• TABLE OF CONTENTS
HIDE
 Title Page
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 Introduction
 Experimental description
 Theory
 Development of the kinetic...
 Experimental results
 Computer results
 Conclusions
 Computer programs - KINEQ
 Reference
 Biographical sketch
 Copyright














Title: Kinetic and experimental study of argon and argon-nitrogen mixtures excited by fission fragments
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 Material Information
Title: Kinetic and experimental study of argon and argon-nitrogen mixtures excited by fission fragments
Series Title: Kinetic and experimental study of argon and argon-nitrogen mixtures excited by fission fragments
Physical Description: Book
Creator: Davis, John Franklin,
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Bibliographic ID: UF00090221
Volume ID: VID00001
Source Institution: University of Florida
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Resource Identifier: alephbibnum - 000178698
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Table of Contents
    Title Page
        Page i
    Acknowledgement
        Page ii
        Page iii
    Table of Contents
        Page iv
        Page v
    List of Tables
        Page vi
    List of Figures
        Page vii
        Page viii
        Page ix
    Abstract
        Page x
        Page xi
        Page xii
    Introduction
        Page 1
        Page 2
        Page 3
        Page 4
    Experimental description
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
    Theory
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
    Development of the kinetic model
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
    Experimental results
        Page 71
        Page 72
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
        Page 91
        Page 92
        Page 93
        Page 94
        Page 95
    Computer results
        Page 96
        Page 97
        Page 98
        Page 99
        Page 100
        Page 101
        Page 102
        Page 103
        Page 104
        Page 105
        Page 106
        Page 107
        Page 108
        Page 109
    Conclusions
        Page 110
        Page 111
    Computer programs - KINEQ
        Page 112
        Page 113
        Page 114
        Page 115
        Page 116
        Page 117
        Page 118
        Page 119
        Page 120
        Page 121
        Page 122
        Page 123
        Page 124
        Page 125
        Page 126
        Page 127
    Reference
        Page 128
        Page 129
        Page 130
        Page 131
        Page 132
        Page 133
    Biographical sketch
        Page 134
        Page 135
        Page 136
    Copyright
        Copyright
Full Text










KINETIC AND EXPERIMENTAL STUDY OF
ARGON AND ARGON-NITROGEN MIXTURES
EXCITED BY. FISSION FRAGMENTS






By


JOHN FRANKLIN


DAVIS III


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




UNIVERSITY OF FLORIDA


1976














ACKNOWLEDGMENTS


I would like to begin by expressing my deepest and most sincere

thanks to the Chairman of my Supervisory Committee, Dr. Richard T.

Schneider, forwhom I will always hold the highest regard. The interest

and help from the other members of the committee, Dr. Hugh D. Campbell,

Dr. Chester Kylstra, Dr. Edward E. Carroll and Dr. Gray Ward are also

greatly appreciated.

The technical problems associated with building the experimental

apparatus and performing the experimental work required the support of

nearly the entire Nuclear Engineering Sciences Department. To Dr. M.

J. Ohanian, Department Chairman, and to the entire faculty, I owe my

deepest thanks for their continual support. With respect to the actual

experimental work, most of the precision matching was done by Ralph

Jones. Ernie Whitman did most of the heli-arc welding and gas filling

with the assistance of Dudley Carter. Joe Mueller was always helpful

with the big jobs such as building the optical table, beam trap and

various shielding. Ken Fawcett was always helpful in solving all the

quirks of the data system electronics and in cleaning up my mistakes.

From these men I received valuable information and ideas that I could

never have received in the classroom. The reactor staff, under the

direction of Dr. Nils J. Diaz and Les Constable, along with Henry Gogun

and George Fogle, were always helpful with their ideas and support and

gave me every consideration. Willy Nelson, with his friendly smile and

timely procurement of supplies, was appreciated. Don Price, Harvey









Norton, Gordon Renshaw, and Richard McGinley of Radiation Control were

extremely helpful and always willing to do things immediately. I am

also very grateful to the girls in the office, Joan Boley, Barbara

Davis, Lois Carroll and Mickey Kaselnak for always keeping me and every-

one else straightened out. I am also indebted in Lois Carroll for her

patience in typing this manuscript.

This experimental work may have never been performed if not for

one man, Dr. R. N. Davie. His hustle and dedication was an inspiration

and it was my greatest pleasure to work with him. B. G. Schnitzler and

G. R. Shipman were always helpful with their ideas and discussion. The

computer efforts were benefited by the help of David Sterritt and Eric

Holtzclaw.

Finally, I express my deepest gratitude, appreciation, and love
to my wife for her patience, drafting of the figures and for putting up

with my stupidity, missing of deadlines, redrawing of figures, not

going to the beach, late hours, my mad ravings in the night and my lazi-

ness. Her continual encouragement and support was essential to the

accomplishment of.my educational goals.














