GAMMA, FAY DISTRIBUTION FROM

NEUTRON EXCITATION IN CESIUM

APPROVED:

P> f o ^ Major Professor

4 „ // *£ .T,.

Minor Professor

J_ t _s Directs^ of Che Department 01 Physics irectpje,

St* "Dea "o't"'the Graduate School " "*

GAMMA RAY DISTRIBUTION FROM

NEUTRON EXCITATION IN CESIUM

THESIS

Presented to the Graduate Council of the

North Texas State University in Partial

Fulfillment of the Requirements

For the Degree of

MASTER OF SCIENCE

By

Richard Morgan Bowers, B. S.

Denton, Texas

January, 1969

TABLE OF CONTENTS

Page

LIST OF TABLES tv

LIST OF ILLUSTRATIONS v

Chapter

I. INTRODUCTION 1

II. INSTRUMENTATION AND PROCEDURE 7

III. RESULTS AND CONCLUSIONS 15 APPENDIX 21

BIBLIOGRAPHY 38

iii

LIST OF TABLES

Table Page

I. Radial and Absorption Corrections 21

II. Differential Excitation Probabilities 22

iv

LIST OF ILLUSTRATIONS

Figure Page

1. Target Holder and Detector 23

2. Block Diagram of Instrumentation ..... 24

3. Delayed Monostable Multivibrator 25

4. Shadowcone and Scatterer 26

5. 0 to 0.5 MeV Cesium Spectrum 27

6. 0.5 to 5 MeV Cesium Spectrum 28

7. Carbon Spectrum 29

8. Empty-Can Scattering Spectrum 30

9. Cesium Minus Background Spectrum 31

10. Correction Curve 32

11. Compton Edge of Standard Sources 33

12. Differential Excitation Probabilities 34

13. Van de Graaff Accelerator 35

14. Target Assembly 36

15. Scatterer and Shadowcone 37

CHAPTER I

INTRODUCTION

133 The study of the energy levels of Cs can be made

133 by both the indirect and direct excitation of Cs . One

133 example of an indirect excitation of Cs is the electron

133

capture of a 7.2 year Ba nucleus (1, 3, 8, 11, 12, 14,

15, 18, 20). This naturally occuring process produces an

excited Cs^^ nucleus which will decay by occupying certain

lower energy levels. With the transition of the nucleus

from one energy level to a lower energy level, a gamma ray

is emitted with energy equaling the energy level difference.

However, there are two distinct disadvantages to this method.

First, there is no possibility of observing energy levels

above 0.437 MeV, since this is the highest energy level to

133

which Ba decays. Secondly, there may be certain energy

levels which would not be observed due to forbidden tran-133

sitions. Another example of an indirect excitation of Cs 133

is the beta emission of a 5.3 day Xe nucleus (14). 133

The direct excitation of Cs can be accomplished by

coulomb excitation and inelastic scattering of neutrons.

133

Exciting Cs by coulomb excitation (4, 6, 16, 17) involves

bombarding the nucleus with alpha particles of insufficient

energy to penetrate the nucleus. The nucleus is elevated

to an excited state by the coulomb field of the alpha

particle. The disadvantage of this method is that only a

small amount of energy c$n be transferred to the cesium.

By inelastic scattering of neutrons (2, 13, 19), a greater

amount of energy can be transferred to excite the cesium

nucleus to higher energy levels. The uncharged neutron can

readily penetrate the coulomb barrier of the nucleus to form

a very short-lived compound nucleus. When this nucleus

decays, it expels a lower energy neutron and leaves the

nucleus in an excited state. The decay of the excited nuclei,

regardless of means of excitation, will produce specific

133

gamma rays characteristic of Cs . The disadvantage of

inelastic neutron scattering is that other materials are also

excited to higher energy levels. This and the presence of

competing reactions, such as (n,2n), (n,p), and (n,oc),

contribute to a high background.

