical Society of America 39(1949)532. "On the Preparation of Hard Oxide Films with Precisely Controlled Thickness on Evaporated Al Mirrors".
Ha69 S. Hayakawa, Cosmic Ray Physics, Wiley Interscience, 1969.
Ha77 W. Hampel and 0. Muller, Meteoritics 1^2(1977)249. "Spallogenic 53Mn in the Mundrabilla Iron Meteorite".
He81 W. Henning, W. Kutschera, M. Paul, R.K. Smither, E.J. Stephenson and J.L. Yntema, NIM 184(1981)247. "Accelerator Mass Spectroscopy and Radioisotope Detection at the Argonne FN Tandem".
He81 G. Herzog, private communication. The chemical similarity of Be and Al is an example of the emperical 'rule of diagonals'.
Hi73 J.C. Higdon and R.E. Lingenfelter, Nature 246(1973)403. "Sea Sediments, Cosmic Rays and Pulsars".
Ka80 D. Kadko and L.H. Burckle, Nature 287(1980)725. "Manganese Nodule Growth Rates Determined by Fossil Diatom Dating".
Ki79 L.R. Kilius, R.P. Beukens, K.H. Chang, H.W. Lee, A.E. Litherland,
138
D. Elmore, R. Ferraro and H.E. Gove, Nature 282(1979)488. "Separation of 26Al and 26Hg Isobars by Negative Ion Mass Spectroscopy".
Ki80 L.R. Kilius, R.P. Beukens, K.H. Chang, D. Elmore, R. Ferraro, H.E. Gove and K.H. Purser NIM 171(1930)355. "Measurement of 10Be/9Be Ratios Using an Electrostatic Tandem Accelerator".
K181 J. Klein and R. Middleton, to appear in the Proceedings of ASysmposium on Accelerator Mass Spectroscopy, Argonne, 1981. "Search for Anomalously Heavy Isotopes and a Super Heavy Element."
K182 J. Klein, R. Middleton and H. Tang, submitted to NIM."Modifications of an FN Tandem for Quantitative 10Be Measurements".
Kr62 V.E. Krohn, J. of Applied Physics 33(1962)3523. "Emmission of Negative Ions From Metal Surfaces Bombarded by Cesium Ions".
Kr78a S. Krishnaswami, J.K. Cochran, K.K. Turekian and M.M. Sarin, Colloques Internationaux du C.N.R.S. No. 289, La Genese des Nodules de Manganese, pg 251. "Time Scales of Deep Sea Ferromanganese Nodule Growth Based on 10Be and Alpha Track Distributions".
Kr78b S. Krishnaswami and J.K. Cochran, Earth and Planetary Science Letters 40(1978)45. "Uranium and Thorium Series Nuclides in Oriented Ferromanganese Nodules: Growth Rates, Turnover Times and NuclideBehavior".
Kr81 S. Krishnaswami, private communication.
Kr82 S. Krishnaswami, A. Mangini, J.H. Thomas, P. Sharma, J.K. Cochran,K.K. Turekian and P.D. Parker, to appear in Earth and Planetary Science Letters, 1982. "10Be and Thorium Isotopes in ManganeseNodules and Adjacent Sediments: Nodule Growth Histories and NuclideBehavior'1.
Ku79b T.L. Ku, A. Omura and P.S. Chen, Marine Science 9, Marine Geology and Oceanography of the Pacific Manganese Nodule Province. Bischoff and Piper eds., Plenum Press, 1979. "10Be and Uranium Series Isotopes in Manganese Nodules from the North Pacific".
La62 D. Lai, Journal of the Oceanographical Society of Japan, 20 thAnniversary Volume, 1962, pg. 600. "Cosmic Ray Produced Radionuclides in the Sea".
La62a D. Lai and B. Peters, in Progress in Elementary Particle Physics and Cosmic Ray Physics Vol. VI Wilson and Wouthuysen eds., North Holland, Amsterdam, 1962. "Cosmic Ray Produced Isotopes and Their Application to Problems in Geophysics"
• La67 D. Lai and B. Peters, Handbuck der Physik 46(1967)551. "Cosmic Ray Produced Radioactivity on the Earth".
