BETA RAY SPECTRA OF NEUTRON DEFICIENT ISOTOPES

OF A.NTIMONY AND TELLURIUM.

Bric~ Norman Clark

A thesis submitted to the Faculty of

Graduate Studies and Research of McGill University

in partial fulfillment of the requirements for the

degree of Doctor of Philosophy.

Radiation Laboratory,

McGill University.

August, 1951.

TABLE OF CONTENTS

Page No •

.lCKN'ovri..El)GMENTS • • . • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 1

SUMMA.R.Y •••••••••• _. ••••••• ~ ••••••••••••••••••• • • 11

INTRoDUCTION •••••••

Previous Work

THE ORY

• • • • • • • • • • • • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • • • • • • • • • • • •

1

2

1. Neutrino Hypothesis ••••••••••••••••••••••• 5

2. Fermi Theory •••••••••••••••••••••••••••••• 6

3. Forbidden Spectra ••••••••••••••••••••••••• 8

4. FT values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . 10

METHODS OF OBSERVATION

1. Beta Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2 • Gamma Rays • . . . • • . . . • • • • . • • . .. . . . . . . . • • . . • • . • 13

1. Spectrometer • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 14

2. Current Regulator •••••••••••••••••••••••••• 14

3. Deteotors ••••••••••••••••••••••••••••••••••

4. Formvar Windows • • • • • • • • • • • • • • • • • • • • • • • • • • • •

15

16

5. Cyclotron Magnet Field ••••••••••••••••••••• 17

6. Aligilment • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 17

7. Calibration of the Instrument • • • • • • • • • • • • • • 18

8. Chemis try . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . 18

MEASUREMENTS AND DISCUSSION

1. Measurements on Tel17 • • • • • • • • • • • • • • • • • • • • • •

2. :Measurements on TellS ••••••••••••••••••••••

3. Observations on sbllS • • • • • • • • • • • • • • • • • • • • • •

CONCLUSIONS • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • BIBLIOGRJ.PHY ............ ' ....................... .

21

24

27

32

33

(i)

ACKNOWLEDGMENTS

The writer wishes to tbank Professer J.S.

Foster, Director of the Radiation Laboratory, for

his interest and helpfûl conversations tbroughout

this work.

The writer is also indebted to his fellow

members in the Radiation Laboratory who showed ao

much co-operation. In particular, tbanks are due to

Dr. A.L. Thompson for se veral helpful discussions

with regard to the chemistry of this work, and to

Mr. Allen Johnson for doing sQ.me of the calibration

rune. Mr. Stanley Doig and,other members of the

shop have always been most co-operative, as bas Mr.

Robert Lorimer, who bas done any glassblowing

associated with this wor~~

(ii)

With the aid of a thin lens spectrometer of

~ 118 ( ) resolution 1.7~ the beta spectra of 51sb 3.5 minutes ,

52Tell7 (70 minutes) and 52Tell6 (3 hours) were examined.

The activity of Sb118 with an end point of 2.65 Mev was

shown to be of a forbidden nature. However, attempts to

straighten the Fermi plot by corrections have to date

been unsuccessful. The 70 minute Te1l 7, not previously

reported, was shown to bave an end point of 2.00 Mev and

the mass was identified by observing the growth and decay

of a known conversion line from its daughter, Sbll7• The

new 3 hour Te116 bas a beta end point of 2.51 Mev, and

its mass is 116, as shown by the known 1.45 Mev positron

end point of its daughter, sb116•

l.

INTRODUCTION

The energies available in the McGill cyclotron

enable one to investigate by means of (p,xn) reactions,

neutron deficient isotopes lying far below the line of

stability of the elements. This thesis is concerned

with investigations of (p,xn) reactions on antimony.

The tellurium activities resulting from bom­

bardment of antimony were separated chemically and

studied by gross decay, aluminium absorption, and

finally by means of a thin lens beta-ray spectrometer.

Two new tellurium activities were round and in-

vestigated, • third half life of 30 minutes was observed

in chemically separated tellurium and again in an un­

separated sample of antimony bombarded in the cyclotron.

However, this latter activity has not been investigated

further.

52Tell7 was observed to have a halt life of 70

minutes and its maas was identified by observing growth

and subsequent decay of a known 50sbll7 conversion line.

Tell7 emits positrons and bas an end point of 2.00 Mev.

No conversioh lines have been observed, nor as yet have

radiator experimenta been performed to look for higher

energy gammas. High energy gammas are :expected since

gross decay behind a lead absorber 1.5 mm. thick, in an

apparatus which effectively eliminates annihilation

radiation, yielded a 70 minute half life.

2.

Another new tellurium isotope, Tellô, bas been 1

round to have a half life of 3 hours and to emit positrons

with an end point of 2.51 Mev. Its mass was assigned by

the fact that a continuum having a similar end point to

Sbllô was observed and also from considerations of the

yield for various bombarding energies. A 92.1 Kev gamma

ray bas also been observed with TellS. Another continuum

believed to be due to camplex decay in Sb116, with an

end point of 0.54 Mev, was observed. However, the

assignment of this 0.54 Mev end point is not definite.

Since the known 6 day TellS decays by K capture

and is known to produc'e.sb118 (3.5 minutes) it provides

an excellent means of investigating the short lived

SbllS in a beta-ray speotrometer. This was dona and its

positron continuum was seen to bave an end point of 2.65

Mev, oonsiderably lower than the 3.1 Mev reported by

Linder and Perlman {2). A Fermi plot of this continuum

deviates from linearity at both high and low energies

and is thus seen to be of a forbidden nature. Efforts

to straighten the ourve by corrections applicable for

ebange of spin 2 and change of parity, and also one for

change of spin 3 and no change of parity, failed. As yet

further corrections have not been tried.