The North Texas State University 160 kV Cockcroft-

Walton accelerator will produce 14 MeV neutrons and 2.5 MeV / ^

neutrons from the D(t,n) He and D(d,n) He reactions

respectively. Young (21) and McAnally (9) made their

investigations with the 14 MeV neutrons, whereas Lamb (7)

used the 2.5 MeV neutrons. McDonald (10) and Dawson (5)

used both the 14 MeV and 2.5 MeV neutrons. The present

study will use only the 14 MeV neutrons produced by the D-T

reaction.

In earlier investigations by Young, Lamb, McDonald and

Dawson, a single-channel analyzer and digital scaler were

used to locate the gamma rays of cesium. The detector

consisted of a Dumont model 6292 photomultiplier tube with

both a Gsl (Tl) and a Nal (Tl) crystal. This system had an

energy resolution of about 10 percent. Energy levels of

0.079, 0.175, 0.44, 0.56^ and 0.75 MeV were reported. In a

more recent investigation, McAnally used a Ge(Li) detector in

conjunction with a room-temperature, solid state preamplifier

and multichannel analyzer in order to improve the energy

resolution. He used cesium chloride, sodium chloride, and

pure sodium as scatterers. Some cesium gamma rays were

resolved by a complex subtraction process necessitated by

the use of compounds in the scatterer. This procedure left

some doubt about the existence of higher energy gamma rays.

Several refinements have been made since the last

investigation. A coincidence-gating technique between the

3 MeV alphas of the D(t,n) He^ reaction and the gamma rays

detected by the Ge(Li) detector was used to reduce the high

background count. This made interpretation of the results

simpler and more accurate, and allowed longer runs without

overloading the multichannel analyzer. A liquid-nitrogen

cooled preamplifier, as well as a new high-stability

amplifier was incorporated to improve the energy resolution.

Also, pure cesium was used as a scatterer rather than the ;

cesium compounds which have been used in the past.

The purpose of this investigation was to analyze the

gamma rays resulting from excitation of Cs*"^ by the

inelastic scattering of 14 MeV neutrons and to determine

the relative intensity of each gamma ray.

CHAPTER BIBLIOGRAPHY

1. Arya, A. P., "Gamma-Gamma Directional Correlations in CS133," physical Review, CXXII (1961), 549.

2. Chien, J. P., and Smith, A. B., "Fast Neutron Scatter-ing from Beryllium, Sodium, and Aluminum," Nuclear Science and Engineering. XXVI (1966), 510.

3. Crasemann, B.. Pengra, J., Lindstrom, I. E., "Radiations from BaJ-33}M physical Review. CVIII (1957), 1500.

4. Davis, R. H., Divatia, A. S., Lind, D. A., and Moffat, R. D., "Coulomb Excitation of Elements of Medium and Heavy Mass," Physical Review. CIII (1956), 1801.

5. Dawson, H. R., "D-D and D-T Neutron Excitation of Energy Levels in t" unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1961.

6. Fagg, L. W., "Coulomb Excitation of Sb, Cs, and Ba," Physical Review. CIX (1958), 100.

7. Lamb, B, L., "Gamma Rays from Neutron Excitation of Cs1JJ," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1959.

8. Mann, K. C., and Chaturvede, R. P., "The Decay of Ba133jn Canadian Journal of Physics. XLI (1963), 932.

9. McAnally, M. A., "Gamma Rays Resulting from the Neutron Scattering in Cesium," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1959.

10. McDonald, P. F. , "Gamma Rays from Cs -33 by inelastic Scattering of Neutrons," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1960.

11. Ramaswamv. M., Skeel, W., and Jastram, P., "Decay of Nuclear Physics. XVI (1960), 619.

12. Ramaswamy, M., Skeel, W. , and Jastram, P., "The Decay Energy of Bal33," Nuclear Physics. XIX (1960), 299.

13. Schectman, R. M., and Anderson, J. D., "Inelastic Scattering of ,14 MeV Neutrons," Nuclear Physics, LXXVII (1966), 241.