TABLE OF CONTENTS
Page

ACKNOWLEDGMENTS ii

LIST OF TABLES vi

LIST OF FIGURES vii

ABSTRACT x

CHAPTER

I. INTRODUCTION 1

Motivation for Study 1
Interest in FF Produced Plasma 2

II. EXPERIMENTAL DESCRIPTION 5

Design Considerations 5
Overall System 6
Optical System 10
Instrumentation System 13
Reactor 15
Data Procedures 15
System Calibration 19

III. THEORY 22

Fission Fragment Interaction 22
Energy Deposition 25
Simplified Energy Deposition Calculation 25
Fission Fragment Energy Deposition in a
Long Gas Cylinder Wrapped with a Thick
Fissile 30
Argon 33
General Considerations 33
Recombination 40
Rare-Gas Molecules 48
Nitrogen 49










Page
IV. DEVELOPMENT OF THE KINETIC MODEL 55

General Considerations 55
Collision Phenomena 56
Computer Technique 68

V. EXPERIMENTAL RESULTS 71

Argon 71
Nitrogen 72
Argon-Nitrogen 73
Temperature Effects 80
Spatial Effects 84
Measurements of Rate Coefficients 88

VI. COMPUTER RESULTS 96

VII. CONCLUSIONS 110

APPENDIX: Computer Program KINEQ 112

LIST OF REFERENCES 128

BIOGRAPHICAL SKETCH 134














LIST OF TABLES

Table Page

1, Average Properties of U235 Fission Fragments 23

2. Energy Deposition Attenuation Factor as a Function of 33
P r /Rg

3. Properties of the Six Groups of Argon 39

4. Measured Recombination Coefficients in Argon 42

5. Properties of Nitrogen Excited States 53

6. Reactions in Argon-Nitrogen Mixtures 57

7. Observed Spectra of 760 torr Nitrogen 74

8. Relative Intensity of Second Positive Group of N2 79

9. Relative Populations of Second Positive Group of N2 79














LIST OF FIGURES


Figure


1. Overall Layout for MCFIG Gas Irradiation Studies

2. Multipurpose Capsule for the Irradiation of Gases
(MCFIG)

3. Test Tube Assembly

4. Optical Setup

5. Spatial Sampling Scheme

6, Instrumentation System

7. Thermal and Epithermal Neutron Flux Distribution in
the UFTR Horizontal Through Port

8. MCFIG Temperature as a Function of Time After Reach-
ing Full Power

9, Relative Spectral Sensitivity of Momochromator and
Photomultiplier Compared to that for the Entire
Instrumentation System

10. Fraction of Energy Deposited in Volume for FF of
Initial Energy E0 from Equation (9)

11. Fraction of Energy Deposited in Volume for FF of
Initial Energy E from Equation (10)

12. Energy Deposition at the Center Line for Various Radius
Tubes from Equation (14)

13. Chart of the Lowest Excited States of Argon

14. Preferential Electron Excitation of the Noble Gases
Kr, Ar and Xe

15. Dissociative Recombinations Process in Argon

16. Potential Energy Curvers for Some Electronic
States of N2 and N2+


Page








Page
17. Potential Curves for Ar+, Ar and N2 64

18. Energy Flow Diagram for the Ar-N2 System 65

19. Measured Populations of Argon 2p Level and
Nitrogen N2C State 75

20. Measured Populations of Argon 2p Levels 77

21. Measured Populations of Argon 2p Levels 78

22. Relative C31 Vibrational State Populations for 100%
N2 and 10% N2 in Argon at 760 torr 81

23. Vibrational State Populations of N2C versus Nitrogen
Concentration (ppm) 82

24. Temperature Dependence of Spectral Line Intensities
for 103 ppm N2 in Argon at 760 torr 83
3
25. Spatial Variation of Relative Intensities for 10 ppm
N2 85
26. Spatial Variation of Relative Intensities for 100%
N2 and 103 ppm N2 in Argon 87

27. Measured Transition Rate of Argon 2p2 Level versus
Pressure 91

28. Comparison of Argon 2p Emissions versus Nitrogen
Concentration (from ThTess) and Model Predictions
from Equation (37) 92

29. Relative Intensity of the Ar(2) Group Emissions Versus
Nitrogen Concentration 95

30. Populations of Excited Argon and N Species versus
N2 Concentration for 760 torr Totaf Pressure 97

31. Populations of Excited Argon and N Species versus
N2 Concentration for 760 torr Totaf Pressure 98

32. Populations of Argon Excited Species versus N2
Concentration for 760 torr Total Pressure 99

33. Comparison of Measured and Calculated Relative
Intensity of Argon 2p Levels versus Pressure.
Energy Deposition of the Form in Figure 11 with
the Peak at 4 x 1016 eV/cm3-sec at 300 torr. 101

34. Populations for 500 ppm N versus Pressure for
Energy Deposition ?9 the Frm in Figure 11 with
the Peak at 4 x 10 eV/cm -sec at 300 torr. 102


viii







Page

35. Populations of Excited Species for 2% N2 versus
Pressure, Energy Deposition of the Form Shown in
Figure 12 for a Radius of 0.6 cm, 104

36, Calculated Energy Deposition Rate and Population
Inversion versus Pressure for 2% N2. Energy
Deposition of the Form Shown in Figure 12 for a
Tube Readin of 0,6 cm agd Fission Density of
3.375 x 10 Fissions/cm -sec. 105

37, Population of Argon Excited Species versus Pressure
for 2 ppm N2. Energy Deposition of the Form in
Figure 12 for a Tube Radius of 1,0 cm. 107

38. Population Inversion Between Ar(3)-Ar(2) Groups
versus Pressure for a Constant Energy Deposition
Rate per Particle, 108













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



KINETIC AND EXPERIMENTAL STUDY OF ARGON AND ARGON-
NITROGEN MIXTURES EXCITED BY FISSION FRAGMENTS

By

JOHN FRANKLIN DAVIS III

AUGUST, 1976

Chairman: Dr. Richard T. Schneider
Major Department: Nuclear Engineering Sciences

Optical emission from Argon and Argon-Nitrogen mixtures excited

by fission fragments are studied in an effort to better understand the

fission fragment energy deposition into the gas. A model of the energy

flow in the gas is developed and compared with the experimental results.