PREVIOUS WORK

In the acoompanying historical account no treat­

ment is given Te116 and Te117, since they have never been

reported. Linder and Perlman (2) mentioned a 2.5 hour

positron activity but oould give no maas assignment nor

end point value. Some attention, however, haa been

given the maas aaaignments of Te118, sbll7 and Sb116

,

aince the masa aaaignmenta in this present work depend

a great deal on their accuracy.

J.R. Riaser et al (1), by bombarding indium

with alpha particles, obtained a half life in antimony

of 3.6 minutes which they identified as Sb118 • Linder

and Perlman (2), bombarding antimony wi th 200 Mev ,

deuterons, obaerved a aix day activity accompanied by

positrons which they ahowed to be due to a 3.5 minute

àntimony daughter. The positron end point they ob­

served to be 3.1 Mev, both by absorption in beryllium

and alao from the end point on an energy distribution

curve taken with a low resolution beta-ray spectrometer.

They also observed some gamma ray activity which, they

postulated, was due to the 3.5 minute antimony.

The maas assignment of Sbll8 waa made from the

fact that 40 Mev deuterons on antimony produced the six

day tellurium isotope, but in a. yield 1/40 that of an

accompanying 4.5 day tellurium activity, thus indicating

the six day activity is a lower mass than the 4.5 day one.

40 Mev is too small to form 116 and probably even 117.

Further, by bombarding 95% pure Sn120 with 18 Mev

deuterons, the 39 hour daughter of the 4.5 day tellurium

was identified as Sb119 • Thus the 4.5 day tellurium is

shown to be Te119 and the 6 day activity, since it can•t

be Te117, must be TellS and its 3.5 minute daughter is

therefore Sbll8.

4.

J.J. Livingood and G.T. Seaborg (3} reported 117

a 3 hour half 1ife of antimony which was probably Sb

but reported nothing further on it. Coleman and Pool (4),

by the use of a pair of X-Ray cameras in conjunction with

X-ray decay curves, observed antimony activities of 2.8

hours, 5.1 bours and 39 hours from the bombardment of

tin by deuteroQs. The maas assignments of antimony 117,

118 and 119 respectively were done by relative abun­

dances and due to the large number of tin isotopes were

open to same question. But by means of the X-Ray camera

both the 2.8 hour activity and the 39 hour activity were

shown to be K capture. The 2.8 hour activity has also a

conversion 1ine at 0.46 Mev, whi1e no electrons or gamma

rays at all were round with the 39 hour activity. The

5.1 hour activity, which decays by K capture, showed a

0.20 Mev conversion line and a 1.5 Mev gamma ray.

G. Timmer (5) bombarded In115 with alpha particles

producing Sb118 , 117 and 116, which he examined in a bata­

ray spectrometer. Conversion lines of a 156 Kev gamma

ray showed a half life of 2.8 hours. Those of a 260 Kev

gamma showed a 5.1 hour half lite and a positron continuum

with an end point of 1.45 Mev bad a half life of one hour.

By bombarding stacked foils in the cyclotron and

later analysing them in the beta-ray spectrometer, excit­

ation curves were obtained for all tbree activities, thus

establisbing the masses as sbll8_ 5.1 hours, sbll7_2.8 hours

and Sbll6_ 1 hour. The ~bll8 and 119 resulta are thus in -

5.

agreement with those of Coleman and Pool. In an

earlier paper Timmer (5) also reported the existence

or an approximately 700 Kev gamma ray accompanying

the Sb116•

'l'HE ORY

1. NEUTRINO HYPOTBESIS

The tact that in bata decay electro~s or con­

tinuously varying energies are observed was very dis­

turbing to physicists, as up until the observation of

this phenomenum all nuclear processes were sharply

monochromatic.

In cases where the difference in energy between

the initial and final nucleus is known this difference

corresponds to the maximum energy or the continuum. How­

ever, calorimetrie experimenta proved that the mean

energy of the emitted betas corresponded more nearly to

the average energy of the continuum than to the maximum,

and since radioactive nuclei.:. have a de.finite mass and

energy state, the law of conservation of energy seemed to

be contradicted.

To preserve the law of conservation of energy and

satisfy other difficulties with regard to spin and

statistics, Pauli postulated that a particle of approx1-

mately zero mass, following Fermi statistics and having a

spin 1/2, was also emitted in bata decay. Thus, considering

6.

nucleL composed of only protons and neutrons, when an

electron is emitted we have a neutron changing into a

proton plus an electron and a neutrino which can carry

off the missing energy. Because of its having no maas

nor charge the neutrino is practically impossible to

observe.

2. FERMI TBEORY

Fermi (6) developed ·Pauli 1 s hypothesis by

considering the neutrons and protons as simply different

states of a particle call~d the nucleon, and by assuming

an interaction term between these nucleons and a virtual

.field of the beta particle and the neutrino, he obtained

the following energy distribution function for beta

particles&

P{W) dW : g2 2 2

3 _,f 3 1 M 1 F (Z,W) p(W0 -W) W dW 211" c

(1)

P(W) dW : probability per unit time for the

emission of a beta partlcle ln the energy range W to W+dW,

g = Fermi constant,

fM/~ matrlx element for the transition,

F(Z,W) = Fermi functlon whlch allows for the effect of

the nuclear coulomb field,

p = momentum of the beta particle in units of mc.,

W = energy of the beta particle ln mc2 unlts, in­

cluding the rest maas of the electron,

7.