14. Stewart. M. G., and Lu, D. C. , "Nuclear Levels of Cs^33," Physical Review. CXVII (1960), 1044.

15. Subba Rao, B. N., "Internal Conversion Coefficient of the 81 KeV Transition in Cs^-33," Nuclear Physics. LXXXVI (1966), 443.

16. Temmer, G. M., and Heydenburg, N. P. , " Coulomb Excitation of Heavy and Medium-Heavy Elements by Alpha Particles." Physical Review, XCIII (1954). 351.

17. Temmer, G. M., and Heydenburg, N. P., "Coulomb Excitation of Medium-Weight Nuclei," Physical Review, CIV (1956), 967.

18. Thun, J. E., Tornkvist, S., Nielsen, K. B., Snellman, H., Falk, F., and Mocoroa, A., "Levels and Transitions in Cs^-33," Nuclear Physics. LXXXVIII (1966), 289.

19. Williams, G. H., and Morgan, I. L., "A Ge(Li) Gamma-Spectrometer for (n, n') Measurements," Nuclear Instruments and Methods. XLV (1966), 313.

20. Yin, L. I., and Wiedenbeck, M. L., "Gamma-Gamma Directional Correlation Measurements in the Decay of Bal33," Nuclear Physics. LIV (1964), 86.

21. Young, J. C., "Gamma Ray Response of a Csl (Tl) Crystal to 14 MeV Neutrons," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1958.

CHAPTER II

INSTRUMENTATION AND PROCEDURE

Several changes and modifications have been made in

instrumentation since the investigation by McAnally (3).

These improvements were specifically designed to improve

energy resolution and t<? reduce the effects of background

radiation. Some equipment was added to make existing

equipment function more effectively.

Initial investigations were made with the North Texas

State University 160 kV Cockcroft-Walton accelerator. Due

to high voltage arcing, poor focusing and low beam current,

the necessary statistics could not be obtained. In order

to improve upon these conditions, the High Voltage

Engineering, model PN-400 Van de Graaff generator (Figure 13)

at Southern Methodist University was used. Due to the

higher beam currents, the tritium target (Figure 14) was

water-cooled to carry off the heat that would have other-

wise driven the impregnated tritium from its titanium disc.

The target was electrically insulated so that the beam

current could be monitored by a microammeter.

A PL 5-100-50 H toroidal surface barrier detector

(Figure 1) was used to detect the alpha particles from the 4

D(t,n) He reaction. The signal from the alpha detector was

amplified and discriminated for use in coincidence counting

8

and neutron monitoring (Figure 2). Alpha counting was made

more reliable with the use of a seven-binary scaler which

produces a single pulse for every 128 pulses fed to it.

This pulse was in turn fed into a five-decade scaler which

was followed by another four-decade scaler.

The Tennelec model TC 130 preamplifier in the gamma

channel was modified so that its two low-noise, impedance-

matching FET transistors could be kept at a constant low

temperature and could be connected as close to the Ge(Li)

detector as possible, in order to minimize lead capacitance.

The power supply for the detector and preamplifier was

provided by a Tennelec model TG 907 variable bias voltage

supply.

Detection of gamma rays is accomplished with the use of

a lithium-drifted germanium detector (Type LG 4.0-5)

manufactured by Nuclear Diode, Inc. (Figure 15). This

newly designed solid state detector has the distinct advantage

of giving better resolution since approximately 3 eV of

energy is expended in producing an electron-ion pair as

compared to 32 eV of energy for a sodium iodide crystal (1,2).

