Title: Delayed coincidence spectroscopy of fission fragment excited gases
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Title: Delayed coincidence spectroscopy of fission fragment excited gases
Physical Description: ix, 104 leaves : ill. ; 28 cm.
Language: English
Creator: Shipman, George Robert, 1944-
Copyright Date: 1976
 Subjects
Subject: Ionized gases   ( lcsh )
Spectrum analysis   ( lcsh )
Nuclear Engineering Sciences thesis Ph. D   ( lcsh )
Dissertations, Academic -- Nuclear Engineering Sciences -- UF   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: leaves 102-103.
Additional Physical Form: Also available on World Wide Web
General Note: Typescript.
General Note: Vita.
Statement of Responsibility: by George Robert Shipman.
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Bibliographic ID: UF00097515
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000202981
oclc - 03928617
notis - AAW9747

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DELAYED COINCIDENCE SPECTROSCOPY OF FISSION FRAGMENT
EXCITED GASES


















By

GEORGE ROBERT SHIPMAN


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF TIE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY



UNIVERSITY OF FLORIDA


1976













ACKNOWLEDGMENTS


As with any research effort, this dissertation has not

been the result of one person's efforts. The patient and

fatherly concern of Professor R. T. Schneider in the con-

tinued support of this project is gratefully acknowledged.

The willingness of my chairperson, H. D. Campbell, to pro-

vide help and insights into the theoretical aspects of

fission fragment produced plasmas was balanced by the equal

and complementary efforts of E. E. Carroll who suggested the

path through two apparently insoluable experimental diffi-

culties. Mr. Ken Fawcett, a man with an uncanny ability to

revive aged and senile electronic equipment, provided for

the care and well-being of the detection system. Mr. Ernie

Whitman and Mr. Joe Mueller provided help and expertise in

numerous and diverse fields. I have benefited from discus-

sions with many of my fellow students but especially Drs.

Bob Davie, John Davis and Jim Fuller. I have also had the

benefit of four friends and helpers during the course of

this research. Dave Sterritt and George Strickland devised

and wrote the programs for the data reduction while Dudley

Carter helped in almost every phase of the work. I pass

this system and the remaining problems on to Tom Maguire

who has been of immense help in the later stages of this

work. I wish to thank my wife, Barbara Jo, for typing and
ii







and retyping the rough draft and for her nearly patient

encouragement during the long term of this effort.


iii















TABLE OF CONTENTS


ACKNOWLEDGMENTS . . .

LIST OF FIGURES .. ..

ABSTRACT. . . . .

CHAPTER


1. INTRODUCTION . . . . . . .

1.1 Background . . . . . .
1.2 General Description . . . .
1.3 Previous Work . . . . .
1.4 Organizational Plan . . . .

2. THEORETICAL CONSIDERATIONS .. ...

2.1 Introduction . . . . ..
2.2 Fission Fragment-Gas Interaction
2.3 Rate Equation for C3Hu. . . .
2.4 Relating Solutions to Observables

3. SYSTEM AND CALIBRATION . . . .

3.1 Introduction and Design Criteria.
3.2. System Description. . . . .
3.3 Calibration . . . . . .
3.4 Output . . . . . .
3.5 Scanning Mode . . . . .

4. ANALYSIS AND RESULTS . . . . .

4.1 Introduction . . . . .
4.2 Programs . . . . . .
4.3 Analysis . . . . . .
4.4 Pulse Height Distribution. . .

5. OTHER GASES AND GAS MIXTURES . . .


5.1
5.2
5.3
5.4
5.5
5.6


General Comparison of Mixture
Neon/Nitrogen Mixtures .
Helium/Nitrogen Mixtures. .
Argon/Nitrogen . . .
Carbon Tetraflouride. . .
Population Inversion Study. .


Page
. . ii


. . . viii
111


. .


. 1

. 1
S 2
. 4
S 4

S 6

S 6
S 7
S18
.20

S22

.22
S23
S29
S31
S32

S35

. 35
S35
S37
S41

S47


Spectra


III









6. CONCLUSIONS AND FUTURE WORK . .

6.1 Conclusions . . . . .
6.2 Future Work . . . . .
6.3 Scanning Mode Recommendations


Page
81


. . 81
. . 82
. . 84


APPENDICES


. . . 86
. . . 89
. . . 92


1. ENERGY DEPOSITION . . . .
2. NE-111 AS A CALIBRATION SOURCE .
3. GAMMA VERSUS TLP FOR START PULSE .
4. GAS PURITY . . . . . .
5. FISSION FRAGMENT TRANSIT TIME. .
6. 252Cf CHARACTERISTICS . . .

BIBLIOGRAPHY . . . . . . .

BIOGRAPHICAL SKETCH . . . . .


. . 102


104














LIST OF FIGURES


Page
FIGURE

1. Typical I(t) Curve . . . . . . . 8

2. Typical Decay Curve Showing Gamma Coincidence
Peak . . . . . . . . . . 9

3. Pressure Variation of Decay Time. . . ... 12

4. Decay Time Versus Pressure for 337.1nm . 13

5. Inverse Decay Time Versus Pressure for 337.1nm 14

6. Metastable Transfer in He/N2. ..... ... 15

7. Double Decay Curve . . . . . . 16

8. Relevant Energy Levels of He, Ne, A, and N2 17

9. System for Lifetime Measurement . . .. 24

10. Scan of Pure N2 . . . . . . . 33

11. Peak Intensity Versus Pressure for 337.lnm.. 40

12. Integrated Intensity Divided by Decay Time
as a Function of Pressure . . . . 42

13. Integrated Intensity Versus Pressure . . 43

14. Fission Fragment Energy Spectrum. . . ... 45

15. Fission Fragment Distribution from TLP Pulse Heights 46

16. Scans of Nitrogen Impurities Ne, A, He, and
Pure N2 . . . . . . . . . 48

17. Scan of Nitrogen Impurity in Pure Neon. . 50

18. Decay Time Versus Pressure for 585.2nm . 51

19. Decay Time Versus Pressure for 585.2nm. . 52

20. Decay Time Versus Pressure for 585.2nm. . 53







Page
21. Decay Time Versus Pressure for 585.2nm . 54

22. Decay Time Versus Pressure for 337.1nm . 55

23. Decay Time Versus Pressure for 337.1nm . 56

24. Decay Time Versus Pressure for 337.1nm . .57

25. Decay Time Versus Pressure for 337.1nm . 58

26. Decay Time Versus Pressure for 391.4nm . 60

27. Decay Time Versus Pressure for 391.4nm . 61

28. Decay Time Versus Pressure for 391.4nm . 62

29. Scan of Nitrogen Impurity in He .. . . 63

30. Scan of He with 1% N2 . . . . .. 64

31. Relative Intensity Versus Pressure for
391.4nm . . . . . . . . . 65

32. Scan of He with 10% N2 . . . .. . 67

33. Metastable Transfer of He/N2 . . . .. 68

34. Decay Time Versus Pressure for 391.4nm . .. 69

35. Scan of N2 Impurity in He, Excitation Period Only. 70

36. Scan of Nitrogen Impurity in Argon . . . 72

37. Scan of Argon with 10% N2 . . . . 73

38.. Fast and Slow Decay in A/N2. . . . .. 74

39. N2 Impurity in CF4 . . . . . . . 76

40. Population Densities on N2(B) and N2(C) Versus
Time . . . . . . . . . . 79

41. Measured 1st and 2nd Positive Populations. .80

42. Using Gamma Versus TLP as Start Pulse. . . 93








vii








Abstract of Dissertation Presented to the
Graduate Council of the University of Florida
in Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy



DELAYED COINCIDENCE SPECTROSCOPY
OF FISSION FRAGMENT EXCITED GASES

By

George Robert Shipman

December, 1976

Chairman: Hugh D. Campbell, Ph.D.
Major Department: Nuclear Engineering Sciences

A time resolved single photon counting system was used

to study the emission spectra of N2 under fission fragment

bombardment. The source of fission fragments was 0.01pg

of 252Cf evaporated on a platinum disk. The 252Cf emitted

two groups of fission fragments with energies of about 80

MeV and about 105 MeV at a rate of 6,000 per second.