The experimental set up consisted of a cylindrical gas capsule

containing a planar foil coated with 93% enriched U02 mounted along the

inside wall of the capsule. This device is inserted into the center

core region of the University of Florida Training Reactor and subjected

to a neutron flux of approximately 4 x 101 n/cm2-sec. The resultant

energy deposition into the gas was ~4 x 1016 eV/cm3-sec at 3.9 mm from

the source. Measurements of the absolute light intensities of the radi-

ation emitted in the wavelength region between 3000 to 8500d are made.

The populations of excited levels of fission fragment excited Arl, ArIl,
+
N2 and N2 are measured as a function of N2 concentration in one atmo-

sphere of Argon.

The N2 second positive group was a prominent emission in all








spectra with Ar-N2 mixtures and largest for an Ar/N2 ratio of 10.

Atomic transitions from Argon 2p to Is levels (Paschen notation) are all

strongly quenched by the addition of N2 while the ArII lines remained un-

affected. Also prominent in the Argon emission is a continuum at 2250R

which was equally quenched by the addition of N2. Approximately 3% of

the energy deposited is emitted in the 2250R continuum of molecular argon

and less than 0.3% is emitted in atomic transitions.

To evaluate the data, a sophisticated rate equation model is

developed. The excitation of the primary gas, Ar, by the incident frag-

ments and secondary electrons is modeled. Considered are the argon ion,

Ar and six groups of excited levels of the atom where each group of

excited levels is treated as a single excited level and assigned a transi-

tion probability that is an average over the gA values of the levels of

that group.

To account for recombination effects, the molecular ion is divided

into two groups, Ar2' and Ar2". The Ar2' comprises the lowest excited

levels of the molecular ion which dissociate upon recombination into the
lower excited levels of the atom. The Ar" comprises all higher states

of the molecular ion and dissociates upon recombination into the higher

excited states of the atom and also has collisional losses to form Ar '

upon collisions with argon or nitrogen.

The addition of nitrogen provides a path for collisional transfer
of excitation from the argon to the nitrogen. The nitrogen also changes

the electron energy distribution, and collisionally relaxes the vibra-

tional levels of the molecular ion. Both these effects change. the ex-

cited species produced upon dissociative recombination.








The populations of the species present in the Ar-N2 system are

found by solving a system of nonlinear simultaneous questions. Con-

sidered in the model are 89 reactions for 20 species where each species

equation is formed using all the source and sink reactions for that

species.

Computer solutions of the excited state populations densities

are obtained for a number of cases upon varying the nitrogen concentra-

tion, pressure, and energy deposition form. The model solutions are

within a factor of ten of the measured populations (within experimental

error) and adequately predict the relative change in magnitude of the

measured populations versus nitrogen concentration. For 2% concentra-

tion of N2 in argon, the model indicates that 4.86% of the deposited

energy is transferred to the N2(C3 n) state. Approximately 86% of the

excitation produced ends in the formation of a nitrogen molecule in the

N2(A3 u) state, which, because of its long lifetime, serves as an effi-

cient energy storage medium.














CHAPTER I

INTRODUCTION


Motivation for Study


The excitation of gases by energetic charged particles has been

of interest to radiation physicists for a long time. When a charged

particle passes through a gas, a portion of the incident energy is

used in the formation of ion pairs and a further portion is converted

directly into excited states of the gas. The spectroscopic analysis of

the radiation emitted from gases excited by ionizing radiation has

proven to be a valuable tool in understanding the processes involved.

Despite efforts of researchers for many years, the interaction of charged

particles with gases is still not fully understood.

The emitted radiation from the gas is not only a result of the

charged particle interaction, but also dependent upon the interaction

processes in the gas itself. The problem may be divided into three im-

portant parts: ,(1) energy loss by the incident particle through excita-

tion of the stopping gas; (2) effects taking place within the pure gas

after the excitation has been produced; and (3) subsequent effects re-

sulting from the presence of impurities. A better understanding of these

processes is important in the development of nuclear-pumped laser experi-

ments.
1,2,3
Previous investigators'3 have found significant emissions from

impurity atoms present in their gases upon excitation by charged particles.









In many cases, the emission from the impurity atoms was the dominant

emitting species. The energy deposited in the excited states of the

major gas studied was collisionally transferred to the impurity gases.

The present research is devoted to understanding the excitation

and energy flow in the rare gases excited by fission fragments. In addi-

tion, this research is seeking to utilize the energy available in the

excited states of the rare gases to efficiently transfer this excitation

to another radiating system, be it atom or molecule,and to ultimately

achieve laser action in that system.