However, due to the lower atomic number for germanium than

for iodine, the Ge(Li) detector has a smaller collecting

efficiency, which for the process of photoelectric

absorption goes as Z~*. Likewise, the photoelectric cross

section for germanium will decrease for increasing gamma

ray energy. Since this will appear to decrease the relative

intensities of the higher energy gamma peaks, corrections

must be made in the energy spectrum for this effect and for

self absorption within the cesium scatterer. The detector

consists of a pure germanium crystal that has been impregnated

with controlled amounts of lithium, and which has a reverse

bias applied across it. This process produces a uniform

depletion layer that extends practically throughout the

germanium crystal. Photoelectric absorption or scattering of

gamma rays within the detector produces high energy

electrons which dissipate their energy by creating free-

charge carriers. These carriers are swept from the detector

by the applied reverse bias voltage and amplified for

analyzation. As the applied bias voltage is increased, the

probability that the carriers will recombine before they are

swept out of the detector is reduced. However, leakage

currents through the detector are in turn increased which

produce a high noise level.

Since a Ge(Li) detector is no longer useful after

11 2 receiving a dosage of about 10 neutrons/cm , it is

protected from direct neutron radiation by a ten-inch,

six-degree copper shadowcone (Figure 4). However, little

can be done to reduce the number of scattered neutrons

entering the detector. To eliminate any activated copper

gamma rays, a 3/4-inch-thick piece of lead is placed behind

the copper cone. The cesium scatterer (O.D 4", I.D 2")

is rigidly attached to this end of the cone (Figure 15).

10

By carefully aligning the toroidal alpha detector, it is

possible to relate the cone of counted alpha particles

directly to only those neutrons which were incident on

the scatterer.

After proper preamplification, the signals from the

alpha detector and the signal from the Ge(Li) detector are

sent into separate Sturrup model 2801 linear amplifiers for

amplification and pulse shaping. From the amplifiers the

alpha signal and the gamma ray signal are sent into separate

Sturrup model 501 single-channel analyzers for noise

discrimination, time adjustment, and additional pulse shaping,

The output of the coincidnece analyzer is first fed into a

delayed monostable multivibrator with variable pulse-width

control (Figure 3) and then into the coincidence input of a

model ND-150 M Nuclear Data 1024-channel dual-parameter

analyzer. The 0.6 microsecond delay in conjunction with

the variable delays of the single-channel analyzers are used

to allow for a two-microsecond time delay in the analyzer

circuitry. The second channel of the Ge(Li) preamplifier

is sent into a Tennelec model TG 200 amplifier with controls

for signal amplification, differentation, and integration

to enhance optimum resolution from the multichannel analyzer.

This signal is sent into the multichannel analyzer, where

it is gated by the alpha-gamma coincidence pulse before

being analyzed. The entire 1024 channels of the analyzer

are used for locating gamma ray peaks of cesium.

11

The scatterer was prepared by pouring 200 grams of

molten cesium into a thin-walled toroidal aluminum can

under an argon atmosphere. In order to provide an air tight

seal, it was covered with a layer of paraffin which kept

the cesium evenly distributed. In order to correct for

neutrons scattered into the Ge(Li) detector by the scatterer,

a similar aluminum scatterer was filled with an equal amount

of paraffin and a graphite ring. Graphite was used because

it scatters neutrons into the detector, as cesium does,

without producing any gamma rays below 4.43 MeV. The

background spectra were taken for the same number of alpha

particle counts as for the corresponding cesium spectrum.

Taking into account the difference in density and neutron

scattering cross section for cesium and graphite, the

graphite scattering can was irradiated for 37 percent of the

initial number of alpha particle counts and a similar empty

scattering can for the remaining 63 percent. Accidental

spectra were taken for the cesium, carbon, and empty

scattering cans by delaying the alpha particle pulse so that

it would be out of coincidence with the gamma ray pulse. *

Each additional spectrum was normalized to the alpha

particle count of the original spectrum and subtracted from

it.

At the present time, the only means by which the

analyzer will record data is with the IBM typewriter. Punch

cards were made from this data to be used in conjunction with

12

an IBM computer, model 1620. Computer programming for

background subtraction was performed by George Pepper.

The results were then printed out for hand plotting.

To optimize energy resolution over the five-MeV energy

range under investigation, two sets of spectra were taken.