W = maximum energy of the emitted betas (in­o

cluding rest mass,

~ = Plank's constant divided by 2~,

C = velocity of light.

In a magnetic beta-ray spectrometer, the number

of electrons measured on any current setting divided by

the momentum at that setting is proportional to the

number of particlès emitted per unit momentum interval.

Therefore Equation (l) can be written more suitably as

(2)

where p is the momentum of the emitted particle and all

other units are as previously defined. From Equation (2)

we can see that a plot of

li!!:;, versus W

should gi ve a straight line if we assume J !4/2 is indepen-

dent of energy. This procedure is known as a Fermi or

Kurie plot. For the great majority of beta spectra in­

vestigated to date the Fermi plot is a straight line

which intercepta the energy axis at the maximum energy

of the beta continuum. Thanks to recent investigations

(7}{8) it now appears that this line is straight dawn to

very low energies. If the decay is complex--that is, if

more than one continuum is present--the plot will depart

from linearity. However, by suitable subtraction--that

is, by allowing for the fact that the square root of the

8.

number is plotted--such curves can often be resolved

into two or more straight lines.

When making a Fermi plot the value of the

Fermi function must be determined for the particular

z considered as a function of energy. This is a very

difficult calculation since a gamma function is in­

volved for which no adequate tables are available. One

therefore has recourse to approximation. I.Feister (22),

at the Computation Laboratory of the National Bureau

of Standards, has calculaied the Fermi function for all

Z•s. Pending publication of the entire work he has

published an evaluation of some approximations to the

Fermi function. The Bethe and Bacher (23) approximation

for Z in this region of the periodic table is accurate

to 0.45%, and for energies above 1.5 Mev .is accurate to

within 0.05%. This approximation is therefore adequate

and is the one used in this work.

3. FORBIDDEN SPECTRA

Aside .from these complex spectra, some spectra

are found which are known to be simple and yet a straight

line Fermi plot is not obtained. This lack of linearlty

is due to the tact that in these particular cases fM/ 2 is

not independant of energy, and auch spectra are classified

as forbidden.

Fermi, when consldering the interaction between

nucleons, electron and neutrino, assumed a simple inter -

action term. It has been shown that there are only five

9.

possible types of interaction which satisfy the neceaaary

invariant conditions, and '.f'cr all:owe<f spèetr,a : these all

give the aame energy distribution. However, the differ­

ent matrix elements obtained from the varioua types of

forces conaidered,place conditions upon the change of

spin and parity of a particular beta transition. For

example, the scalar interaction requirea that in an al­

lowed be ta transition no change in spin or pari ty be

allowed. Ho~e~er, since all possible types of forces

do give the correct distribution, a atudy of these allow­

ed spectra is lesa likely to lead to knowledge of the

interaction forces than is the study of forbidden spectra.

E.J. Konopinski and G.E. Uhlenbeck (9) have

considered the theory of forbidden spectra, as bas also

Gruelling (10). They bave obtained theoretical correction

factors for varying changea of spin and parity wbich

would be expected for eaoh type of possible force. When

the suitable correction is applied to the Fermi funotion

of a forbidden apectrum the Fermi plot will yield a

straigbt line. c.s. Wu (ll) has listed eleven forbidden

spectra which bave been straigbtened by the first for­

hidden tensor or axial vector correction for a spin

change of two. Since these two forces are the only ones

which permit a first forbidden apectrum to have a spin

change of two and the theoretical corr-ection has straight­

ened the Fermi plots, i t se ems that one of the se types

of force will form at least part of the correct interaction

10.

term. Konopinski (12) bas given an excellent review

of this theory and has listed the corrections ap­

plicable to first or second forbidden spectra.

4. FT VALUES

If Equation {1) is integrated over all values

of energy, and aga in assuming 1 M/ 2 inde pendent of

energy, one can obtain an expression for the mean life

given in Equation {3).

1 y (3)

where Y = the me.an life of a nucleus undergoing beta decay.

If instead of the mean life, one uses the half

life,T, and writes F for the term under the integral sign,

one oan write ~his equation as

(4)

K = a constant.

By the use of approximations for F, values for FT can be

oomputed and have been tabulated for most known radio­

active nuclei;> and it bas been round possible by this

means to classify beta transitions.

Thus FT values of 1000 to 5000 are considered

super-allowed, those from 104 to 106 are considered

allowed. Groupings of first forbidden, etc., have FT

values or the order 100 times larger than the next lower

11.

state. However such classification of nucleii by this

method, due to uncertainties in values of the matrix

element, are quite sketchy and do not definitely establish

the order of any particular beta transition. A recent

review has been given on FT values by E. Feinberg and G.

Trigg (13), and A.M. Feingold (14) has given a compre­

hensive table of FT values of known radioactive isotopes.

METHODS OF OBSERVATiON

1. BETA RAYS

One may observe the half life of bata particles

with a thin window Geiger counter, and by taking obser­

vations with the counter behind various thicknesses of

absorbing material (generally aluminium) determine the

maximum energy of the bata particles. This is usually

done by analysing the resulting curve with techniques

developed either by Feat~er (15), or by Bleuler and

Zuenti (16). The absorption technique bas the advantage

of speed and only a weak source is required. However, if

there is more than one continuum or soma conversion linas

only the maximum energy will be obtained with any degree

of reliability.

If larger amounts of activity are available one

may, at the sacrifice of being able to observe only a much

smaller solid angle of the activity, resolve more complex

radiations by a magnetic bata-ray spectrameter.