The system used a time-to-amplitude converter (TAC)

which converted the time difference between two timing

signals into a voltage. The "start" pulse was supplied

by a plastic scinti'llator which detected the prompt fission

gamma rays. The "stop" pulse was generated when a single

photon struck the photomultiplier mounted on the spectro-

graph. The probability that a photon occurred was pro-

portional to the population of the excited state giving

rise to the observed spectral line. The number of photons

in each time interval was thus related to the excited

state density in that same time interval. After accumulat-

ing enough counts to give acceptable statistics, a curve

viii







was generated which plotted the number density of excited

states as a function of time-after-fission. The portion of

the curve which occurred after the fission fragment had

passed out of the field of view (the 'afterglow.) was least

squares fitted to a decreasing exponential.

It was observed that the excited state population

decayed more rapidly as the pressure was increased. Ex-

trapolation to zero pressure allowed determination of the

radiative lifetime while the rate of change of decay time

with pressure was related to the collisional quenching

cross section. The resolving time of this system was less

than two nanoseconds.

The radiative lifetime of N2 (C3u) was found to be

42ns1.5ns and the collisional transfer rate constant to
-11 3 -1
be 1.0x10 cm sec

Several other noble gas-nitrogen mixtures are described

in a more qualitative manner.














CHAPTER I

INTRODUCTION


1.1 Background

This dissertation is divided conceptually into three

parts. First, there is the description of a new system

which was developed for the analysis of gases excited by

non-periodic, transient, randomly occurring events. Second,

this system was used to make a careful study of one parti-

cular excited level in one particular gas. Third, a more

qualitative description is given of some measurements which

were done on various gas mixtures exhibiting both resonant

and nonresonant behavior.

The work which led to this dissertation was initially

conceived as an alternative to an in-core reactor experi-

ment. The study of fission fragment excited gases had been

going on for some time in connection with both the gaseous

core reactor and the nuclear pumped laser program. 2 3 4

A typical experiment involved placing a 235U lined cylinder

within the core of the reactor, filling the cylinder with a

chosen gas or gas mixture, and observing the emitted photons

with a spectrographic system. 2 There were, however, sever-

al limitations inherent in this type of experiment. Because

of the nature of the reactor environment, stringent safety

precautions had to be maintained. These precautions, while

1





2

necessary, severely limited access to the experimental

device and restricted data acquisition, system modification,

and system adjustment. Excitation using the spontaneous

fission fragment emitter 252Cf was seen as an attractive

alternative. This isotope provides a safe, small, self-

powered source of energetic fission fragments closely

resembling in all respects the fission fragments emitted by

235U (see Appendix 6). At first it was thought that the

extremely low flux, about nine orders of magnitude less

than in the reactor, would make the spectroscopic diagnos-

tics an insuperably difficult problem. The resolution of

this problem proved to be the adoption of single photon

counting techniques.

1.2 General Description

Preliminary intensity measurements indicated that only

about one photon was observed for each 100 fission fragments

which passed through the region of observation. In addition,

it was soon realized that the time that a fission fragment

took to cross the chamber (about four nanoseconds) and,

hence, the length of the excitation pulse was extremely

short as compared to the mean time between fission fragments

(about 0.3 milliseconds) thus allowing a unique assignment

of an observed photon to a particular fission fragment.

Since we could tell which fission fragment caused which

photon, a measurement of the time interval between the

passage of the fission fragment and the emission of the

photon allowed a determination of the lifetime of the atomic





3

or molecular level from which the photo originated. We

had thus created a fluorescence decay system utilizing a

totally new type of excitation. Since the probability that

a photon will be observed at a certain point in time is

proportional to the population density at that time, a

history of the photon flux is a history of the excited

state population. After the time intervals between many

fission fragment-photon pairs had been' measured, such a

photon flux history was obtained. Determination of the

interval between excitation and photon emission required

accurate knowledge of when the fission fragment transited

the excitation region. In all previous fluorescence decay

systems, the excitation source had been triggerable, and

this trigger had provided the timing "start" pulse. Here,

however, the excitation was a randomly occurring, non-

periodic, transient event. Two independent methods were

developed for providing the timing "start" pulse, both of

which were found to give identical results.

What originally started as an alternative to a reactor

experiment evolved into a system of great versatility,

capable of providing information not only about fission

fragment excited gases, but also about the fluorescence

decay of atomic and molecular levels.

The extent to which the information obtained in this

experiment can be scaled to the reactor situation has not

yet been resolved. Several studies' 2 3 have shown that

population density increases linearly with reactor power





4

over several orders of magnitude. Populations probably

remain linear until electron-electron interactions begin to

dominate the thermalization of the swarm, a density which

has not yet been reached in gases up to one atmosphere.

1.3 Previous Work

Previous work germane to this dissertation has appeared

in a diverse range of fields.- This work has been recently

reviewed4 and the interested reader is referred thereto.

If, from the vast body of literature available'upon the

subject of the interactions of fast charged particles with

gases, we restrict our attention to those which are con-

cerned with the passage of fission fragments and the sub-

sequent emission of photons, we find that they are, with

one exception, concerned with the steady state case of

excitation within a reactor. That exception is the work of

Axtmann5 and his students at Princeton. They have also

studied the C3Hu level of nitrogen, but with quite a dif-

ferent experimental technique and obtained essentially

identical results.

1.4 Organizational Plan

The plan of this dissertation will be to first develop

in Chapter 2 the theory necessary to understand the optical

emission of fission fragment excited gases in a general way,

and then look at the rate equations for the C3Hu level in

pure nitrogen. In Chapter 3 will be described the experi-

mental apparatus and its calibration. Chapter 4 will be an

analysis of the results for pure nitrogen, including values





5

for the radiative and collisional rate constants. Chapter

will describe in a semiquantitative way the measurements

which have been made on other gases and gas mixtures. After

a short concluding discussion, a series of appendices will

examine in greater detail some questions which relate to the

analysis rather than interpretation of the results.














CHAPTER 2

THEORETICAL CONSIDERATIONS


2.1 Introduction

The primary theoretical motivation for this disserta-

tion was the desire to understand in what respect plasmas

created by the passage of fast charged particles resembled

(and how they differed from) electrically produced plasmas.

During the early stages of the research, which eventually

led to a nuclear pumped laser, it was widely believed that

plasmas' created by ionizing radiation would differ in

fundamental respects from electrical plasmas. As more

insight was gained, however, it was realized that the bulk

of the excitation and ionization was caused by the shower of

fast secondary electrons rather than by the fission fragment

itself.6 Thus the excitation process was seen to be elec-

tron impact excitation with the following difference: the

electron energy distribution was nonmaxwellian, with peaks

at high energies corresponding to the "injection" energy of

the secondary electrons.7 Since, in the nuclear plasma

case, the electrons are always slowing down from their

initial birth energy, while, in an electrical plasma, the

electrons are being constantly accelerated by the electric

field, we may expect some differences in the excited state

populations.








The basic operational problem was that of finding

relations between the observable quantity N(t), the number

of photons which were emitted at time "t" after fission,

and the properties of fission fragment excited gases in

general or atomic and molecular properties of particular

gases. We shall first describe .the interaction of the

fission fragment with the gas in a general way, and then,

these relations will be derived'by starting with the

elementary rate equations for pure nitrogen. The parameters

most amenable to analysis were T, the l/e decay time; Trise,

the l/e rise time; ENi, the sum of all photons observed

independent of time; and Nmax, the value of the peak in-

tensity (see Fig. 1).

2.2 Fission Fragment Gas Interactions

Californium was chosen as a fission fragment source

since it underwent spontaneous fission and, thus, elimi-

nated the need for a neutron supply as is the case with

uranium. When a spontaneous fission occurred,the first

particles which entered the gas were prompt gamma rays.

These gammas were emitted within 10-11 seconds of fission,

but caused little excitation of the gas in the experimental

chamber. They could, however, cause a small ionization

pulse in the residual gas within the photomultiplier. This

pulse was amplified by the dynodes and recorded by the

counting system. The gamma pulse occurred well before the

peak of the light pulse, and caused no increase in back-

ground during the decay period. (see Fig. 2). At the time of










8



















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10

the gamma burst, the fission fragments began to enter the

gas. As the fission fragments passed through the gas, they

struck atoms of the gas mixture causing excitation and

ionization. The resulting excited states and ion pairs

were not all produced in interactions with the fission

fragment. The bulk of the excitation and ionization was

produced by the secondary electrons ejected during the

ionization.

Once the fission fragment had stopped, either by giving

up its energy or through hitting the wall, no more energy

was transferred to the gas, and relaxation proceeded (see

Appendix 5). Though no more energy was entering the gas as a

whole, the populations of individual levels may still have

continued increasing. This continued increase may have

been due to dissociative recombination, cascading, reso-

nance trapping, or metastable transfer. In these cases it

was difficult to say exactly when the excitation of a given

level ended, and the afterglow commenced. Thus, while the

decay rate in the afterglow was determined by the radiative

and collisional lifetimes, if excitation processes continued

after the fission fragment had passed from view, then what

was being measured was the excitation or transfer rate

rather than the decay rate.