The first set pertained to energies from 0.1 to 0.5 MeV,

since this was the most likely energy region for gamma ray

emission. The second set had energy ranges from 0.5 to

5.0 MeV to investigate the existence of gamma rays above

0.7 MeV (3). Each set of spectra contained a cesium

spectrum and background spectra using carbon and empty-can

scatterers under similar conditions.

In the 0.1 MeV cesium spectrum, a total of 4.03 billion

alpha particles was registered in 26 hours. The 0.5 to

5.0 MeV cesium spectrum required a total of 6.1 billion

alpha particles in a twenty-hour period. Necessarily, a

corresponding alpha particle count was taken for both

background spectra. Since each alpha particle that was

counted represents one neutron striking the cesium scatterer,

a one-to-one correspondence could be made between the two

particles.

In calculating relative intensity, self absorption

within the cesium scatterer must be taken into account.

Since the cesium gamma rays have an equal probability of

going off in any direction, only a fraction of the total

gamma rays can be detected. However, this probability will

13

be approximately a constant for any cesium nucleus and

should not alter the relative intensities of the gamma

rays. The intensity of each gamma ray must be adjusted

according to its energy for self absorption and detector

collecting efficiency.

3 3 57 Sources used for energy calibrations were Ba , Go ,

_ 60 _ 137 , 22 . , ,

Co , Cs , and Na . Using standard sources, an energy

resolution of 6.0 keV was obtained with the Ge(Li) detector.

CHAPTER BIBLIOGRAPHY

1. Hollander, J. M., "The Impact of Semiconductor Detectors on Gamma Ray and Electron Spectroscopy," Nuclear Instruments and Methods. XLIII (1966), 65.

2. Mayer, J. W., "Semiconductor Detectors for Nuclear Spectrometry," Nuclear Instruments and Methods, XLIII (1966), 5T.

3. McAnally, M. A., "Gamma Rays Resulting from the Neutron Scattering in Cesium," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1959.

14

CHAPTER III

RESULTS AND CONCLUSIONS

Representative cesium spectra obtained from this

experiment are shown in Figures 5, 6, 7, 8, and 9.

Exponentially decreasing counts of 12,000 to 400 for the

0 to 0.5 MeV spectrum (Figure 5) produced statistical errors

of 0.9 percent to 5 percent. Similar results were observed

in the 0.5 to 5 MeV spectrum (Figure 6). Following

subtraction of the carbon background spectrum (Figure 7),

the empty-can spectrum (Figure 8), and accidental spectra,

these two spectra were combined to produce the cesium

spectrum shown in Figure 9. Statistical errors for this

spectrum ranged from 1.6 percent to 15 percent. In the case

of the cesium scatterers, the ratio of the true to accidental

coincidences was approximately four to one. However, the

accidental spectrum showed no outstanding features.

From the gamma ray spectra shown, it was evident that

only three peaks were prominent. All three of these peaks

are attributed to the background since they were found in

the cesium spectrum as well as in the background spectra.

Likewise, the peak heights were directly related to the

time of neutron irradiation. The first peak at 0.51 MeV

was apparently the result of positron annihilation in

surrounding materials. The second peak at 0.69 MeV was

15

16

assigned to the (n,n') reaction of germanium. The third

peak was predominantly the 0.85 HeV gamma ray from the

(n,n*) reaction of germanium.

Unfortunately, these peaks were not removed completely

when the background spectra were subtracted. The primary

reason for insufficient background removal was the

activation of the 4.43 MeV peak of carbon and its subsequent

escape peaks. These peaks enhanced the background count

at the higher energies at the expense of the lower energies.

Another factor was the inelastic scattering of neutrons by

carbon rather than elastic scattering. Apparently, these

two effects caused the background compensation to be a

little less than it should have been, as witnessed by the

three peaks. It turns out that this error does not affect

the analysis very much since the analysis depended on the

slope of the cesium pulse height spectrum rather than its

magnitude.

Apparently, within the excitation range of 14 MeV

133

neutrons on Gs , there are a great many energy levels.