12.

Speotrometers oonsiat normally of two types:

the lens type and the 180 degreé foousaing type. A lena

type utilizea the faot that the magnetio field along the

axis of a aolenoid ooil doea for electrons what an

optioal lens does for light. It foousses them on its

axis and shows ohromatio aberration, thus fooussing the

electrons at different spots aooording to energy. A

detecter plaoed on the axis will register electrons only

of a partioular energy and by varying the magnetic field

it will register relative intensities of electrons of

various energies, or more partioularly momenta, as a

funotion of magnetio field. The 180 degree type, hist­

orioally the older instrument, utilizes the faot that

mono-kinetio electrons travelling in a oirotilar orbit

will be fooussed on a spot, provided· their angle of di­

vergence from the source is not large. By varying the

magnetio field electrons of various energies may be

focussed on the detecter, or the magnetic field may be

left constant and the detecter moved. A photographie

plate can be used to deteot all .energies at once. This

latter means is very useful for aoourate observation of

the energies of conversion linas.

In any beta ray speotrometer work, and particu­

larly when very low energies are oonsidered, a number of

precautions in preparation of sources should be observed.

These eonsist mainly in seeing that an electron loses as

little energy as possible in travelling through matter.

13.

Some. considerations are: (1) to use carrier free sources

wherever possible, if not possible, as little carrier as

is practical; (2) how the activity is applied to the

source, evaporating from a liquid solution often produces

crystals which vary widely in thickness; and (3) the

amount and atomic weight of whatever source backing is

used should be low in order to reduce back-scattering to

a minimum •

.. Gamma rays can be measured direètl:y- by absorption,

or by using a radiator and analysing the resulting electrons

by aluminium absorption techniques. Considerably greater

accuracy can be obtained by means of the beta spectro-

meter or spectrograph. If the gamma rays are of low or

moderate energy and the source has a sufficiently high

atomic number, one can observe directly internal conversion

lines which, with a knowledge of the various X-ray levels,

gives very accurate values for gamma ray energies. These

linas also provide an excellent means or getting half-life

measurements since all radiation is thus known to be due

to one isotope. If no internal conversion lines are ob-

served there remains the possibility of using a radiator.

The activity is then placed in a capsule of material of

low Z, of such thickness that no beta rays can penetrate

it, then a thin piece of radiating material is placed on

top of it. Gamma rays penetrating the capsule will knock

14.

out electrons from the various shells of the radiating

material, the electrons will be observed and, with a

knowledge of the electron energy levels of the radiator

atoms, the gamma ray energies may be determined.

APPAIU.TUS

The beta-ray spectrometer used in this work is

of the thin lens 'type 'a.escribed by Deut~ch, Elliott and

Evans (18), designed and constructed at tne Chalk River

Laboratories of the National Research Council and es-

tablished in this laboratory througb the efforts of Dr.

J .L. Wolfson- (19). · Some modifications to the current

regulation were made by Dr. G.D. Douglas (20), and Dr.

J. Moon (21) rewound the colla. For deta11ed descriptions

of the apparatus the reader is referred to these authors.

A photograph of the instrument is included in Figure 1.

The colla are wound with 4140 turns of rectangular

wire 0.200tt x 0.050", insu].ated with Q;F Formex in four

rough1y equal sections. Between each section is a 0.046u

sheet or copper with tabs projecting through the brasa

wal1s of the magnet spool to the water coo1ing coi1s

wound on the outside or the spool.

2. CURRENT REGUUTOR

The power supply consista of a 20 horsepower,

200 volt, 3 phase, 1200 r.p.m. induction motor coup1ed

15.

direct1y to a 12 horsepower DC motor used as a generator.

The current regulation is provided by two circuits. A

full wave rectifier power supp1y utilizing two type 866

mercury vapor tubes supplies the exciting field of the

generator. This is placed in series with a bank of five

6A3 triodes in parallel. An amplified error voltage is

applied to the grids of these tubes in such a way as to

reduce the error.

A. second f'eed back loop of eight 6.&S7 triodes

in parallel is in series with the magnet coil, and the ,,

same error voltage is applied to the grids of these tubes.

Since the eight 6!87 can carry only two amperes they are

paralleled by a variable resistance which can be used to

carry currents over two amperes.

3. DETECTORS

The Geiger counters previously used on the

spectrometer had a rather poor slope. Since glass counters

were being prepared in the laboratory, giving plateaus lOO

volts w1de and with a slope of 1%, it was decided to

adapt this type for the spectrometer.

The counter is prepared from a glass tube with

the anode formed by painting silver on the inside walls.

The glass walls of the tube are uniform and merely slide

into a heavy brasa block containing 0-Rings for a vacuum

seal. On the face of the block is screwed a brass plate

in which is the hole for the counter window. If one desires

16.

to vary the aize of the window it is only necessary to

change this plate for one with a suitably sized hole.

Since silver is highly photo sensitive the counter was

painted with glyptal to make it light tight. This ar­

rangement can be sean in the photograph of the spectro­

meter in Figure 1.

Used in this mannar the counter gave a plateau

at least 100 volts wide, with a slope of from 5% to 10%.

A counter filling of 6 to 7 cm. of argon and 0.5 to 1 om.

of ethylene in normal operation lasted only from one to

two months, after which refilling restored it.

4. FORMVAR WINDOWS

The counter window is 3/16tt in diameter oovered

with a formvar film which needed no further support. To

make the formvar films, formvar is dissolved in ethylene

dichloride and a drop of the solution dropped on a dish

of water. If one is careful of the concentration of the

solution the drop will spread over the surface giving a

thin film wbich can be picked up on any suitable frame.