For this approach to work, the transfer rate from the excited

levels of the rare gas to the secondary gas must be large. Also, only

one or two excited states of the secondary gas should be populated. The

excited states so produced should be somewhat metastable against radia-

tive decay and strongly resistant to quenching by collision and, of

course, the lower levels should be depopulated rapidly by either radiation

or collision processes,

The scope of the present research is restricted to the study of

the argon-nitrogen system. A kinetic model is developed in an effort

to describe the energy flow in the rare gas argon and effects of the

addition of various quantities of nitrogen. It is hoped by this research

that this model and technique of analysis will be applicable to other

rare gas-secondary gas systems with the ultimate result of the production

of an efficient direct nuclear pumped laser system.


Interest in FF Produced Plasma


Fission fragment interaction with a gas or gas mixture is of a com-

plex nature and presents many experimental difficulties. Although there









exists good deal of data on fission fragment stopping powers and ran-

ges in gases, the excitation of gases by fragments has not been studied

much until relatively recently. Motivation for research on fission frag-

ment produced plasmas is presently directed towards several basic goals.

Of primary interest is the search for efficient means for transferring

energy from isotopes or reactor sources into laser systems. The concept

and review of past work related to this area has been presented quite

adequately elsewhere and the reader is referred to these articles for a

more complete review.4,5,6,78

The feasibility of direct nuclear pumping by fission fragments

has been demonstrated conclusively fairly recently in He-Xe, CO, and

Ne-N2.9,10,11 Unfortunately these results are in the infrared and only

by using high flux pulsed reactors. Two other direct nuclear pumped

lasers have been reported in Xe and HF using gamma rays from an under-
12,13
ground nuclear explosion.113 These two experiments have proven that

gamma-ray pumping is feasible, although not very practical.

It is clear that the next immediate challenge is to build a laser

system which will operate at lower flux levels and in a continuous mode.

As mentioned previously, in addition to the above two goals, it is de-

sirable to produce laser action in the visible or near ultra-violet.

A second motivation for research of fission fragment produced

plasmas is in the development of a plasma core reactor.6 Inherent in a

plasma core reactor is high power densities. The removal of a part of

this energy by radiation would be of interest for a variety of applica-

tions. It is clear than an understanding of the radiative processes

within such a plasma would be essential to the system design and imple-

mentation.









Finally, the relatively high energies of fission fragments and

their availability in nuclear reactors makes the study of fission

fragment produced plasmas of interest in the field of applied radiation

chemistry, for the promotion of chemical reactions. This concept has

been of interest for several years and was reviewed recently by

Stannett.14

Clearly, studies of the excitation and kinetic processes involv-

ing the interaction of charged particles with gas systems must rely on

information from a large number of different .experimental studies. No

attempt is made to summarize the experimental studies relevant to this

research. However, of particular relevance is the study of fission

fragment, alpha, and proton exictation of gases which has been adequately

reviewed by Walters, Theiss and Davie.1'15'16

The analysis of the kinetic processes involved is difficult,

due to the complexity of the problem. This complexity has plagued other

researchers as one notes the discrepancies between experiments and

experimentally determined values (found in the literature) for various

rate processes. The available information has been assimilated and used

to develop a model that is consistent. A description of the processes

involved and their corresponding rates is taken from electron beam ex-

periments, flowing afterglow studies, photoionization studies, theoreti-

cal considerations, mass spectrometer studies, analysis of available

absorption and emission spectra and electron impact cross section

studies. This information is more thoroughly discussed in Chapters III

and IV.














CHAPTER II

EXPERIMENTAL DESCRIPTION


Design Considerations

The experiments consisted of spectroscopic studies of the

emission spectra in the visible and near ultraviolet of ultra-pure argon

and argon-nitrogen mixtures under irradiation by fission fragments. Pre-

vious research of this nature associated with nuclear reactors has been

plagued by the high noise level produced by gammas on the photomulti-

plier detector and the presence of impurities in the gases studied. The

primary objectives in these experiments was to improve the signal to

noise ratio over previous research with the added requirement of high

system and gas purity. Another objective of the system design, as de-

fined by R. N. Davie for his experimental work, was the capability to

measure spectra at different distances from the fissioning source.5

This required a planar source of fission fragment and the appropriate

optical system to sample a well-defined region at a desired distance from

the source. In addition, it was desired to perform an absolute system

calibration. Thus, a simple optical system was also important. Other

factors considered were the low expected light output, possible radiation

damage to the in-pile parts of the system, possible heat dissipation

problems from the fissionable material, and UFTR safety requirements.

This last factor introduced the.requirement that the experiment

provide double containment of the fission products. The resultant design









eliminated any gas handling while the system was in the reactor. This

limitation coupled with reactor availability, limited the amount of

experimental data collected.