We have now followed the energy of the fission frag-

ment in a general way from the time the fragment entered

the gas until some of its energy reached a particular

atomic or molecular level. The energy thus stored in a





11

level may leave by a variety of routes. The energy

pathways for deexcitation depend upon pressure in a com-

plicated way, because collisional effects are a function

of pressure. At very low pressures where collisions may

be ignored, the major loss of energy from a level is due

to radiative decay. As the pressure increases, collisional

self-quenching will cause the lifetime of the level to

decrease (see Figs. 3 and 4). By extrapolating the lifetime

versus pressure curve back to zero pressure, the radiative

lifetime may be determined. Fig. 5. Thus, the rate at

which the lifetime changed with pressure may be used to

determine the collisional transfer coefficient for the

observed level. This discussion of deexcitation assumes

that no impurities are present. With impurities, the

kinetic behavior becomes a great deal more complicated.

Impurity quenching may cause the lifetime of a level to

decrease from what it was in the pure gas case, while the

effects of transfer from an impurity with a resonant

metastable level can cause a dramatic increase (see Fig. 6).

If two different deexcitation paths exist having quite

different time scales, the decay curve may show two decay

times, one for short times and one for long times (see Fig. 7).

With the exception of a small amount of work on CF4,

the bulk of this study was carried out on the gases

helium, neon, argon, and nitrogen. Partial energy level

diagrams for these gases are shown in Figure 8. This

figure illustrates the energetic resonance between the







12







800 torr

2
337.1 nm





C,

U
C,

fo




C7
C-)-


L-



Lo 100 torr






























0 25 50 75
TIME (ns)

Figure 3. Pressure Variation of Decay Time










13










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CC



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2 + +
B Eu level of N2 and the metastable levels of helium and

also the argon metastable, C3Hu resonance. These reso-

nances lead to intense first negative and second positive

emission respectively. By observing mixtures of the three

inert gasses with nitrogen, we were thus able to examine

the cases of neutral excitation (A/N2), ion creation and

excitation (He/N2), and no interaction (Ne/N2).

2.3 Rate Equation for C3Hu

In this chapter we will confine ourselves to the

relatively simple case of the C3Hu level of N2 with no

foreign gas present. For this restricted case, there was

no cascading8, metastable transfer or resonance trapping,

and thus the rate equation is very simple, consisting of

two deexcitation terms, collisional and radiative decay,

and one excitation term, collisional excitation from the

ground state. The collisional excitation "turns off" at

tf, the time at which the fission fragment left the ob-

servable region or had lost so much energy that it could

no longer produce excited states. The rate equation thus

separated in a very natural way into two parts, one before

tf, and one after.5

dN2 dEd *
dt = Qdt 2 k2PN2 Ot

dN2 *
-- = klN2 k2PN2 tft dt
where, N2 is the population of the C3Hu level, Q is the

number of C3Hu states produced per unit energy deposited

in the gas, Ed is the energy deposited by the fission






19

fragment, P is the pressure, tf is the time at which the

fission fragment collides with the wall or has slowed to

the point where it can no longer cause excitation and ioni-

zation. kI and k2 are the rate coefficients for the

deexcitation reactions shown below:

N2(C3 Hu) N2(B3 u) + hv
k2 observed states.
N2(C3Hu) + N2(X1Zg+) -
kI is the sum of all transition probabilities for the C3Hu

state,
1 -I
k AL = sec
S UL Tr
where, Tr is the radiative lifetime of the state, and k2 is

the two-body collisional quenching rate with units (sec-l

torr-1)

After tf, the population decreases monotonically, and

Equation 2 may be solved to give

-(kl+k2P)t
2(t) = N2maxe (3)

Solving the rate equations for 0
form for the energy deposition of the fission fragment.

While much has been written about the form of this func-

tion9 10, for nitrogen the energy deposition is adequately

described by the square law for electronic stopping

E = E (1-r/Re) 2, (See Appendix 1) (4)

which, in terms of velocity is

v = v0(1-r/Re), (5)

where E and v are the energy and velocity of the fission

fragment at the distance r from the source. EO and v0 are







the initial energy and velocity of an average fission

fragment. Re is the extrapolated electronic range, Re =

SRT 2
M, where S is the range in mg/cm ; R is the gas constant;

T the absolute temperature; P the gas pressure; and M the

molecular weight of the gas. Integrating Equation 5 over

the flight time and track length of the fission fragment

gives
Rerf
tf e n(l-1 ) (6)
V Re

Solving Equations 3 and 6 together gives

dEd -Zt
-ZE0e

where Z = 2 v0/Re. Thus, for 0

ZEQ -Zte- (kl+k2P)t} (7)
N2(t) = (k) {e e 2(
2 k i ~k+k2

2.4 Relating Solutions to Observables

Since the intensity of I(t) equals klN2(t), determin-

ing the decay time of the intensity in the period tf
gives the excited state lifetime directly. Since Equations
ZEoQkl
3 and 7 must be equal at tf, Im; = klN2mnx k+k2P-Z

{e-(kl+k2P-Z)tf_-}. As goes to infinity, N2max s just

the total number of excited states produced by the passaqe

of the fission fragment

N2max([_') = EOQ(l-e ) .

Integrating klN2(t) from 0 to gives the observable inte-

grated intensity
S kN -Ztf
I = 0 klN2(t)dt = klEOQT(1-e ).





21

Physically, N2max is just the maximum excited state

density that is obtained, and I is that fraction of the

excited states which decay by radiation. Note that

I/T = I2max(T) .













CHAPTER 3

SYSTEM AND CALIBRATION


3.1 Introduction and Design Criteria

The design of the experimental apparatus was deter-

mined in large part by the nature of the isotope source.

The source emitted about 3000 fission fragments each

second into the gas, and each fission fragment produced

about 104-105 photons. Because of the very small solid

angle subtended by the monochromator slit, it was ex-

pected that, on the average, on'e photon would be counted

for each 1000 fission fragments. This extremely low

light level demanded that single photon counting techniques

be employed. Since thermally emitted electrons or dark

counts could not be distinguished from single photons,

the photomultiplier was cooled to decrease random back-

ground caused by thermionic emission of electrons from the

photocathode. The key to the noise reduction and the

advantage of this system over other single photon counting

spectroscopy systems was the use of two photomultipliers

in a delayed coincidence circuit which virtually eliminated

the possibility of counting a random noise signal generated

in only one of the tubes. The low flux level also required

the ability to maintain suitable gas purity for the ex-

tended times necessary to establish acceptable statistics

22







(see Appendix 4).

The experimental apparatus (see Fig. 9) may be conveni-

ently divided into three sections:

The experimental chamber,

The vacuum and gas handling system, and

The detection system.

The operation and calibration of each component section will

first be described, followed by a discussion of system

characteristics.

3.2 System Description

The experimental chamber was a 3.8cm i.d. double-sided

Varian high vacuum flange. The side toward the total lumi-

nosity photomultiplier (TLP) had a pyrex window which was

optically coupled to the TLP. The side facing the monochro-

mator had a sapphaire window within 2mm of the entrance slit

of the monochromator. The flange had two ports, one which

connected with the gas handling and vacuum system, and the

other'through which the calibration source or high voltage

electrode could be inserted. The 252Cf source was cemented

to the inside of the flange in such a way that a fission

fragment emitted normal to the surface traveled along the

long axis of the monochromator slit. In this way the light

input to the monochromator was maximized.

The vacuum system was entirely stainless steel with

copper gaskets. A combination of roughing, oil diffusion,

and ion pumps were used to attain an ultimate vacuum of
5xl-9mm Hg before filling with a test gas. The system was
5x10 mm Hg before filling with a test gas. The system was













TO GAS HANDLING AND
VACUUM SYSTEM


SCINT.
FOR GAMMAS


TIME TO
PULSE HEIGHT
CONVERTER (TAC)


MULTICHANNEL
ANALYZER (M1CA)


System for Lifetime Measurement


Figure 9.





25

then filled with research grade gas and the pressure

measured to within 1 torr with a Wallace and Tiernan gauge.

Ultek high vacuum valves were used to completely isolate

the system during the period of counting, which, in some

cases, took as long as 72 hours.