However, ethylene dichloride does not distribute itself

readily or uniformly over water and it was more convenient

to place the drop in the corner of a square dish•and to

help the spreading by drawing a wire through the drop as

it started to spread. Thus a large thin surface is ob­

tained from which one can pick the more uniform section.

A film of 26 auch layera can pass 7 Kev electrons and, on

17.

a·3/16" diameter window, broke at a pressure gradient

of 24 cm. of Hg. In normal use at 6 to 10 cm. of Hg

no abnormal drift in counter characteristics is observed.

5. CYCLOTRON MAGNET FIELD

The instrument is lined up along the magnetic

meridian so that only the vertical field of the earth

bas any serious effect on the focussing property of the

spectrometer. The vertical field is neutralized by a

set of neutralizing coils. The field of the cyclotron

magnet has an appreciable affect on the spectrometer,

giving a field about 3/5 that of the vertical field of

the earth. By a system of trial and error with a

Thorium B source, a degaussing current was found which

gave Thorium F line widths ldentical to those obtained

with the magnet off, and Thorium Aline widths only

slightly wider. Thus for energies above approximately

100 Kev the spectrometer can be operated with the

cyclotron magnet on.

6. A.LIGNMENT

The instrument was lined up mechanically by means

of the six setscrews on the side of the coil, so that the

axis of the brasa pipe was lined up on the axis of the

magnet coil as accurately as possible. This was done by

means of the dial indicator of a type used in a machine

shop, set on an arro so as to indicate distances from the

18.

outside edge of the coil to various spots along the

brasa pipe. Once lined up by this means the instru­

ment was very close to aotual alignment. The final

adjustments were made by adjusting the source bolder

and observing the line widths of the various thorium

linas. This proved somewhat tedious and an arrange­

ment for moving the counter, which could be moved

without disturbing the vacuum or the source, would be

much fas ter.

7. CA.LIBRA.TION OF THE INSTRUMENT

. '•

The instrument was cali br a ted on thé Thorium F

line using the value of G. Lindstrom (26). The Thorium

L Line was measured to be 2605., differing from Lind­

strom•s value of 2607.18 by 0.1%. Agreement on the

1.331 Mev gamma ray of Co60 was satisfactory, considering

the uncertainty in allowing for the uranium radiator

tbickness.

8. CHEMISTRY

The antimony was normal1y bombarded as a powder

he1d in a small thin-walled aluminium tube. When small

lumps of antimony were bombarded they were ground to

powder form before the chemistry was per~ormed. The

cbemical separation of tellurium is essentia1ly tbat used

by Linder and Perlman (24). The chemistry consista of the

1:9.

following steps:

l. The antimony is dissolved in hot hydro­

fluoric acid with concentrated nitric acid added drop­

wise until solution is complete.

2. The solution is evaporated to dryness at

low beat and under reduced pressures. Care must be ·

used in this evaporation process since if the precipi­

tate is heated to much an insoluble precipitate is

formed.

3. fhe precipitate is dissolved in 3 N hydro­

cn.loric ac id.

4. Tellurium carrier is added as a soluble

chloride.

5. so2 is bubbled into the solution for ten

minutes and the precipitate of tellurium is centrifuged

out.

The tellurium carrier was in a standard solution

and the amount of carrier added was determined by the

number of drops used. l- 2 mg. of carrier w~re used in

most sources. The tellurium activity is then transferred

to the spectrometer source mounting by evaporating it on

from a nitric acid solution.

The source backing was of three types, Mica of

about 5 mg./cm.2 was used on the Sb118 source. Fine glass

of the order of 1 mg./cm~ or less was used in the study

of the other continua.··. Since the half lives are

relatively short for any particular source, either the

20.

conversion lines or the continuum was studied.

When conversion lines were the chief interest a

metallic foil of columbium of 2.5 mg./cm. 2 was

used. Columbium has a high Z but it is impervious

to nitrio acid and simplified grounding of the

sources.

21.

MEASUREMENTS AND DISCUSSION

1. MEASUREMENTS ON TE11 7

Tellurium produced by bo.mbardment o~ antimony

with 65 Mev protons in the McGill cyclotron showed a new

half life when followed by gross decay. Table I contains

the resulta of a series of half life measurements.

Hal~ Life in Hours Mean Value

1.167 1.14 1.192 1.168 1.185 ,.

1.170 - 0.020 brs.

70.2 ! 1.2 min.

Aluminium absorption curves were taken on a

sample immediately after chemical separation and also

20 hours later, so as to evaluate the long lived beta

decay. Both absorption curves were analysed by the Bleuler

and Zuenti method. This method consista in finding the

thickness d of aluminium needed to reduce the intensity to n ..

its l nth part. Then, from a system of standard curves, 2

an energy corresponding to this d is read off. These n

standard curves, calculated by Bleuler and Zuenti, are of

a semi-theoretical nature based on an allowed Fermi

spectrum with Z :: 20 as a standard substance. This differa

from Feather mainly in that he used the forbidden· spectrum

of RaB as a standard spectrum. Table II contains the

analysis of the aluminium absorption of the short lived

activity.

22.

TA.B.ΠII

Counts/200 Thickness dn Energy n sec. Mg./cm.2 mm. Mev.

0 41.80

1 20.90 140 0.518 2.30

2 10.45 276 1.022 2.05

3 4.23 363 1.345 1.80

4 2.61 436 1.614 1.90

5 1.30 500 1.851 1.98

Mean Value 2.00 ~ 0.19 Mev.

 te1lurium source examined in the thin lens

spectrometer showed a number of conversion linas. Table III

contains those of interest here.