Overall System


The overall system which resulted from the above design consi-

derations is shown in Figure 1. The heart of the system is the multi-

purpose capsule for the irradiation of gases (MCFIG) which was a high

quality vacuum tight system containing the gases under study with a flat

plate coated with a 3 micron thick 93% enriched U02 along the inside

wall of the capsule. The MCFIG was.designed for increased safety over

previous gas irradiation studies and the necessity for gas purity as

most gas contamination is due to its container. The MCFIG was essentially

a piece of stainless steel tubing with a sapphire window welded on one

end and a valve and thermocouple port for the other,as shown in Figure

2. The MCFIG was inserted into the center core region of the UFTR via

the horizontal through port (HTP) and subjected to a maximum flux of

1.6(10)12 n/cm2-sec. The initial argon data was taken with a short MCFIG

which subjected the sapphire window to high radiation fluxes. Lumines-

censeand radiation damage to the sapphire resulted. Consequently, the

MCFIG was rebuilt with a longer tube which moved the window out of

the high flux region. All of the argon-nitrogen mixtures experiments

were performed with this extended MCFIG. The luminescence and radiation

damage of the sapphire in the high flux was studied in some detail by

Davie and the reader is referred to his dissertation for complete re-

sults.15









W ---- E


THERMOCOUPLE FUEL BOXES
FUEL BOXES
READOUT
TEST TUBE
SHIELDING THERMOCOUPLE
OUTPUT
7 / f


LENS
ASSEMBLY


LENS


VACUUM.
PUMP


SAPPHIRE
WINDOW
ON MCFiG


REACTOR --


WINDOW
INSIDE
END CAP


SHIELDING


BEAM
TRAP


N2 COOLANT


LEAD
SHIELDED
PM TUBE


LENS


MONOCHROMATOR


OVERALL LAYOUT FOR .MCFIG GAS IRRADIATION


STUDIES


Figure 1:



















STL.


I I -UO2 SOUl
rri--rc ,--
LrJ- --& I I- uo -so--
L L_ _


Figure 2. MULTIPURPOSE CAPSULE

IRRADIATION OF GASES


FOR THE
(MCFIG)


-SAPPHIRE
















HELIUM
BOTTLE
FOR
FLUSHING





VACUUM GAGES


WINDOW OR
THERMOCOUPLE
FEEDTHROUGH
AS REQUIRED

WEST END CAP


CONFIGURATIONS

I) CHECK.GAS SYSTEM INTEGRITY FOR
LEAKS: A AND B CLOSED, C OPEN.
2) EVACCUATE TEST TUBE: A AND C
OPEN, 8 CLOSED.
3) ISOLATE TEST TUBE DURING REACTOR
OPERATION: B AND C CLOSED, A OPEN.
4) FLUSH SYSTEM WITH He A AND B
OPEN, C CLOSED.


EAST END CAP


TEST TUBE


4ACFIG -


LENS ASSEMB


WINDOW


PUMP
EXHAUST


TO REACTOR STACK
VIA APPROPRIATE
FILTERS


Figure 3: Test Tube Assembly









The MCFIG was contained in an aluminum test tube which extends

through the HTP and was sealed at each end by use of end caps with quartz

windows for light transmission as shown in Figure 3. Two lenses which

were part of the optical system were also mounted inside this tube. After

the MCFIG was loaded in the test tube from the west end of the reactor and

the end cap replaced, the test tube was evacuated to approximately 100

microns of Hg using a roughing pump located at the west end and then the

system was isolated prior to reactor startup. This system provides secon-

dary containment should the MCFIG leak. When a thermocouple was located

in the MCFIG, it was monitored by instrumentation at the west end of the

reactor. As the reactor was brought up to power, the light produced in

the capsule due to the fission fragment interaction is monitored by a

monochromator with an attached photomultiplier tube at the east end of

the reactor. The PM signal was sent to an instrumentation system to be

described in a following section.


Optical System

A schematic of the optical system is shown in Figure 4. The lens

system was focused for an image at the center of the reactor core with

a 1:1 image maintained throughout. A front surface mirror was mounted

on a precision rotation module external to the reactor which permitted

sampling different distances from the planar source by the monochromator.

The actual region sampled by the monochromator is quite complex; it is

shown approximately in one dimension in Figure 5. A treatment of the

problem has been accomplished by Davie.15

The energy deposition by a fission fragment into a gas volume is

a direct function of the gas pressure. However, the energy deposition in


















I" MCFIG
UFTR SAPPHIRE WINDOW
COREl 1.5" DIAMETER
ENTER LENS
11.0" -I 4 f= 20" LENS


1.5" DIAMETER
f=20" LENS


8.0
5.5" --


FUSED SILICA TEST
TUBE WINDOW
-ROTATABLE MIRROR


S5.5"
--- 40.0" -
40.0" -'


REACTOR


2" DIAMETER
f=IO" LENS


SPECTRO-
GRAPH


20.0" \

57.5"- \1


Figure 4: OPTICAL SETUP (top view)







3 Micron UO
Coated Plane
Surface


Region Sampled by
Monochromator
(Adjustable by Rotating
Adjustable Mirror)





Aperture Provided by
The Sapphire Window







MCFIG Structure
Including Window
Mounting


Figure 5: Spatial Sampling Scheme









the region sampled by the monochromator is also a function of the dis-

tance from the source. Therefore, it is imperative in future experiments

with fission fragments to include a spatial calibration to insure con-

sistency in analysis of data for different pressures and gases.