The detection system may be described in terms of

its four component functions:

a. Determine when a fission fragment has passed

through and excited the gas (the total lumi-

nosity photomultiplier, TLP).

b. Determine when an excited atom or molecule

has decayed by the emission of a photon

(the specific luminosity photomultiplier,

SLP).

c. Measure the time interval between the exci-

tation and decay (the time to amplitude

converter, TAC).

d. Digitize, accumulate and store measurements

for successive fission fragments (the multi-

channel analyzer, MCA).

a. When the fission fragment passed through the gas, a

large burst of photons was produced. These photons were

of all wavelengths and were spread in time between the

fission and the final decay of the longest lived of the

atomic and molecular states which, in some cases, could

involve times as long as 80 microseconds. When these

photons struck the photocathode of the TLP, they produced







a current pulse which triggered abuilt-in discriminator.

The discriminator, which used zero crossover timing,

generated a fast negative logic pulse when half the total

charge had been collected. To use this signal for timing,

-it was necessary to know how it was related in time to the

fission event. This time relationship was determined in

the following manner. Within 10-1 seconds of fission, a

burst of prompt gamma rays occurred and was detected by a

fast plastic scintillator mounted on a photomultiplier

placed underneath the experimental chamber. The time

interval between the gamma burst or fission and the peak

of the total luminosity pulse was thus determined (see

Appendix 2). In addition, the lifetime of an excited state

was measured using first the gamma detector and then the

TLP to start the timing(see Appendix 3). The measured life-

times in both cases were found to be identical. Thus, the

output of the TLP was related to the time of fission and,

hence, to the time of excitation by a fixed and known time

lag.

b. The photons from the gas passed throCgh a sapphire

window and into the entrance slit of the monochromator.

Because of the close proximity of the monochromator slit

to the source, no system of lenses would have increased the

light input. The monochromator was a Model 218, 0.3 meter

focal length Czerney-Turner, manufactured by the McPherson

Instrument Corporation, with the grating blazed for 300nm.

Mounted directly behind the exit slit was an RCA 8575





27

photomultiplier contained within a thermo-electrically

cooled housing. The system was calibrated for relative

wavelength sensitivity using an Eppley Tungsten filament

lamp certified by NBS. No absolute calibration was at-

tempted. The output of the anode was dropped across a

50 ohm load resistor to give fast time response and the

resulting pulse sent to an Ortec Model 260 time pickoff

unit. The time pickoff unit consisted of a pulse trans-

former, a wide-band transitor amplifier, a tunnel diode

discriminator, and a line drive buffer. The unit acted

essentially as a fast, low level timing discriminator.

When an input pulse exceeded the discriminator setting, a

NIM standard fast negative logic pulse was generated.

c. The time-to-amplitude converter measured the time

interval between the leading edges of logic pulses fur-

nished to its start and stop inputs and generated an analog

output pulse that was proportional to the measured time

interval. The start pulse initiated the charging of a

highly linear capacitor,and the stop pulse terminated the

charging. The voltage across the capacitor was thus pro-

portional to the time interval between the two signals and

served as the output. Since the TAC was triggered by the

leading edge of the input pulse, it was necessary for good

time resolution that the pulses have very fast rise times

and that they exhibit a small dynamic range. These criteria

were satisfied by using NIM-standard fast negative logic

pulses. The start pulse was supplied by a zero crossover






28

discriminator built into the base of the TLP, an Ortec

Model 264 photomultiplier timing discriminatory and pre-

amplifier. This unit also provided a linear or analog

output which was used in conjuction with an Ortec Model 410

linear amplifier to set the discriminator level above the

alpha particle peaks so that the system responded only to

the fission fragments. Since both the TLP and the SLP were

responding to photons produced at about the same time, the

pulses from the SLP were delayed by a nanosecond delay unit

to allow time for the zero crossover discriminator in the

TLP to determine the time of fission. The arrival of the

start pulse opened a time window whose width could be set

from 50 to 80,000 nanoseconds. During this time window,

a photon hitting the photocathode could stop the TAC,

thereby producing an output pulse. If no photon was re-

ceived before the end of the time window, the TAC automati-

cally reset and was then ready to receive another start

pulse. The fraction of start pulses which were followed

by a stop pulse was thus related to the number of photons

present in the experimental chamber. The detailed behavior

of. the number of true stops as a function of time accurately

reflected the time decay of the photon density and,, thus,

the population of the excited level.

d. The output of the TAC was sent to a TMC Corporation

multichannel analyser (MCA). Using a Model 210 pulse

height logic unit, the height of each output pulse from the

TAC was measured and sent to a channel whose address cor-







responded to the measured pulse height and, thus, to the

measured time-after-fission. Counts were accumulated for

a period of time determined by the arrival rate of photons

at the SLP. The fewer photons per second being counted,

the longer the measurement had to be made in order to ac-

cumulate adequate statistics. The period of counting could

be varied from five minutes up to 72 hours, the upper limit

being set by the necessity for maintaining adequate gas

purity (see Appendix 4). After measurement was completed, the

contents of the MCA memory was read out to an X-Y plotter

for qualitative analysis and the digital values punched on

paper tape for subsequent computer analysis.

3.3 Calibration

To determine the minimum decay time that this system

could measure, a small chip of an "ultra fast" plastic

scintillator (NE-111) was suspended in the middle of the

excitation region (see Appendix 2). The measured decay time

of 1.67.06ns showed no broadening compared with the

published value of 1.7 ns. Since no scintillator could be

found with a shorter decay time, the minimum that this

system could measure is still unknown. The TAC was

capable of resolving signals 10ps apart. The system was

ultimately limited by the photomultipliers which have rise

times on the order of Ins.

The minimum resolution in a real experiment is limited

generally by the statistical accuracy of the number of

counts which can be measured in the time over which condi-






30

tions may be expected to remain unchanged. To determine

the variation due to counting statistics and errors associ-

ated with filling the system to a chosen pressure, decay

measurements at two different pressures were repeated ten

times each. The standard deviation of what should be

identical measurements was 0.138ns at 800 torr and 0.324ns

at 100 torr, or about 4% in each case.

Identification and elimination of the gamma background

proved to be a major experimental difficulty. In our pre-

vious experiments in the. reactor, signal noise caused by

gamma rays striking the photomultiplier had required

extensive shielding, a massive low-pass filter, and extreme-

ly slow scanning speeds. The original reason for consider-

ing a coincidence system was the freedom it seemed to offer

from gamma background. Since the gammas are discrete

particles, and thus capable of striking only one photomul-

tiplier at a time, requiring coincidence between the outputs

of the photomultipliers should have eliminated the possibil-

ity of responding to gamma noise. Examination of early

decay curves, however, showed .a peak occurring shortly after

fission which was finally traced to a coincidence between

the fission fragment signal from the TLP and a gamma noise

pulse in the SLP. Because the prompt gammas, unlike decay

gammas, do not occur at random but are emitted within 1011

seconds of fission, coincidence does occur between the

fission fragment and the gammas (see Fig. 2). These gamma

pulses were thought to be due to ionization of the residual







gases within the tube and to scintillation of the quartz

window. The background level was found to be greatly in-

creased by opening the system and exposing the photomulti-

pliers to light, even with no voltage applied. After such

an exposure, about six hours was required before the back-

ground level approached steady state. Extensive tests

were made to determine how.the signal to noise ratio

changed with photomultiplier voltage. The signal to noise

ratio was found to increase monotonically as the high

voltage was reduced. If the voltage was reduced too

much, however, the fission fragment could not be distin-

guished from the alpha particles. The optimum situation

was when the voltage of the two photomultipliers was the

same and equal to 2200 volts.

3.4 Output

After a lifetime measurement had been completed, the

data was in the form of integer values between zero and

220 located in the memory of the MCA. Each channel cor-

responded to a time interval determined by the full scale

setting of the TAC. For example, system calibration showed

that when the full scale setting of the TAC was 50ns and

64 channels of the MCA were being addressed, 10ns covered

a range of 12 channels. Thus, each channel corresponded

*to 0.83.04ns. The channel by channel accumulated count

was read out along with the corresponding channel number

either in analog form onto an X-Y plotter, in decimal form

on adding machine tape, or punched in binary coded hexi-

decimal on paper tape. In practice, the data was always





32

displayed in analog X-Y form for quick analysis. If the

data appeared to be of interest, it was punched on paper

tape and the paper tape later read into the IBM 1800

computer and stored in a file on a magnetic disk. This

data was then available for immediate analysis and future

reanalysis.