H Gauss cm.

1268

1412

TABLE III

Energy Kev.

125.8

151.0

Sn Binding Energies Kev.

t 29.18 K

+ 4.3 L

Energy Kev.

155.0

155.3

Timmer, whose re.sul ts were mentioned previously, • 1

reported a 156 ~ev gamma-ray of Sb117 • ~he half life of

the Hf = 1268 line was therefore followed, and it was ob­

served to grow and then decay with a half life of 2.8 hours.

Now a daughter activity follows the decay law

(6)

23.

where A o • activity of parent substance at t - 0 ' -1 t = time,

)1 = decay

;12 • decay

Now sin ce in this

approaches

constant of

constant of

case ).1 >-\ ;l A o

2 l

parent, and

daughter.

for t large, this equation

(7)

Subtracting Equation (6) from Equation (7) gives

~11 = (8)

If the growth curve were plotted on semi-log

paper it would approach a straight line which could be

projeoted back through t = 0, thus obtaining a plot of

Equàtion (7). If now the ordinates of the original curve

are subtracted from those of the straight line another

straight line should result. This line would represent

Equation (8), from which the half life of the parent

oould be determined. This analysis was performed on two

auch growth curves, one of which is given in Figure .3.

Each gave a parent activity of 1.17 hours and a daughter

activity of 2.8 hours.

Table IV contains the end point measurements . for three Fermi plots of the positron continuum.

TABLE IV

Energy in Mev. Mean Value

1.98 1.99 2.02 2~00 '!:. 0.021

24.

Figure 4 is a composite plot of the three Fermi

plots normalizeà at an energy value of E = 1.47 Mev. In

all these plots there was evidence of a higher energy

continuum of low intensity due to Tellurium 116. Utilizing

the few points between 2 and 2.5 Mev on the Fermi plot,

plus the knowledge of the end point, this activity was de­

termined and subtracted giving the activity due only to

Tell7. Figure 4 contains the curves atter these sub­

tractions.

2. MElSUREMÈNTS ON TE116

In a spectrometer source obtained from a 65 Mev

bombardment, a line was observed at Hf = 863 gauss cm.,

which showed a 2.9 hour half life with no growth observed

at all. This line was observed to be lesa intense than the

one at ~ = 1268, due to s~17, by a ratio of 3 to 1. How-/

ever, a source from a 75 Mev bombardment showed this 863

line to be larger by a ratio of 2 to 1. These resulta are

shown in Figure 5. The resulta of a series of half life

measurements on this line are given in Table v.

TABIB V

Ralf Life in Hours Mean Value

2.900 2.900 2.900 2.833 2.883 :!' 0.030

This half lite is remarkably close. to that for

Sb117 • However no growtb at all was observed on this line,

25.

while on the same spectrometer sources growth was ob-117 served on the Sb line, so this is quite obviously

not due to the same isotope. Further, K-L differences

·give better agreement for tellurium than for antimony,

as shown in Table VI.

T.A.BΠVI

H Energy Sb Binding Energy En er gy Deviation Gauss cm. Kev. Kev. Kev. ~

863.5 61.86 -t 30.46 K 92.32

1039.4 87.50 + 4. 70 L 92.20 0.1

Sn Binding Energz

863.5 61.86 + 29.18 K 91.04

1039.4 87.50 _,. 4.47 L 91.97 1.0

A Fermi plot of a source prepared from a 75 Mev

bombardment is given in Figure 6. This plot shows com­

plexity. The end point values obtained.are listed in

Table VII.

T~LE VII

Energy in Mev.

0.54 1.47 2.51

Timmer' s re sul ts, mentioned previously, gi ve,,an

end point of 1.45 Mev for Sb116 • The 1.47 Mev end point

is thus in agreement with his results. Also, since the

26.

yield or this isotope goes up by a factor of six in relation­

ship to Tell7 when the bambarding energy is increased from

65 to 75 Mev, it is apparent tbat the mass of this isotope

is less tban 117. This isotope was observable in spectro­

meter sources from 65 Mev bo.mbardments. One would not

expect it, therefore, to be of maas 115 since to obtain this

at least a (p,7n) reaction is required and from resulta on

Te117, a (p,5n) reaction which does not appear at 50 Mev,

this is not sufficient energy.

From these considerations and the agreement in end

point energies, it seems apparent that this isotope is

Te116

• Wb.en the momentum distribution was calculated from

the three straight lines of the Fermi plot and the areas

measured it was found that the area under the 2.51 Mev

distribution was twice the sum.·of the other two areas.

This may be explained by the .t'act that from the time the

~ellurium was separated until counting started was forty

minutes. This is insufficient time for the antimony

products to have reached equilibrium with the parent

activity. Another possibility is that Sb116 also decays

by K capture. -._If this latter possibility is so it bas not

been observed as yet.

There is a possibility also that 0.540 Mev is

due to Sb116, since hard gamma rays are reported for Sb116

by Timmer at a value of about 700 Kev. He gave no

specifie value and it may be possible that the gamma

energy is a little higher. Thus one might obtain a 900

Kev in cascade with the 0.54 Mev beta. Since Timmer bas

published no Fermi plot, but only his momentum distribution,

27.

it appears tbat he achieved his end point byexamination

of this distribution. In this case he would mias the

0.540 continuum and therefore the possibility of it being

due to Sb116 still remains. Using a radiator in the

spectrometer to examine the gamma rays accompanying this

decay is desirable. However, as yet this bas not been

done. Since the 2.51 Mev end point is one of high

energy and of atrong 1ntens1ty, it ia not possible that

this one could be missed by Timmer and it is therefore

due to the TellS isotope.