Hence, there are so many gamma rays excited that they cannot

be resolved even with a Ge(Li) detector. This is not too

surprising a result as it is known that in the region of Cs13^

and with 14 MeV of excitation energy, one would expect

nuclei with many closely spaced levels. A great many levels

might be excited making available a large number of decay

paths. This experiment, however, definitely establishes that

17

there are gamma rays from cesium, and one can make an esti-

mate of their yield in the energy range between 0.2 and

2.7 MeV. The lower energy limit was set by the dead layer

in the Ge(Li) detector as well as self absorption in the

scatterer and rising background. The upper limit is set by

insufficient knowledge of the gamma rays from carbon which

would have both primary and escape peaks in the 3.4 MeV

region and above. This yield was expressed in terms of a

differential excitation probability

o-(E) = Y(E) / N <J> T

where Y(E) was the differential gamma ray yield Ay/fiE, y was

the gamma ray yield, N was the number of nuclei per cubic

centimeter in cesium, (j) was the neutron flux,1 and T was

the thickness of the cesium scatterer. Since the yield and

the flux were taken over the same area and for the same

period of time, only the total counts of each were required.

However, several corrections had to be made to achieve a

true yield.

It was found experimentally that the average count rate

of a gamma ray source from the maximum and minimum angles

which could be subtended from the Ge(Li) detector to the

cesium scatterer was a good approximation to the count rate

of a source whose angle subtended through the centroid of

the cesium scatterer. Because of the six-degree angle of

the incident neutrons, the activated cesium was in the shape

of a parallelogram.

18

Another correction had to be made for self absorption

in the cesium (Figure 10). Weighted-mean values for dis-

tances a particular energy gamma ray had to travel in the

cesium to reach the Ge(Li) detector were calculated (Table I).

These distances were calculated from the photoelectric,

Compton, and pair production cross sections in cesium.

Since the Compton scattering cross section for germanium

is much greater than the photoelectric cross section and

there are no resolved photoelectric peaks in cesium, a

composite spectrum (Figure 11) of the Compton edges of known

intensity gamma ray sources was taken for comparison. The

pulse height spectrum of each gamma ray was approximated to

a constant value from zero energy up to the energy of the

Compton edge and a count of zero in channels above that

energy. This provided a means for translating the differen-

tial of the multichannel spectra into a gamma ray yield for

cesium as a function of energy (Table II). An advantage of

this method was that the detector efficiency for various

energy gamma rays was already incorporated in the composite

spectrum. However, the highest energy source available

was the 2.56 MeV gamma ray of thorium, which set another

limit on the high energy end of the composite spectrum.

In determining the composite spectrum, standard sources

were placed a fixed distance from the detector along a line

through the center of one of the rectangular cross sections

in the cesium toroid. An inverse-square correction had to

19

be made for the differences between this position of

standard sources and the weighted-mean distance of the

emitter atoms in cesium. Ratios of detected intensity to

initial intensities in cesium were calculated from the

inverse exponential of the absorption coefficients and

weighted-mean distances. In producing a true yield, the

ratio of the cesium count differential per 5 keV to the

composite spectrum yield for the same energy was multiplied

6

by a standard of 1.95 X 10 disintegrations and was then

divided by the appropriate radial and absorption corrections.

Differences in energy between a photoelectric peak and its

Compton edge were noted in Table II.

After all corrections were completed, the excitation

probabilities per 5 keV in cesium were found (Figure 12).

Values ranged from 1.38 millibarns at 0.22 MeV to 0.37

millibarns at 2f5 MeV. Correction factors were chiefly

responsible for the small hump near 1.3 MeV. At this

energy range the composite peak began to flatten while the

absorption coefficients began to plateau. From the inherent

statistical and experimental errors of this investigation,

credible error values could be made ranging from 10 percent

at the 0.2 MeV level up to 20 percent at the 2.5 MeV level.