La80 W.A. Lanford, P.D. Parker, K. Bauer, K.K. Turekian, J.K. Cochran
139
and S. Krishnaswami, NIM 163(1980)505. "Measurements of 10Be Distributions Using a Tandem Van de Graaff Accelerator".
Li65 M.E. Lipshutz, P. Signer, and E. Anders, J. of Geophysical Research 70(1965)1473. "Cosmic Ray Exposure Ages of Iron Meteorites by the 21Ne/26Al Method".
LiOO Livingstone and Blewett, Particle Accelerators.
Li76 W.C. Lineberger, IEEE Trans, on Muc. Sci. NS-23(1976)934. "Atomic Electron Affinities".
Li81 A .E . Litherland, R.P. Beukens, L.R. Kilius, J.C. Rucklidge, H.E. Gove, D. Elmore and K.H. Purser, NIM 186(1981)463. "Ultra Sensitive Mass Spectrometry with Tandem Accelerators".
MaOO B. Mason, Handbook of Elemental Abundances, Gordon and Breach, New York, 1971.
Ma54 R.B. Mason and P.E. Fowle, J. of the Electrochemical Society 101(1954)53. "Anodic Behavior of Aluminum and its Alloys in Sulfuric Acid Electrolytes".
Ma80a A. Mangini, Personal Communication, "Report: Progress in 10BeMeasurements".
Ma80b T.S. Mast and R.A. Muller, to be published in Muc. Sci. Applications, "Radioisotope Dating with Accelerators".
Me60 J.R. Merrill, E.F. Lyden, M. Honda and J.R. Arnold, Geochimica et Cosmochimica Acta 18(1960)108. "The Sedimentary Geochemistry of the Beryllium Isotopes".
Me78 P. Meyer, Nature 272(1978)675. "The Cosmic Ray Isotopes".
Mi74 R. Middleton and Adams, 118(1974)329.
Mi76 R. Middleton, IEEE Trans on Nuc. Sci. NS-23(1976)1098. "A Review of Negative Ion Sources".
Mi77 R. Middleton, NIM 144(1977)373. "A Survey of Negative Ions from a Cesium Sputter Source".
Mi79 R. Middleton, R.W. Zurmuhle, J. Klein and R.V. Kollarits, PRL 43(1979)429. "Search for Anomalously Heavy Isotopes of Oxygen".
Mi80 R. Middleton, Proceedings of SNEAP 80, Univ. of Wisconsin, pg. 134. "A Versatile High Intensity Negative Ion Source".
Mi81 R. Middleton, J. Klein and H. Tang, to appear in the Proceedings of A Symposium on Accelerator Mass Spectroscopy, Argonne, 1981."Instrumentation of an FN Tandem for the Detection of 10Be".
MoSla M. Monaghan, Dissertation review for the Dept, of Geology at Yale
140
University, 1981. "Exposure Ages of Land Surfaces".
Mo31b R.K. Moniot, T.H. Kruse, W. Savin, G.S. Hall, T. Milazzo and G.F. Herzog, submitted to the 12 th Lunar and Planetary Science Conference, Houston, 1981. " 10Be in Stony Meteorites by AcceleratorBased Mass Spectroscopy".
Mu77a R.A. Muller, L.W. Alvarez, W.R. Holley, E.J. Stephenson, Science 196(1977)521. "Quarks with Unit Charge: A Search for AnomalousHydrogen"-.
Mu77b R. Muller, Science 196(1977)487. "Radioisotope Dating with a Cyclotron".
Ne77 D.E. Nelson and R.G. Korteling, Science 198(1977)507. "Carbon-14: Direct Detection at Natural Concentrations".
No70 L.C. Northcliffe and R.F. Schilling, Nuclear Data TablesA7(1970)233. "Range and Energy Loss Tables".
0v69 J.C. Overley, P.D. Parker and D.A. Bromley, NIM 68(1969)61. "TheEnergy Calibration of Tandem Accelerators".