It might be mentioned here that at 70 Mev a

tbirty minute activity bas been observed by gross decay

in low intensity in chemiea~ly separated tellurium. No

further work bas yet been done on this.

3. OBSERVATIONS ON SBll8

In the activities atudied by gross decay the

short lived activities tailed off into a six day back­

ground. This activity is due to a six day TellS isotope

decaying by K êapture to SbllS whicb bas a half life of

3.5 minutes and a beta end point of 3.1 Mev, as reported

by Linder and Perlman (2). We have here a means ot

studying Sb118 in a beta spectrometer, since with a Te118

source SbllS bas effectively a six day half life. Te118

and all other tellurium activities present in a sample two

days old decay by K capture, and Sbll8 will be the only

positron continuum present.

28.

A tellurium source was prepared from an antimony

block bombarded for six hours at 50 Mev. The Fermi plot

of the spectrum given in Figure 7 gives an end point of

2.65 t 0.01 Mev but is not the type expected of an allowed

transition.

Each point on this curve, with the exception of

two at the very highest energy, representa a count of

8000 or higher. Statistical fluctuations are therefore

small. Further, in a study of a short lived activity

obtained from bombarding an antimony block, degraded protons

raised the six day activity to appreciable proportions.

This activity was evaluated for energies up to 1 Mev and

this same curvature was obtained for the Fermi plot.

This spectrum seems to be of a forbidden type,

similar to that for 91 as given by Langer and Priee (17).

In her review of beta spectra previously mentioned, c.s. Wu

lista eleven spectra, all of which are of this type and

can be straightened by multiplying the Fermi function by~

of Equation ( 9-).

2 2 o(_: (W -1 )+(W -W) 0

(9)

All terms are as previously defined.

This correction is one for first forbidden spectra

with a spin change of two and change of parity, which is

only possible for tensor or axial vector forces. This

correction was applied to the spectrum of Sbll8 and, as

shown in Figure 7, for energies down to l Mev the curve is

straightened out. However, for energies below 1 Mev the

curve drops off appreciably.

29.

E. Feinberg and G. Trigg (13), in a recent

review of ft values, pointed out that for nuclei:: of

even atomic weight a group of eight nuclei. with log

(ft) values in the vicinity of 4.4 (allowed), six of

these lie in the very near vicinity of Z : 50. In­

cluded in this group is Sbl18. A.M. Feingo1d ( 14),

in his table of log ft values, listed the value for

SbllS as 4.7. This value, based on measurements of

Linder and Perlman, predicts an allowed (unfavored)

type of transition. Further, Shull and Feinberg (25)

point out that for nucleL. satisfying the o<:..-type of

eQrrection (W 2- 1 ) ft is of the order of 1010• This

value for SblÎS is approximately 3 x 106, which is

quite low.

The deviation at low energies could bardly

be due to source thickness sincê 1 Mev is relatively

high energy for such affects, besides the fact that

affects of source thickness would give excess electrons

rather than too few. It is therefore concluded that

the Cl\. -type correction is not sui table for this case.

E.J. Konopinski, in his review article

previously mentioned, in discussing the first order

corrections, points out that for Z larger than Z , c

given by Equation (10),

where oe:..: 1/137 fine structure constant,

(10)

30.

R = nuclear radius in units h/mc,

A = atomic number

and W0

: end point energy (including rest mass) in

units mc2 •

All first order corrections except the oC-type become

independant of energy and the Fermi plot is therefore a

straight line. This is due to the fact that in the cor-

rection factor a term c~i>2 becomes of a dominant

magnitude.

In this case Z0

= 15. The one second order

correction term unaffected by this term was also tried,

but it gave an even poorer fit than did theo( correction.

It seems that Z in this case is not sufficiently greater

than Z0

to nullify the other corrections and therefore

some of the other first order corrections should be tried.

The type of forbidden spectrum most often ob­

served is that of the o(-type. This spectrum is not of

this type., nor is 1 t expected to be second forbidden

because of its small f t•. value which, in fact, is even

small for first forbidden spectra. Since 50sn118 is seen

to have even Z and even neutron population one would ex­

pect tt to have zero spin. 51sb118, which decays to

sn118 if it is to be first forbidden would then be 50 ' expected to have a spin change 1 ~ 0 or lesa .likely 0 _,..o.

There bave been a number of conversion lines

observed which, sine~ they show appreciable decay in a

few days, are due to either Tell9, TellS or their

antimony daughters. However, their intensity is low and

31.

with the high positron continuum their half lives and

energies are not precise1y determined.

Table VIII contains the more certain of these

1ines, with the more intense ones marked with an *•

TABLE VIII

H Gauss cm.

479.7 * 800.

959.

1242. * 1385.

1530.

1796. * 1960.

2040.

Energy Kev.

19.85

53.54

75.4

121.3

148.

176.

231.2

260.

285.

32.

CONCLUSIONS

A 0.54 Mev beta continuum was observed but as

yet bas not been assigned, other than that it is due to

Tell6 or Sbll6. There is evidence that Te117 emits a

hard gamma ray. Lead absorption experimenta are inadequate

for resolving it due to the presence of a large number of

gamma rays of other isotopes. Further work of the nature

ot radiator experimenta and observation of conversion lines

with a helical baffle in the spectrometer seems advisable.