3.5 Scanning Mode

The system was also capable of being used as a very

sensitive scanning spectroscopic system (see Fig. 10). To

scan, the system was set to respond to all photons of the

correct wavelength regardless of the time of occurrence

and then to step the spectrograph and a counter simultaneous-

ly so that the number of photons at each wavelength was

counted. In this mode of operation, the time window of

the TAC was set wide enough to include the entire decay

curve of the excited level. Any photon of the correct

wavelength that was associated with a fission fragment

thus gave rise to an output pulse. Using the internal

single channel analyzer (SCA), this output pulse was con-

verted into a NIM standard 5-volt slow logic pulse and

supplied to the input of a multiscaler logic unit in the

MCA. The multiscaler unit addressed each channel of the

MCA sequentially and stepped when it received a trigger

pulse. This trigger pulse was supplied by a recycling

timer which would supply trigger pulses with any chosen

interval. This trigger pulse was also supplied to a

stepping motor connected-to the wavelength drive of the















337.1 N2


357.6 N2


340.4 N2


WAVELENGTH


Scan of Pure N2


Figure .10 .





34

spectrograph and served to step the spectrograph and multi-

scaler simultaneously. Because.of the wide slits on the

spectrograph, it was found to be sufficient to sample the

intensity every 0.2nm corresponding to 46 steps of the

stepping motor for each trigger pulse. The length of time

counted at each wavelength varied with the intensity of the

light. When scanning a gas, such as an argon-nitrogen mixture

which is very bright, 10 seconds per channel proved suffi-

cient while for some poor emitters, several hours at each

channel were necessary.

An interesting feature of this mode of operation was

the ability to make a scan at any point in time. That is,

by setting the TAC to respond only to photons which occurred

between, say, 10 and 18ns after fission, we had a spectro-

graph with an 8ns shutter which opened 10ns after fission

(see Fig. 35). A spectral scan at several times after fission

could provide a complete time history of the intensity at

all wavelengths.













CHAPTER 4

ANALYSIS AND RESULTS


4.1 Introduction

The basic question addressed in this chapter is that

of relating the raw data to the fundamental properties of

the fission fragment excited gas. How the system charac-

teristics affected the raw data was discussed in Chapter 3.

Here we shall assume that the value of N(t) has been cor-

rected for instrumental influence and try to determine how

these values relate to the radiative and collisional decay

coefficients. We shall first discuss how the computer

programs processed the raw data and then proceed to a

detailed discussion of the four output parameters T, N(t),

SUMNi, Nmax, and their interpretations (see Fig. 1). Finally,

some consideration will be given to general problems re-

lating to any measurement carried out with this system.

4.2 Programs

After an experiment had been performed and a count

completed, the data was stored in the memory of the MCA.

This data was immediately punched in hexadecimal form onto

paper tape and then read into an IBM 1800 computer and

placed in a file on a magnetic disk. All the data and the

current working programs were kept on this disk and, to-

gether with the lab book, provided a complete record of the

35






36

experiment. Once the data was on the disk, there were two

programs which could be called upon for analysis: SUMNi

which printed the sum total of counts between any chosen

limits thereby giving a number proportional to the number

of excited states which decayed by radiation in the time

interval specified, and SLOGP.

SLOGP first plotted logNi, the log of the number of

counts, versus the channel number i. Since the decay curve

was expected to be a simple exponential decay, the SLOGP

plot was visually examined to determine if there was a

straight line region and, if so, what channels comprised it.

In addition, this plot allowed a good determination of just

where background occurred. Once the limits of the decay

region had been determined, the program performed a least

squares fit of the experimental data to an equation of the

form Yi = Ae-ti/T+B in the region chosen from the log plot

and specified by the operator. The operator supplied an

initial value of background chosen to be just below the

value obtained from the log plot. An iterative calculation

was then performed for the number of iterations and with

the iteration interval specified by the operator. For each

value of the background, a least squares fit was made and

the value of the error function calculated. After the

iteration was complete, the value of background for which

the error function was a minimum was chosen, and both the

raw data and the chosen fit were plotted along with the

numerical values for A, T, B, and the error function. If








the fit appeared to be flawed, new values of starting chan-

nel, stopping channel and background could be chosen and a

new curve superimposed upon the old for comparison.

4.3 Analysis

Once the data had been analyzed by the programs, there

were four numbers available:

1. T, the time it took for the excited state

population to decrease by a factor of l/e.

2. N(t), proportional to the number of excited

states decaying at time t.

3. SUMNi, proportional to the number of ex-

cited states which decayed between the time

A and the time B.

4. Nmax, proportional to the greatest emission

rate.

The decay time determined by the least squares fit of

the raw data was the sum of the radiative and collisional decay

times. Since the radiative decay time is an atomic con-

stant, while the collisional decay time changes with

pressure, it is possible to determine the two effects

separately. As the pressure decreased, collisions became

less frequent,and the collisional decay rate decreased

until at zero pressure, the decay of the excited states was

entirely determined by the radiative decay.

The decay time T is equal to l/(kl+k2P), 'so that by

plotting 1/T versus pressure, the data should fit a straight

line with an intercept kl and a slope k2. Deviation from a





38

straight line, indicat-ing nonlinear behavior, was ob-

served at the highest pressure studied (800 torr). This

deviation indicated that three-body collisions were be-

coming important and any studies carried out at this

pressure or greater should include a quadratic pressure

term in the rate equation. Figure 5 shows the results for

the C3fu level of pure nitrogen with the determined values

of k1 and k2 shown. This curve represents the analysis of

82 decay time measurements.

There were three sources of difficulty which may cause

errors in the interpretation of the lifetime curves. These

three problems all took the form of additional excitation

terms in the rate equation other than direct excitation,

some of which persisted sufficiently late in time that

there is no true "decay" period.

Resonance trapping will cause the population of a

level connected to the ground state by an optically allowed

transition to be maintained at an artificially high level

compared with the optically thin case. The two most

satisfying solutions to this problem are either to solve

the equation of radiative transfer simultaneously with the

rate equations or to choose a level for which there is no

radiation trapping.

Cascading causes the population of an intermediate

level to continue to be augmented by the. decay of higher

lying levels for some time after the end of the excitation

pulse. This problem may be solved by either solving the







coupled sets of rate equations for all the levels in the

gas or by choosing a level which has no significant cascade

contributions.

Collisional transfer between excited levels in a

decaying gas causes an irreversible flow of excitation

energy toward the lowest excited state within a closely

spaced group of levels. The collisional transfer rate, and

hence,the collisional quenching coefficient, is explicitly

solved for in this dissertation.

It should be noted that one source of uncertainty

usually present in plasma spectroscopic diagnostics was

avoided here. No assumptions about equilibrium or depar-

tures therefrom were necessary. The measurement of the

decay time was independent of the relative populations of

the various levels subject to the three preceding cautions.

In fact, one could, hardly imagine a more nonequilibrium

gas either spatially or temporally than that being present-

ly considered.

The curves of intensity versus time yielded the

temporal variation of the population. It may be noled Llha

this variation was in fact the solution of the rate equations

for the particular level observed. The rise time of the

population was nearly constant for all levels in all gases

at all pressures, and this surprising result is discussed

at more length in Appendix 5.

Plotting Nmax versus pressure (Fig. 11) we find that

the maximum excited state density occurred at about 350

torr but did not exhibit a particularly sharp maximum.






























--i





r 0

E ro


14
o 4


m
sL cIJ
E













* 0)
o !-i


En



V) Q)

LU




-
SI
LU
Ql >1


(:) 0- e1





0 -


4




CD -
C\J -1














co C~-
(AA





41

From Section 3 we see that SUMN/T plotted against pressure

should show identical behavior which, in fact, it does (see

Fig. 12).

As shown in Section 3, the sum of the counts in all

channels can be related to the ratio of the radiative decay

constant and the collisional decay constant. Alternatively,

if kI and k2 are known from the decay curve, SUMN can be

related to the total number of excited states created by

the fission fragment passing through the gas, QE(l-e-Ztf).

Figure 13 shows the variation of the integrated intensity

with pressure. The curve indicates a rather narrow peak

occurring at about 175 torr. Comparing Figures 11 and 13,

we may observe that a pulsed laser might operate best at

a different pressure than would a CW laser. Since, in the

pulsed case, we are interested in the greatest instantane-

ous population, we would choose the peak in Figure 11,

while for the steady state case, we are concerned with the

average population density and would follow Curve 13.