Pa80 M. Paul, W. Henning, W. Kutschera, E.J. Stephenson and J.L. Yntema, Physics Letters 94B(1980)303. "Measurement of the26Mg(p,n)26Al(g,7.2xl05 yr) Cross Section via Accelerator MassSpectroscopy".
Pi49 J.P. Pierce, Theory and Design of Electron Beams, Van Nostrand,1949.
Pr73 G. Proca, Rev. of Sci. Instruments, 44(1973)1365.
Pu79 K.H. Purser, A.E. Litherland and H.E. Gove, NIM 162(1979)637."Ultrasensitive Particle Identification Based Upon ElectrostaticAccelerators".
Pu81 K.H. Purser, P. Williams and A.E. Litherland, Proceedings of the 10 th International conference on Electromagnetic Isotope Separators, Zinal, Switzerland, Sept. 1980. NIM 186(1981)487. "Isotope Ratio Measurement at Abundance Sesitivities < 1:1015, A Comparison Between Mass Spectoscopy at keV and Mev energies."
Ra78 G.M. Raisbeck, F. Yiou, M. Fruneau and J.M. Loiseaux, Science 202(1978)215. Ml0Be Mass Spectrometry with a Cyclotron".
Ra79a G.M. Raisbeck, F. Yiou and C. Stephan, J. de Physique Lettres 40(1979)L-241. "26Al Measurement with a Cyclotron".
Ra79b G.M. Raisbeck, F. Yiou, M. Fruneau, J.M. Loiseaux and M. Lieuvin, Earth and Planetary Science Letters 43(1979)237. "10Be Concentrationand Residence Time in the Ocean Surface Layer".
Ra79c G.M. Raisbeck, F. Yiou, M. Fruneau, J.M. Loiseaux, M. Lieuvin and
141
J.C. Ravel, Nature 282(1979)279. "Deposition Rate and Seasonal Variations in Precipitation of Cosmogenic 10Be".
Ra80 G.M. Raisbeck R. Yiou, M. Fruneau, J.M. Loiseaux, M. Lieuvin, J.L. Reyss and F. Guichard, Earth and Planetary Science Letters 51/1980)275. "10Be Concentration and Residence Time in the DeepOcean".
Ra81 G.M. Raisbeck and F. Yiou, NIM 186(1981)483. "Electromagnetic Isotope Separation as a Method of Improving the Detection Limits of Accelerator Spectrometry".
RaSla G.M. Raisbeck, F. Yiou, M. Lieuvin, J.C. Raval, M. Fruneau, and J.M. Loiseaux, recent data to appear in the Proceedings of A Sysmposium on Accelerator Mass Spectroscopy, Argonne, 1981.
Re76 J.L. Reyss, Y. Yokoyama and S. Tanaka, Science 193(1976)1119. ll26Al in Deep Sea Sediment".
Re80 J.L. Reyss, Y. Yokoyama and F. Guichard, Earth and Planetary Science Letters, in press (1980). "Production Cross Sections of 26A1, 22Na, 7Be from Argon and of 10Be and ''Be from Nitrogen:Implications for Production Rates of 26A1 and 10Be in the Atmosphere".
Ro64 F. Rohrbach, CERN 64-50, N.P.A. Division Internal Report. "Some Studies of High Voltage Breakdown Across Large Gaps: Investigationof the Properties of Oxide Coated Aluminum Electrodes".
Sa77 F. Sauli, CERN 77-09, May. 1977. "Principles of Operation of Multiwire Proportional and Drift chambers".
Sc77 P. Schlotz, Messung Spallogener Nuklide imi Mundrabilla Eisenmeteoriten, Diplomarbeit, Max-Plank-Institut fur Kernphysik, Heidelberg, 1977.
Sh78 P. Sharma and B.L.K. Somayajulu, in La Genese des Nodules de Manganese, Colloques Int. du CNRS £289, Gif Sur Yvette, 1978.