An alignment device was fabricated to insure proper operation of

the optical system prior to each MCFIG insertion. It consisted of a

light source encased in an aluminum tube the same diameter as the MCFIG

with a ground glass window at one end. The window was masked by a

pattern containing precisely positioned holes 0.1 inch apart and making

a cross pattern. This device was inserted into the UFTR HTP with a known

pattern orientation such that the ground glass and pattern were at the
reactor core center. A 1:1 image was then verified and the rotatable

mirror set to center the pattern on the monochromator slit. Then using

the monochromator on zero order the output of the PM tube was monitored

as the mirror was rotated to obtain a calibration for the rotation module.

The region sampled in the core could be repeated within 0.01 inches.


Instrumentation System

A schematic of the instrumentation system is shown in Figure 6.

The system consists of a model 218 McPherson 0.3 meter scanning mono-
chromato. coupled with an EMI 9558QB(S20) photomultiplier tube. The

relatively low light levels being monitored made it necessary to use

large entrance and exit slit settings. For most of these experiments,

a setting of 150 Pm was used and a linewidth of 3.5A was experimentally

determined. The output of the PM was input to a Keithley model 410
micro-mi ro ampmeter. The 0-5 volt output was routed through a low








SUPPLY


EMI 9558QB
(S 20) PM TU


LIGHT INCIDE T
ON SLIT FROM
MCFIG


KEITHLEY
160 DIGITAL
MULTI METER


N2 OUTLET


SIGNAL CABLE


LEAD
FOR


SHIELD
PM TUBE


KEITHLEY
MODEL 410
MICRO-MICRO
AMPMETER


COOLED N2 GAS
FROM HEATING N2
TO BOILING POINT


MCPHERSON
.3 METER
SCANNING


HP 7000AM
x-y RECORD-
ER


Figure 6: Instrumentation System


C\









pass RC filter and used to drive a Hewlett Packard Model 7000AM X-Y re-

corder and Keithley Model 160 digital multimeter. The PM was located in

a lead shield to reduce the gamma radiation level and was cooled by

liquid nitrogen to reduce dark current.

The large RC low pass filter (time constant = 4 sec) was used to

convert the gamma noise to a steady state background level from which the

signal could be distinguished. Small changes in this level existed due

to reactor power variations. A big disadvantage to this approach to

improve signal to noise is the long data acquisition times necessary.

For example, a 20 a/min sweep speed on the monochromator required 5

hours minimum time to observe the spectra from 2000-9000 W.


Reactor

As previously described, the experimental system was designed
around the University of Florida Training Reactor (UFTR). The UFTR is

an Argonaut type reactor licensed tooperate at 100 kW. The UFTR per-

formance characteristics have recently been documented by Zuloaga.17

Of particular interest in this study was the neutron flux distribution

in the UFTR horizontal through port which is shown in Figure 7. The

maximum thermal neutron flux was found to be 1.9 x 1012 neu/cm2-sec at

100 kW. For all of our studies, the MCFIG was located directly below

the center vertical port (CVP) which unfortunately was not the point of

maximum flux. Thus, in these experiments, the MCFIG was exposed to a

thermal flux of approximately 1.6 x 1012neu/cm2-sec.


Data Procedures

As indicated earlier, the necessity for gas puritywas imperative.












1012


















loll
101


50 40 30 20 10 0 10 20 30 40


DISTANCE


FROM CVP


(inches)


Thermal and Epithermal Neutron Flux Distribution in the UFTR Horizontal Through Port.


Figure 7:










The MCFIGs were baked and evacuated on a pumping station for approximately

two weeks, to a final pressure on the order of 10-8 torr, and then helium

leak tested prior to filling. For all the experiments the capsules were

filled to 760 torr with the appropriate research grade gas at ambient

temperature. Prior to each MCFIG loading, the optical system was cali-

brated as described in the section on the instrumentation system. The

test, post-test and safety-related details of these experiments were docu-

mented in the safety analysis which was prepared for the UFTR Safety Sub-
18
committee in July, 1973.1 After the MCFIG was loaded and the reactor

reached full power, a fast spectral scan was obtained to get a feel of the

magnitude of the spectrum. Due to the MCFIG having a fissioning heat

source in it and relatively poor heat transfer characteristics, it was

found that the MCFIG would heat up during the reactor operation. As a con-

sequence, considerable changes in the spectral intensities occurred after

the reactor reached full power. Figure 8 shows a plot of the temperature

as a function of time .for the case of Xe as the fill gas. Detailed spectral

scans were not taken until the capsule warmed up which was at an absolute

minimum of 20 minutes. After the capsule had warmed up, a detailed scan

was made from 2000-8400A at a scanning rate of 20A/min and a slit width of

150 or 300 pm. Then, selected spectral regions were investigated for spa-

tial effects, reactor power variations and temperature effects. Calibrated

interference filters were used throughout to insure against second order

effects and internal monochromator reflections which proved to be extremely

important in the nitrogen mixture data. The experiments required more

reactor time than originally anticipated which, coupled with reactor

schedule considerations, breakdowns, etc., proved to be the limiting factor

for the number of mixtures studied.










240 -

220

200

180 -

160

140


120

100

80

60

40

20


0 20 40 60 80 100 120


140 160


180 200 220 240 260 280


TIME AFTER REACHING FULL POWER (min)
co
Figure 8: MCFIG Temperature as a Function of Time After,.Reaching Full Power.