4.4 Pulse Height Distribution

One tremendous advantage to using the decay curve for

determining the lifetime or transition probability of an

excited state was that it didn't depend upon the number of

excited states created, but rather upon how those that were

created changed with time. What was being measured then

was not intensity, but the rate of change of intensity

which was much easier to do. The number of excited states

created by the passage of a fission fragment was propor-







12






(U
4-






0

0D C

4-)

oO












o >
aU











- U)





W1


Si0.0
4- (















C *
CC









0-
*D -
** t
E0















s4-
0 a














-0
ctJ
LIJflO
C'-(





0 M







-- L
*_ M











6-

































\J E
z c


*


I


LNnflS 3AIIV1]3


CD (
CO
co

U)










Lo ---
(n







4J





H
.1







to U


D C>
4J


0w
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V C)
L -1i


I *








44

tional to the energy deposited which was, in turn, dependent

upon the initial or birth energy of the fission fragment.

Fission fragments at birth showed a distribution of energies

divided into two rather well defined groups. .The energy

distribution of the 252Cf source used in the experiment,

measured with a surface barrier detector, is shown in Figure

14. Since some fission fragments have a greater initial

energy than others, they will, on the average, create more

excited states in passing through the gas. Thus, the

intensity we observed after the passage of a fission frag-

ment was dependent to some extent upon the particular

particle we chose to observe. Note, however, the rate at

which the excited states died away did not depend on the

energy of the fission fragment which created them. Looking

at the output of the TLP, the amplitude of the total lumi-

nosity pulse was proportional to the number of-photons

resulting from the passage of the fission fragment. At

constant pressure, this was proportional to the energy of

the incident particle.

The pulse height distribution in the gas is shown in

Figure 15.' This distribution was measured in CF4, a gas

which gives a large number of photons per unit energy

deposited and, hence, provided good scintillator resolution.

























1000









H-








100



















ENERGY
Figure 14. Fission Fragment Energy Spectrum




























U







L.L





U-



LL
CO




























PULSE IIEIGIIT



Figure 15. Fission Fragment Distribution from TLP Pulse
Heights













CHAPTER 5

OTHER'GASES AND GAS MIXTURES


5.1 General Comparison of Mixture Spectra

Figure 16 shows scans of the fission fragment excited

nitrogen impurity spectrum of one atmosphere neon, argon,

and helium alongside that of pure nitrogen. Notice that

the argon and helium were scanned five times faster than

the neon and nitrogen, and hence, their peak heights or

intensities should be increased by a factor of five for

direct comparison. That the nitrogen impurity in argon

and helium was so much brighter than either pure nitrogen

or nitrogen in neon, was a consequence of the efficient.

transfer of energy from the metastable energy traps in

argon and helium in to nitrogen. The dominant emission

from helium was the 391.4nm N2 band, while that from argon

was in the 337.lnm N2 band, both of which results follow

immediately from a consideration of Figure 8. The dominant

emission from neon was also the 391.4nm band, and this has

not been explained. The relative peak heights adjusted for

the different scan speeds of the most intense emission

from each gas goes as neon to argon to helium to nitrogen,

2.1 : 1.3 : 13.7 : 1

5.2 Neon/Nitrogen Mixtures

Besides the measurement-of the decay time of the C3 u

47






















CNJ
2-


0)

-C
U C\J

U
) Z

uo o
(Y)


L- CNJ ^ CN



c
r;- Cn a


.c
r
u

u







CC)







C

S 4J
- I


r- --
.c














CJ 0J CD
I n




u O

O O ---




CC
z coU










U

U
-~H
c i



u l
cr -------- t






ro nU)


u

o





49

state of-nitrogen, several other studies were carried out

to illustrate the versatility of the system for the study

of fission fragment excited gases. An extensive study was

made of neon and neon-nitrogen mixtures. Using this mix-

ture provided the opportunity of viewing, simultaneously,

three lines from three different species, atomic neon,

molecular nitrogen, and the nitrogen molecular ion. Figure

17 is a spectral scan of fission fragment excited neon

showing the nitrogen impurity emission. 'Looking first at

the 585.2nm neon line in pure neon, we find a radiative

lifetime of 16.6+.7nm (seeFig. 18). As the nitrogen impurity

concentration increased from 1% to 10' to 50% (Figs.19, 20,

21), there was, no significantchange in the lifetime of the

neon excited state. This might be expected since there

was a very poor energy match between the levels of neon

and those of nitrogen. The values for k2, the collisional

transfer coefficient, appeared to show a systematic de-

crease with nitrogen concentration. If this trend is real,

it means that the nitrogen is less effective than the neon

in quenching the level that gives rise to the 585.2nm line.

Looking next at the C3]lu level of nitrogen with neon con-

centration values of 1%, 10%, 90%, and 99% (Figs. 22, 23, 24,

25), we see that when the nitrogen was the dominant gas,

the lifetime varied strongly with pressure; whereas,when

the neon was dominant, there wa's very little change. This

again tells us that there was not much interaction between

the levels of the two gases.








50




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51

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0 I
i-
- COJ X- Ld

E 0 L- C


Z M II II II
a. m "I


L4
0




D (U)

v U




Ci
w
o o
va 0
















C 23
C-d
0 0-

















(4s









(su) Thu Av33a
/ aa
*' -
















(su cnI tl


























LA


0 I I


X t-0
X
EO c


c m m -
-.) -

: c' "- -- p


58
o
CO
co

















Co

CC








-0

I 4-

o 3


l a)
S::


V) U)


) m



E
-1





C)
/ O 3
















(1a)





rJJ









(0 Av





59

When we turn to the analysis of the 391.4nm and due

to the N2+molecular ion, which has been reported to have a

radiative lifetime of about 65ns, some very strange results

are observed (see Figs. 26, 27,.28). In pure neon, the first

negative band due to nitrogen impurities within the gas

had a lifetime of 90,0003500ns. As the nitrogen concen-

trations increased to 10% and 50%, the lifetime dropped to

10,000400 and finally, to 3800150ns. Anomalous excitation

of N2 has been seen before in laser produced air plasmas,1

argon plasma jets,12 radio frequency plasma torches,13 and

manganese seeded plasma jets,14 but has received no adequate

explanation.

5.3 Helium/Nitrogen Mixtures

Next, let us consider helium-nitrogen mixtures. For

pure helium, the emission was dominated, just as in the case

of electrical excitation, by the first negative bands due to

the N2 molecular ion (see Fig. 29). As the nitrogen concentra-

tion increased to 1% (Fig. 30), the dominant band became the

second positive system, but the first negative was still

visible. This behavior is to be compared with the case of

electrical excitation in glow discharge where the 391.4

emission dominated to much higher concentrations of N2. The

persistence of the N2+ emission may be due to the electric

field inhibiting recombination and, hence, keeping the

population of N2+ elevated with respect to the fission

fragment excited case. The intensity variation of 391.4nm

with pressure for several mix ratios is shown in Figure 31.







60

.0-
o












4-)o
I Ico








c c ,|
-O rcO C

NO0 0 0 I O
x x o

uE * o O
E 0r

. Uj II II II


l- M -z -z p In


S -- -4

0
O






(A 0)




m






kO
S u







o Q



















o o
L4
o o



















(u 3 C

(su) 31Ii AV33G






















5-
5--
0
-4-)







+ C) C
0 + 0 0
I- C'.I ,- 0

r- CO
E X 0
C r-- O --

Z II II II












S0 0 C L0










S









































(s ) I I I I AV3I I
o0 O O o
Lo LD in L


.s-

5--)
4A-



C/,
O r
O






LLJ
:Dw





CD
CD













O4








O













O
0







+ OO C
4-,








C\i

O LCD U
E c

I ~ 0
C;Z N M

II II II


0







*- n
4-4





U)



















0
0)
0- 4
I O)






f14















s ] I,
o u




Saa)


























(su) 3]II AV33G







63





























0 :

s4
CD

1--1



+ L- 04


w4
0J S-
N.J -0







U)
O Zj

S^---------- -I C

Ocr




J I (NJ







(C\ 1




r2-








64




















o +


co



o c,






4J
coX
+ H H














O
-1 0I





co
c1




















Ai -SN3-1-- 33 i d






































































































AIISN31NI 3AIlV13d


3-
T







Q)


LUl









Us-U
wU)

4
4- ,l



4
LU >
nL
S>1





4-)






4-J


J .
*rJ


(Da






[LI


LO

























+
CDm















E
c:
240








+1 T
3= C





66

At 10% nitrogen, all observable emission was due to the

neutral molecule (see Fig. 32).