Previous investigations have been severely hampered by

high background counts and cesium compounds as scatterers.

The subtraction process involved would produce spurious

peaks. It was hoped that the improvements in resolution and

20

detection techniques in this investigation could resolve

the gamma rays from cesium. Even though the efficiency of

the system was high enough to resolve gamma ray peaks in

the background, it evidently was not enough to resolve the

closely spaced energy levels of cesium. In order to achieve

any significant improvement in efficiency and backgroung

reduction, vast modifications of equipment and facilities

would have to be made.

TABLE I

RADIAL AND ABSORPTION CORRECTIONS

-uR ( R N2

E u R uR e R V R 7 C(R,u)

.102 3 3 . 2 . 0301 1 . 0 0 .368 2 . 5 3 . 399 .147

. 2 0 4 3 . 6 2 . 269 . 973 .378 2 . 7 7 .478 . 181

. 2 5 5 1 . 9 5 . 461 . 90 .406 2 . 9 6 .547 .222

.307 1 . 3 1 . 4 9 4 .646 . 525 2 . 9 9 . 588 . 2 9 3

. 358 1 . 1 2 .581 . 603 .546 3 . 0 8 . 591 .322

. 460 .672 .617 . 414 . 661 3 . 1 2 . 6 0 5 . 4 0 0

. 511 .607 . 630 .382 .681 3 . 1 3 .612 .417

.767 . 451 .652 . 294 . 744 3 . 1 5 . 618 . 459

1 . 0 2 .378 . 6 6 1 . 250 .779 3 . 1 6 .622 . 484

1 . 2 8 . 3 3 0 . 6 7 0 .221 . 801 3 . 1 7 . 6 2 5 . 501

1 . 5 3 . 311 .682 .212 .811 3 . 1 8 . 6 3 0 .512

1 . 7 9 . 2 8 8 . 6 8 8 . 198 .820 3 . 1 9 . 6 3 3 . 519

2 . 0 4 .272 .687 .188 .829 3 . 1 9 . 6 3 3 . 5 2 4

2 . 3 0 . 268 .686 . 184 .831 3 . 1 8 . 630 . 5 2 5

2 . 5 5 ———__

. 2 6 5 . 6 8 5 .182 . 8 3 3 3 . 1 8 . 630 . 526

TABLE II

DIFFERENTIAL EXCITATION PROBABILITIES

E (MeV)

Ec (MeV) Ay/AX y

1 *4 (X10*)

1 4 (X10*)

<r(E) (mb)

. 220 .10 4 8 . 8 3900 3 . 5 7 1 2 . 4 1 . 3 8

.260 . 150 4 1 . 6 2400 3 . 7 1 1 1 . 5 0 1 . 3 0

. 410 .250 3 4 . 5 2000 3 . 4 9 8 . 7 3 .986

. 5 7 0 . 3 5 0 2 7 . 4 1600 3 . 2 2 7 . 4 1 .838

.792 . 572 1 6 . 5 1140 2 . 8 2 5 . 9 7 . 676

. 8 6 8 .638 1 1 . 8 850 2 . 7 1 5 . 5 5 .627

1 . 0 2 7 .797 7 . 6 585 2 . 5 4 5 . 2 6 . 594

1 . 1 5 . 921 6 . 1 463 2 . 4 8 5 . 0 3 .569

1 . 3 1 1 . 0 8 4 . 2 340 2 . 4 1 4 . 8 1 . 5 4 3

1 . 4 0 1 . 1 7 3 . 1 267 2 . 2 9 4 . 5 7 .516

1 . 5 6 1 . 3 3 1 . 9 175 2 . 1 7 4 . 2 3 .478

1 . 7 5 1 . 5 3 1 . 6 155 1 . 9 8 3 . 9 4 . 4 4 5

2 . 1 4 1 . 9 1 1 . 2 125 1 . 8 7 3 . 6 0 .407

2 . 3 1 2 . 0 7 . 9 100 1 . 7 8 3 . 3 9 . 3 8 3

2 . 5 3 2 . 3 0 . 8 90 1 . 7 4 3 . 3 1 . 3 7 4

2 . 7 5 2 . 5 2 .7 80 1 . 7 1 3 . 2 5 . 368

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BIBLIOGRAPHY

Articles

Arya, At_P., "Gamma-Gamma Directional Correlations in Gsljjj" physical Review. CXXII (1961), 549.