System Calibration


One of the objectives of this research was to compare the rela-

tive intensities of emission in the observed spectra. This objective

was later expanded to include estimating the population densities of the

emitting species which required an absolute system calibration. This

calibration was complicated by the fact that the radiation source was

volumetric, the source was inside the reactor and, therefore, inaccessible

and the optics sustained radiation damage that was dependent upon the

total irradiation time. The calibration was performed by R. N. Davie and

the results are reported in detail in his dissertation.15 Basically, the

problem was broken into two parts: (1) the collection efficiency of

the optical system for a distributed source, and (2) the relation between

the photomultiplier tube current and the energy collected by the optical

system.

The calibration of the system was limited by radiation damage to

the optical system at short wavelengths and the PM tube in the infrared.

A tungsten lamp calibrated by the National Bureau of Standards and a

deuterium lamp provided the standard sources above and below 3500O

respectively. Figure 9 presents the relative system spectral response.

The population densities calculated .from the absolute system response

was believed to be accurate within a factor of 10 for the wavelength in-

terval of 3000 to 80008.15

Voltage measurements of the line heights as recorded by the x-y

plotter were converted to current as measured by the micro-micro amp-

meter. For 150 pm entrance and exit slits on the monochromator, the

following expression, as derived by Davie, was used to calculate all ex-

cited state populations.15
























































4000


5000 6000


7000


8000


Relative Spectral Sensitivity of Monachromator and
Photomultiplier Compared to that for the Entire
Instrumentation System


10









1










0.1









0.01


0.001


2000


Figure 9:


3000









1.6 IAHX (
SgA V Chc

where
-8
IX = observed line height expressed in amps; 3.0 x 10-8 amps/volt

H = monochroma or and photomultiplier transfer .unction at 7000 R
(2.3 x 10 watts/amp)

X = wavelength of the observed line in

g = statistical weight of the excited level
-I
A = transition probability of the observed transition in sec

V = effective volume (7.4 x 10-5cm3)

C = relative system calibration curve normalized to unity at 7000
0-15
he = 1.99 x 10 joules-A














CHAPTER III

THEORY


To accurately evaluate the experimental data, an accurate de-

scription of the gases being investigated must be developed. The three

basic considerations that are reviewed in this chapter are; first, the

way a particle deposits its energy; secondly, some basic properties of

the gases being investigated; and third, processes by which the deposited

energy is transferred throughout the stopping gas.


Fission Fragment Interaction


Fission fragments are distinguished by their large mass, high

initial energy and high initial charge. Light ions such as protons or

alpha particles are essentially charge invarient over their path where-

as heavy ions exhibit charge variance. Fragments born in fission are

emitted isotrophically and can be divided into a light group and a heavy

group. The average properties of Uranium-235 fission framents are given

in Table 1. These factors differentiate the slowing down processes of

fission fragments as compared to other forms of nuclear radiation.

A good review of both theoretical and experimental work on the

slowing down of heavy ions through matter was conducted by Northcliffe

in 196323 and more recently by Miley.24 When a fission fragment inter-

acts with a gas, it loses energy rapidly by electronic collisions at the

beginning of its tract, when its velocity and charge are greatest. These

collisions with the atomic electrons of the stopping gas produces ioni-

zation and excitation of the stopping gas. The fragment path is not

22














Table 1

Average Properties of U235 Fission Fragments92021,22


Mass number

Atomic number

Initial net charge

Initial energy

Maximum delta-ray energy

Range in air at 150C, 760 torr

Range in U02 (10.9 g/cm3)

Range in U308 (8.25 g/cm3)

Range in U-metal (18.7 g/cm3)


Light Group

95

38

+20e

97 MeV

2.2 KeV

2.7 cm


Heavy Group

135

54

+22e

65 MeV

1.05 KeV

2.1 cm


6.8 im

7.5 um

5.4 um









expected to significantly deviate from a straight line in this region.

As the fragment continues to slow down, it loses part of its charge

through recombination with electrons and the energy deposition rate per

unit path length decreases. When the fragment reaches a velocity lower

than the orbital velocity of the outer electron of the stopping gas, the

capture probability becomes very large. As the fragment continues to

slow down, additional electrons are captured and so on until the fragment

becomes a neutral atom.

A nearly neutral fission fragment can also elastically collide

with the stopping gas. Above about 30 MeV, the inelastic collision

processes for energy loss dominates.25 Below 30 MeV, elastic collision

processes are dominate, increasing the energy deposition rate but not

necessarily the ionization rate. Thus, primary fission fragment excita-

tion is expected to be most significant at higher energies or during the

first part of the fragment path.

The dominance of inelastic coulombic collisions in the slowing

of fission fragments means that the most important energy loss process

is the ionization and excitation of the stopping gas. Much of the energy

of ionization is given to high energy electrons (delta rays). They can

have energies up to the keV range. For delta rays having energies ex-

cedding the ionization potential of the gas, the ionization cross sec-

tions are much greater than those for excitation, and these delta rays

can result in numerous additional or secondary electrons. It is pri-

marily these delta rays and their secondaries that are the most impor-
19
tant source of the excitation produced by fission fragments. Thus,

it is not unreasonable to say that a fission fragment interaction is

like a small instantaneous electron beam. Fission fragment energy









deposition from a fissioning source is, therefore, reasonably approximated

by the sum of a large number of small high energy electron beams, and the

studies of E-beam excitation can be useful in interpreting the experi-

mental results of fission fragment excitation of gases.