Turning to the decay curve with 1/2% N2 in helium,

we see that the difference between 800 torr and 100 torr

was dramatic (see Fig. 33). There was a lot more energy com-

ing out of the gas as light, and it came out for a lot

longer time even though the energy deposited by the

fission fragment was less for a gas that was mostly helium

than for the heavier nitrogen. Figure 34 shows the change

with pressure of the decay time for various mix ratios.

As the amount of N2 decreased, the lifetimes of the N2

level was increasingly determined by the helium metastable

lifetime. As the amount of nitrogen increased,.recombina-

tion took place much more rapidly, causing an additional

depletion of the level.

Figure 35 shows .a scan where the system had been

adjusted to respond only to photons which were emitted

during the first four nanoseconds after fission, while

the fission fragment was still within the field of view,

and excitation was still taking place. There appears to

be no difference between this curve and Figure 29, which

is from photons emitted at any time after fission.

5.4 Argon/Nitrogen

Because of the intense emission observed in the past

from fission fragment excited mixtures of argon and nitro-

gen, 2 15 and also, because of the interest in using argon

as a cover gas -in the gaseous core reactor, preliminary







67





























C0 N



S C\J

o ,-
0







,- c.- i


S- *
CD C
c U



U)
























(@LPs Je u.L) 1IISN313AI 3AIliV 18





















Lf

Ln +
OT C\
O 2:J


1LO
-r-I


0) a'


33











C w
U,--
r!






u)
I-i




4)
m
fO



ai-


(DLPJS jAOULL) SINnOD


Ln







69




















0 0
CD












LC
oC 0
CL



a)




Li U





Ln


-- /1J


a Q)
(13 e- a
/ / i^ u
/ / iyi
/ / uj u
/ / Q; ^-
/ 0- (
/ >

/. -S
I '
/ ^,
/ >1
/ ^


Ln
o O Co


I I I

*4.




























2 CC





.- 0


Srci
0

-- 04-








+ )


cc












o4







UP
LL3
d LF --.





71

measurements were carried out on a 10 to 1 mixture of argon

to nitrogen, the mixture that had been found previously to

give the most light. Figures 36 and 37 show a spectral

scan of the second positive band of N2 which dominated the

visible emission. There was very little emission in the

first negative band.

Looking at the decay curves of the C3Hu level, we

find a very instructive result. The decay curves show two

components, a fast and a slow one (seeFig.38). Quantitatively,

the fast decay approaches 4 5 n s, the radiative lifetime

of the C3Hu level, while the slow decay shows a zero pres-

sure value o f 600 ns. What we are observing is the fast

initial decay of the directly excited nitrogen level and

the slower metastable transfer from the argon 4'P level

which is in resonance with it. As the pressure drops, the

collision frequency decreases, and metastable transfer is

no longer so important vis a vis direct excitation.

5.5 Carbon Tetraflouride

One of the peripheral interests in the optical emis-

sion of fission fragment excited gases had been the

possibility of using a gaseous scintillator as an in-core

monitor of the power level in the LMFBR. Early studies by

Pagano16 indicated that the fluorocarbon CF4 had the dual

advantage of being inert and having high luminosity. Pagano

was not able to determine whether CF4 or some dissociation

product was causing the intense emission which'he observed.

Because of the difficulty of obtaining CF4 (or Freon-14),








72






















c I
00



0 -


(- -4

s---


LA. 1 --
t-0 C a C


-.4
z7


Co I-















jr u]
-*
( S _U t (L> aA


H












rnf








(O I SNJIW13I I IV---
*--i _^ -1l






ro ---- i-_________
n -L -- -- ,______________
F3 vr>
-- Sr

S (







73





















o


0 L
o




c 0





co
o\o






C C
O w

ci 0













-'t-.---_.._.---
Cl
N-
C-

















(ae~2s x2aui[) AIISNJINJI JAII1JU3












A/N 2 10/1
337.1nm N2


800 torr


0 100 300 0


400 torr


200 torr












IX2


300 0 100 300
TItlE .(ns)
Fast and Slow Decay in


100 torr


0 100


300


A/N2


Figure 38.







only a few measurements were carried out. Spectral scans

showed a broad continuum in the ultraviolet which had been

previously observed by Walters.' A decay curve was made

of the C3Hu level of the nitrogen impurity at 586 torr,

and the decay was a simple exponential with a 1/e time of

2'6.2 1.0ns (see Fig. 39). This emission was much too weak to

account for the intense total luminosity emission.

CF4 was the only gas in which the present system was

able to resolve the couble humped fission fragment distri-

bution (see Fig. 15). This is thought to be due to the large

amount of light given out for each fission fragment allow-

ing much better energy resolution.

Just why CF4 should have such a high specific scintil-

lator efficiency is not clear, nor is it known what is

responsible for the emission in the ultraviolet.

We expect the excited states of the molecule to be

rather high due to the tight bonding of fluorine. The

higher the initial excited state is, the greater the proba-

bility usually is of non-radiative deexcitation.6 Dis-

sociation following excitation has been 'suggested as the

reason for the almost total lack of luminescence in other

inorganic polyatomic molecules.17 The same reason has been

given for the absence of luminescence in saturated hydro-

carbons.18 This reason may account for the observed lack

of luminosity on the part of UF6.19 The emission from CF4

remains a mystery.

In all these measurements it was necessary to remember





















S- O
0


co
co C0n
Ln E

00
O n
0 ID
00 00

u 0 C\
-) co) 0









Lno >1




LU Q

~ (N





0 ,




000000 0 0


Lyn

.,0000
0 0 00 0^





n 0 0









SINn03





77

that the number of excited states and, thus, the intensity

caused by the passage of a fission fragment would depend

upon which fission fragment was observed. The initial

energy of fission fragments varies and so does the energy

deposition. If we set the discriminator level so as to

trigger on only the most energetic half of the fission

fragment distribution, we found as expected, that the in-

tensity per fission fragment increased. Thus, all the

measurements described in this dissertation are for the

mean of all particles, and the discriminator had been

carefully set to include all the fission .fragments but to

exclude all the alpha particle's. It should be noted that

in a reactor experiment, the observed emission comes from

both the fission fragments and the alphas, as there is no

way to distinguish excited states caused by the one from

the other. Some differences may, therefore, be expected

in experiments done in the two environments.

5.6 Population Inversion Study

To determine if a population inversion exists between

two levels, it is sufficient to monitor the relative

spontaneous emission coming from each level when, by cor-

recting for the relative transition probabilities, we can

determine the population ratio. Observation of the steady

state excitation does not always reveal the presence of

pulsed inversions since a long uninverted tail may mask an

inversion which occurred early in time.

The most difficult problem associated with this type

of measurement was finding an allowed transition whose







upper and lower levels were both within the purview of

the system. Usually, if one of the transitions was in the

visible, the other was in either the ultraviolet or the

infrared. The laser candidate which we studied was the

second positive band of nitrogen (see Fig. 8). Pulsed laser

emission had been observed previously,20 and so this band

provided a good choice for testing the method. In addition,

we had available a computer solution of the rate equations

of nitrogen (Fig. 40) for electron beam excitation.21 These

solutions were of the form I versus t, exactly the output

of the system. The upper and lower levels of this band

system are the C3Hu levels which could be monitored through

the second positive emission itself, and the B3Hu level

which gives rise to the first positive group (Fig. 41). The

measured populations and the way they changed with time had

the same form as that predicted by the solution of the rate

equations and showed the same decay rate when allowance had

been made for the difference in pressure between the com-

puter solution (3 atmospheres) and the experiment (1

atmosphere).







79








o
o
c (U













,- U




0 0)
c o c












S












W a)
C )
























-cc
0

4O




0-I
0 0
(C D
in l
"---





/ o (



























a C 0 D
/ 0:
/ "]-1


C\J .-
~ .-
0

Ln
+0
+ 0




















Ar/N2 (2.5%)

800 torr



1st Positive
3 T = 300nsec
B
9



V)

C.)
o
LU-


















I I I I I


100 200 300 400 500
TIME (ns)

Figure 41. Measured 1st and 2nd Positive Populations














CHAPTER 6

CONCLUSIONS AND FUTURE WORK


6.1 Conclusions

The conclusions arrived at in this research fall

naturally into three categories: conclusions about the

system, conclusions about pure nitrogen, and conclusions

about gas mixtures and fission fragment excited gases in

general.