Chien, J. P., and Smith, A. B., "Fast Neutron Scattering from Beryllium, Sodium, and Aluminum," Nuclear Science and Engineering. XXVI (1966), 510.

Crasemann, B.^.Pengra, J., Lindstrom, I. E., "Radiations from Ba1JJ," Physical Review. CVIII (1957), 1500.

Davis, R. H., Divatia, A. S., Lind, D. A., and Moffat, R. D., "Coulomb Excitation of Elements of Medium and Heavy Mass," Physical Review. CIII (1956), 1801.

Fagg, L. W., "Coulomb Excitation of Sb, Cs, and Ba," Physical Review. CIX (1958), 100.

Hollander, J. M., "The Impact of Semiconductor Detectors on Gamma Ray and Electron Spectroscopy," Nuclear Instruments and Methods. XLIII (1966), 65.

133 Mann, K. C., and Chaturvede, R. P., "The Decay of Ba ,"

Canadian Journal of Physics. XLI (1963), 932.

Mayer, J. W., "Semiconductor Detectors for Nuclear Spectrometry," Nuclear Instruments and Methods. XLIII (1966), 55.

133 Ramaswamy, M., Skeel, W., and Jastram, P., "Decay of Ba ,"

Nuclear Physics. XVI (1960), 619.

Ramaswamy,, M,, Skeel, W., and Jastram, P., "The Decay Energy of B a X J V ' Nuclear Physics. XIX (I960), 299.

Shectman, R. M., and Anderson, J. D., "Inelastic Scattering of 14 MeV Neutrons," Nuclear Physics. LXXVII (1966), 241.

Stewart, M. G., and Lu, D. C., "Nuclear Levels of Cs*^," Physical Review. CXVII (1960), 1044.

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39

Subba Rao, B. N., "Internal Conversion Coefficient of the 81 Kev Transition in Csl33," Nuclear Physics, LXXXVI (1966), 443. "

Temmer, G. M., and Heydenburg, N. P., "Coulomb Excitation of Heavy and Medium-Heavy Elements by Alpha Particles," Physical Review. XCIII (1954), 351.

Temmer, G. M., and Heydenburg, N. P., "Coulomb Excitation of^Medium-Weight Nuclei," Physical Review, CIV (1956),

Thun, J. E., Tornkvist, S., Nielsen, K. B., Snellman, H., Falk. F., and Mocoroa, A., "Levels and Transitions in Cs133," Nuclear Physics. LXXXVIII (1966), 289.

Williams, G. H., and Morgan, I. L., "Ge (Li) Gamma-Spectrometer for (n,n*) Measurements," Nuclear Instruments and Methods. XLV (1966), 3lTI

Yin, L. I., and Wiedenbeck, M. L., "Gamma-Gamma Directional Correlation Measurements in the Decay of Bal33 » Nuclear Physics. LIV (1964), 86.

Unpublished Materials

Dawson, H. R., "D-D and D-T Neutron Excitation of Energy Levels in Cs^-33," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1961.

$

Lamb, B. L., "Gamma Rays from Neutron Excitation of Csl-33 " unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1959.

McAnally, M. A., "Gamma Rays Resulting from the Neutron Scattering in Cesium," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1959.

McDonald, P. F., "Gamma Rays from Cs^-33 by Inelastic Scattering of Neutrons," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 1960.

Young, J. C., "Gamma Ray Response of Csl (Tl) Crystal to 14 Mev Neutrons," unpublished master's thesis, Department of Physics, North Texas State University, Denton, Texas, 3L 9 58 •