Te117 was observed, its half lite measured and

its maas determined by observing the growth and sÛbsequent

decay of the known 126 Kev conversion line of Sb117 • The

end point of its positron continuum was shown to be 2.00 Mev.

A;Lso Tell6 was observed to have a half life of

2.9 hours. Its maas was established by the observation of

the end point of the positron continuum of its daughter

activity, Sb116 • The end point of the Tell6 positron

continuum was measured to be 2.51 Mev.

Opportunity was taken to observe the short lived

daughter of TellS. TellS decays by K capture and has a half

life of six days. It was therefore possible to observe

Sb118 in the beta spectrameter. The end point of the latter

was determined to be 2.65 Mev. Attempts to straighten its

Fermi plot, which has the appearance of a torbidden spectrum,

have so far been unsuccesstul. From the negative resulta of

attempts to straighten it, from considerations of expected

spin changes, and from its small FT value, it is now expected

to be of a first forbidden type with spin change 1-..,.o.

33.

BIBLIOG&lPHY

(l) Hisser, Lark, Horowitz and Smith. Pbys. Rev. 57,

355. (1940).

(2) M. Linder and I. Perlman. Phys. Rev. 73, 1124. (1948).

(3) J.J. Livingood and G.T. Seaborg. Phys. Rev. 55,

414. (1939).

(4) K.D. Coleman and M.L. Pool. Pbys. Rev. 72,

1070. (1947).

(5) G.J. Temmer. Phys. Rev. 75, 146A and 76, 424. (1949).

(6) E. Fermi. Zeits. r. Physik. 88, 161. (1934).

(7)

(8)

( 9)

(10)

(11)

(12)

(13)

R.D. Albert and c.s.wu. Phys. Rev. 74, 847. (1948).

C.S.Wu and R.D. Albert. Phys. Rev. 75, 315. (1949).

E.J'. Konopinski and G.E. Uhlenbeck. Phys. Rev. 60,

E. Greulling •. Phys. Rev. 61, 568.

c.s. Wu. Rev. Mod. Phys. 22, 386. j

308. (1941).

(1942).

(1950).

E.J. Konopinski. Re,'v. Mod. Phys. 15, 209. (1943). . 1

E. Feinberg and G. Trigg. Rev. Mod. Phys. 22,

399. ( 1950).

(14) A.M. Feingold. Rev. Mod. Phys. 23, 10. (1951).

(15) Feather. Proc. Camb. Phil. Soc. 34, 599. (1938).

(16) Bleuler and Zuenti. He1vitica Physic Acta XIX,

375. (1946)

(17) L.M. Langer and H.C. Priee. Phys. Rev. 75, 1109. (1949).

(18) M. Deutsch, L.G. Elliott and R.D. Evans. Rev. Sei. Inst.

15, 178. (1944).

34.

BIBLIQGRAPHY (Cont 1 d.)

(19) J.L. Wolf' son. Ph.D. The sis (1948).

(20) D.G. Douglas. Ph.D. The sis (1949).

(21) J.H. Moon. Ph.D. Thesis (1950).

(22) I. Falster. Phys. Rev. 78, 375. (1950).

(23) H.A. Bethe and R.F. Bacher. Rev. Mod. Phys.

194 • ( 1936 ) •

(24) M. Linder. u.c.R.L. 432, 52-3.

8,

(25) Shull and Feinberg. Phys. Rev. 75, 1768. (1949).

(26) G. Lindstrom. Phys. Rev. 83, 465. (1951).

. 1

w r­<( _J

CL

0

0

....

"" ... 0

C\1

"" t!) > a: ::;)

u... 0

•o )-cr 0

"" x 0 0

/ 0

0

!'(')

(.!)

u..

0 0

01

- ~ 3S 1 SlNOO~

Y' 0

1 (fi

a: :::;)

0

1 :I:

0

,....

0

,...._

w '\t

1-

1- en (.9 0 1-

....J ~ 0 Cl.. > LL. 0

~ ~

a:: ~ w lL.

0 ... 0

x >-(!)

a:: w z w

0 IX)

0 CD

0 N

w

lO w .....

..... 0

<...9 ...J (/) Q..

.... LL ..J

0 :ii > a:: 0 w ..J u... -

~

0 0

x

>-(!)

0::: 1.1.1 z 1.1.1

S!INO A~\1~118~\1

(/)

t­z :::>

>­c::r:: <l c::r:: t-

ID c::r:: <l

8

7

FIG 7 j A-FERMI PLOT SB

118

S-oc- CORRECTED FERM 1 PLOT 1

-r-

I

- 1

~

A

ENERGY X lOO KILOVOLTS

0 0 c z -t Cf)

x

0 0 0

F I(.J 8

MOME NTUM D ISTRI BUT ION SB 1 IS ~ 5 32 6= GAUSS CM.

20 --22

z 0

.... Q.

0 ~ 4D en CD

<.!) c CD en

2 LL. :l

+ z!! .... .... j -CD "" "" :l en .... .... ~ c

1 1 1

c CD \,)

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0~0 1 0 __ _L_ _ _jllll - 1

0

· ~3-s 1 S!N no ~

0 0

... 01

2 \,)

' 0 2

~

90 z '10 ({r)

0 ( (J .. )

'fO

'1

'1

~

_3

10/o

0

-10%

JO%

0 Eo=3mc 2

-10%

-20%

CoRRrcT/ON FoR z~zo ~ = d,.,(Z) - cL,... (zo)

dA\(ZO) FOit

f"./AI'\c 1 • 1, 3,10

At,. l,fZJ,3,5, 7,9

F 1 G Il