The detection system was similar to existing fluor-

esence decay systems except that no trigger pulse was

necessary, and so sources that were non-periodic, transient,

and randomly occurring could be studied. The system was

extremely sensitive, capable of counting single photons,

and was relatively insensitive to gamma background. The

source, unlike a discharge, did not cause extensive thermal

decomposition or. dissociation of the test gas, nor was it

necessary, as with'a flash lamp, that the level of interest

be connected to the ground state by an allowed transition.

The system could measure times as short as 1.7ns and, per-

haps, shorter. It had repeatability of better than 4% and

a minimum resolving time of about Ins. The minimum re-

solving time was limited by the photomultipliers and could

be improved as photomultiplier transit spread is improved.

The results obtained for the C3Hu level of pure

81







nitrogen were the same within experimental error as those

obtained by others in the past (see Table 1). The gas had

a single decay time and showed no deviation from two-body

quenching until 800 torr. There appeared to be no other

source of excitation to this level other than by the

secondary electrons. The energy deposition by the fission

fragment was found to exhibit an almost perfect square law

dependence and is a maximum when tf = T/Z2

Comparing Figures 11 and 13, we may observe that a

pulsed laser might operate best at a different pressure

than would a CW laser. Since, in the pulse case, we are

interested in the greatest instantaneous population, we

would choose the peak in Figure 11, while for the steady

state case, we are concerned with the average population

density and would follow curve 13.

Looking at other gases and gas mixtures, we found that

CF4 gave the greatest light output with a 10/1 argon/

nitrogen mixture second. For the mixture neon/nitrogen,

where there are no resonant interactions, whatever level

was being observed showed only small changes from the pure

gas case except that the decay time was determined by the

total pressure and not the partial pressure of the observed

gas. The lifetime of the N2+ molecular ion showed extreme-

ly large values when nitrogen was a small impurity (less

than 1%). No explanation for this behavior was found. In

argon/nitrogen and helium/nitrogen, the lifetimes were.

dominated by resonant collisions with metastables. In argon,














TABLE 1


Previously Published Values for C3lu

Reference No. 5,





84

the C3Hu showed some enhancement while in helium, the B2 u+

was tremendously increased.

6.2 Future Work

Much work remains to be done using the isotope excited

fluoresence decay system to further characterize fission

fragment excited plasmas. The inclusion of an electric field

within the experimental chamber would allow the separation

of effects due to ions from those due to neutrals. In

addition, the electric field could be used to increase the

light output, thus, reducing the demands on the spectroscopic

apparatus. Cooling the chamber would allow the study of

gases which undergo thermal decomposition at room temperature.

In order to separate the effects of the fission fragments

from those of the secondary electrons, a beta source (such as

12C) might be included in the chamber. The beta energy dis-

tribution spectrum of 12C closely resembles that of the

secondary electrons created in fission fragment bombardment.

A shutter over the fission fragment source would allow ob-

servations to be made of the gas with electron excitation

alone or with fission fragment plus electron excitation.

Another use of this system is for studying extremely

electronegative gases. These gases, of which UF6 is one of

particular interest, are difficult to excite using electrical

charges, and one must often resort to "seeding" an inert gas

with small fractions of such molecules.

Still another use of this system is in the serach for

possible fission fragment excited laser inversions. Since





85

the system provides time resolution, it is possible to

identify inversions which are quenched at long time and are,

thus, candidates for pulsed operation.

This system would also work well for studying

liquids.

6.3 Scanning Mode Recommendations

A major improvement in the-scanning technique could be

experienced through the use of a minicomputer or micropro-

cessor. The scanning rate could be controlled in such-a

way that when there was no signal (as determined by compar-

ing the count rate against background), the system would be

stepped quickly. When a spectral feature is reached, the

system would step slowly enough to achieve any required level

of statistical accuracy.

Another interesting system could be developed to com-

bine the features of the scanning system and the fluorescence

decay system. In this mode, the system would accumulate a

decay curve, read it out, step to a new wavelength, and

repeat the process. This mode would be of use where there

is emission continuum such as the xenon excimer band where

the lifetime varies across the band. A scan of this nature

would allow one to choose the region best suited to lasing.













APPENDIX 1

ENERGY DEPOSITION


The whole question of how many excited states are

produced by a fission fragment revolves about the exact

form of the energy deposition equation.

The Bohr energy loss equation describes the slowing

down of a fission fragment due to electron collisions as

dE 47TN(Zeff)2e4ZN
dr MeV2 e,

where e = electronic charge,

Zeff = effective charge of. fission fragment with
velocity V traversing a medium of atomic
number ZN,

Le = logarithmic summation term, and

V = velocity.

A semi-empirical expression for Zeff, also due to Bohr, is
1/3
Zeff = Zff (V/V0),

where Zff = the atomic number of the fission fragment,
and

V0 = the velocity of the electron in the first
Bohr orbit.
dE / 2
For a gas, d -kV. Integration gives E = E0(l-r/R)
dr
This expression has been described as giving poor agreement

with experimental data.9 From energy loss measurements in

gases, the exponent has been assigned a value from 1.5 to

1.7. Axtmann0l has derived an expression for the intensity

86





87

of the fission fragment excited gas in terms of the exponent in

the energy deposition equation. Let

Ed = E0 E = E(l-[l-r/R]n) (1)

be the energy dissipated in the gas.

A binomial expression of Equation 1 gives

Ed = E0nB[l-B(n-l)P/2]P, (2)

where 8 = r/ROP0, and 0 refers to one atmosphere. Now the

measured intensity equals

kl
I = (cQEd) F[ k+k2p], (3)

where Q is the number of excited states formed per unit

energy loss in the gas; E is the collection efficiency; F
k1
is the number of fission fragments per second; is
kl+k2P
the probability of light emission by an excited molecule;

P is the pressure in torr; ki and k2 are the radiative and

collisional decay constants with units of sec-1 and mm-1

sec-1 respectively.

Substituting Equation 2 into Equation 3, differentiating
dI
and setting equal to zero gives

k2 n-1
n(l+ -P )(l-8P,)
n 1


= 0,


k2/kl

where n is obtained implicitly in terms of 6, k2/kI and P,

the pressure of maximum luminosity. For'this experiment

S4.11 = 2.335x10-3, and Pm = 300 torr. Thus,
2.2x800 m


1-(1-2.34x10-3x300)- [nx2.34x103 (1+.018x300)(1-2.34x10-3x300)n-1
-(2.018-
.018


1-(m1-P ) "





88

A fit of our experimental data gives n = 1.98 within 1%,

sufficiently close to two to justify using the expression,

E = E0(1-r/R) 2














APPENDIX 2

NE-111 AS A CALIBRATION SOURCE


After the delayed coincidence single photon counting

system had been assembled and tested, a search was made

for a fast, low level light source to calibrate the timing

measurement. The characteristics required of the source

were:

1) The pulse should have as small a half-width

as possible and resemble in shape the

fission fragment pulse.

2) The pulse amplitude should be as near that

of the fission fragment pulse as possible,
6
on the order of 10 photons.

3) The source should be small so that it would

fit in the experimental chamber. The opti-

mum solution would have the source small

enough that it could be inserted in the

chamber through one of the gas access ports

so that the calibration could be done with-

out disturbing the system.

4) The source should have near 4r geometry so

that it could be observed by both photomul-

tipliers simultaneously.

Normally, single photon counting systems are calibrated

89





90

using a tungsten ribbon lamp and filters. This was

clearly not possible in our case since we needed a pulsed

source. Choppers were briefly considered, but the rotation

speeds necessary to achieve chopping speeds of Ins were

considered excessive.

Fluorescence decay systems are normally calibrated

using a quenched spark source. This source was too large,

too bright, and too expensive to fit easily into this

system.

The solution was a small chip of NE-111, a fast

plastic scintillator mounted on the end of a short length

of copper tubing brazed to an 1/8-inch pipe plug which

could be screwed into the gas system access port. The NE-111

was secured to the end of the copper tubing by heating the

tubing and pressing the plastic into the end. The length

of tubing was so chosen that when the plug was screwed all

the way in, the source was in the center of the field of

view.

The shape of the light pulse which NE-111 emits when

struck by a gamma ray is well known, having a full width

at half maximum of 1.54ns and a decay time of 1.7ns. The

output peaks at about 370nm, a wavelength well suited to

most spectroscopic systems.

In use, the calibration-source was placed in the cen-

ter of the experimental chamber and viewed by the detection

system. Since both photomultipliers were seeing the same

light pulse, the measured time interval, corresponding to




91

no time difference, was subtracted from all decay measure-

ments. This zero time interval was caused by differences

in cable lengths, light paths, and photomultiplier transit

times.




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