Sh79 Nuclear Wallet Cards, V.S. Shirley and C.M. Lederer eds., Lawrence Berkeley Laboratory, 1979.
Si73 R. Silberberg and C.H. Tsao, Astrophysical Journal Supplement series #220, 25(1973)315. "Partial Cross Sections in High-EnergyNuclear Reactions , and Astrophysical Applications. I. Targets with Z<28. "
5077 B.L.K. Somayajulu, Geochemica et Cosmochimica Acta 4/1(1977)909. "Analysis of Causes for the 10Be Variations in Deep Sea Sediments".
5078 R.K. Sorem, R.H. Fewkes, W.D. McFarland , and W.R. Reinhard, in La Genese des Nodules de Manganese, Colloques Int. du CNRS #289, Gif Sur Yvette, 1973.
142
Ta79a S. Tanaka and T. Inoue, Earth and Planetary Science Letters45(1979)181. "10Be Dating of North Pacific Sediment Cores up to 2.5Millions Years B.P.".
Ta79b S. Tanaka, in a report on The Natural Radioactive Isotopes of Beryllium in the Environment, held in New Haven 1979. K.K. Turekian and H.L. Volchok eds. 1,1°Be Stratigraphies in the Pacific Sediment Cores Back to 4 Million Years".
Ta80 S. Tanaka and T. Inoue, Earth and Planetary Science Letters49(1980)34. lll0Be Evidence for Geochemical Events in the NorthPacific During the Pliocene".
Th80 J. Thomas, WNSL Internal Report 70. "Measuring the 10Be to 9Be Ratio".
Th81a J. Thomas, A. Mangini and P. Parker, IEEE Trans, on Nuc. Sci. NS-28(1981)1478. "Improvements in an Accelerator Based MassSpectrometer for Measuring 10Be".
Th81b J. Thomas, P. Parker, A. Mangini, K. Cochran, K. Turekian, S. Krishnaswami and P. Sharma, to appear in the proceedings of A Symposium on Accelerator Mass Spectroscopy, Argonne, 1981. "10Be inManganese Nodules".
Tu78 K.K. Turekian, J.K. Cochran and D.J. DeMaster, Oceanus 21(1978)34. "Bioturbation in Deep Sea Sediments: Rates and Consequences".
Tu79a K.K. Turekian, J.K. Cochran, S. Krishnaswami, W.A. Lanford, P.D. Parker and K.A. Bauer, Geophysical Research Letters 6(1979)417.
Ty76 P. Tykesson, H.H. Andersen and J. Heinemeier, IEEE Trans, on Nuc. Sci. NS-23(1976)1104. "Further Investigations of ANIS".
Ur78 University of Rochester Nuclear Structure Laboratory Annual Report for 1978. v .
Ur79 University of Rochester Nuclear Structure Laboratory Annual Report for 1979.
Uw81 University of Washington Nuclear Physics Laboratory Annual Report for 1981.
Vo79 H. Voshage and H. Feldman, EPSL 45(1979)293. "Investigations on Cosmic-Ray Produced Nuclides in Iron Meteorites".
Wa47 R.E. Warren, J.L. Powell, and R.G. Herb, The Reviev; of Sci. Instruments 18(1947)559. "Electrostatic Analyzer for Selection of Homogeneous Ion Beams".
Wa81 T.F. Wang, A computer program to calculate charge state fractionation for a doubly stripped beam in the Tandem. Based on a program by H.D. Betz.
143
Wo62 Wolfendale, Cosmic Rays, Progress in Elementary Particle and Cosmic Ray Physics, vol. VI, North Holland, 1962.
Yi72 F. Yiou and G.M. Raisbeck, PRL 29(1972)372. "Half Life of 10Be".
Yo67 Y. Yokoyama, Nature 216(1967)569. "Terrestial Origin of 26Al and Oceanic Sedimentation Rates".
Yo78 Y. Yokoyama, F. Guichard, J.L. Reyss and N.H. Van, Science 201(1978)1016. "Oceanic Residence Times of Dissolved Be and Al Deduced From Comogenic Tracers 10Be and 26A1".