Energy Deposition


It is not of particular importance in this study to evaluate the

energy deposition from the planar source used in the experimental work.

This problem has been treated elsewhere15 and those results are used

in the present analysis for the energy deposition at one atmosphere.

What is of interest is to develop a description of the energy deposition

as a function of pressure for two cases. First, in order to evaluate

the intensity versus pressure data of Walters, a rather simple form will

be developed. Secondly, the energy deposition at the center line in a

long gas cylinder with a thick Uranium coating is used to investi-

gate effects of pressure and cylinder dimensions for optimization studies

relevant to nuclear pumped laser design.

Simplified Energy Deposition Calculation

Bohr has derived a theoretical relationship describing the slow-
26
ing down of fission fragments. His classical theory gives for the elec-

tronic energy loss

dE 47 (Zeff)2e4ZnLe
dr m V2 (2)
e

where e is the electronic charge, me is the mass of the electron, Zeff

is the effective atomic charge of a fragment of velocity V traversing

a medium of atomic number Z and L is a logarithmic summation term.
Integration gives an approximate solution
Integration gives an approximate solution








r2
E = E (1 ) (3)

Here E is the energy of a fragment of initial energy Eo after traveling

a distance r of its total range R.

Neither the approximate Eq. (3) nor the complete Bohr stopping
power formula Eq. (2) correctly describes energy loss by fission frag-

ments in light materials. However, most of the available range energy

data may be fitted fairly closely to an empirical n-power law of the

same form

E = E(l )n (4)

where 1 < n < 2. Rearrangement of Eq. (4) will give Ed the energy de-

posited in the stopping gas after the fragment has traveled a distance

r,
n
Ed = E E = E [ (1 r (5)

Using the experimental results of C. Leffert et al.27, the range of a

fission fragment of initial energy Eo in a gaseous medium can be written

R = 20.1 Ra(Eo) (vA/m)(T/p), (6)

where T is the gas temperature in degrees Kelvin, p is the gas pressure

in torr, m is the molecular weight of the gas, and A is the average

atomic mass of the gas species. The parameter Ra(E ) represents the
range of the fission fragment in air at standard temperature pressure.

Ra(E ) is known, to within 10% accuracy for fragments of moderate to
high energy,to be28

R (Eo) = 0.12Eo2/3 (7)

By interpolation of the data reported by Kahn, Harmon and
Forgye29 for a 3-micron U source,the relative fragment escape energy
Forgye for a 3-micron UO2 source, the relative fragment escape energy








was found to be 0.56. Using 83 MeV as the average born energy of a

fission fragment, the escaping fragment will have an average energy of

46.5 MeV. Using Eq. (6) and (7), the range of an average fragment in

argon for a 3-micron source at standard temperature and pressure is 1.31

cm. Using the Bragg relationship27

R = RoPo/p (8)

where the subscript refers to one atmosphere, Eq. (5) reduces to the

form

Ed = E[l (1 r ) (9)
o

where Eo = 46.5 MeV

p = pressure in atmospheres

R = 1.31 cm. range of fission fragment of energy E at 1 atmo-

sphere.

A plot of this function versus pressure for n = 1.5 and 2.0 is

shown in Figure 10 for r = 3.7 cm.

To evaluate Walters' data using thisequation one must realize

that his optics did not sample the entire tube diameter. Thus, appli-

cation of Eq. (9) must be modified to

Ed = E [(1 R)n (I 2Pn], (10)
o o

where r1 and r2 are the limits of the region sampled, as measured from the

source. Using his average cord length of 3.7 cm and estimating that Walters

was looking at the central 75% of the volume, rl and r2 are .248 cm and

3.45 cm respectively. A plot of Eq. (10) using the above values is

shown in Figure 11.


















n = 2.0


n = 1.5


1 ATM


IOU 200 300 400 500 600 700

PRESSURE (mmHg)

Figure 10: Fraction of Energy Deposited in Volume for FF of Initial Energy Eo from Equation 9.


0.7!



LU
I-
c-

L 0.5


LU
L>-




0.2E

















n = 2.0


n = 1.5


1 ATM


) 400 500 600 700
PRESSURE (mmHg)


Fraction of Energy Deposited in Volume for FF of Initial Energy E from Equation 10.
b


0.751


0.2!


Figure 11:









Fission Fragment Energy Deposition in a Long Gas Cylinder Wrapped
With a Thick Fissile Foil

Most direct nuclear pumped laser experiments are similar in de-

sign. They consist of a long cylindrical cavity and an external neutron

source. The cavity is usually lined with a thin layer of enriched

uranium or Boron-lO. After the fragment is born, it will lose energy

at a rapid rate in the coating. Of the fragments that escape, some are

stopped in the gas while others may travel completely through the gas

and become imbedded in the wall. Obviously, the energy deposition of

fragments will not only vary with the flux but also with the coating

thickness, size of the cavity, type of gas and gas pressure.

A solution to the energy deposition due to fission fragments

in a long gas cylinder wrapped with a thick fissile foil was developed
29
by Gordon Hansen.29 The problem was simplified by seeking a solution

over the central 25% of the gas volume. The solution was based on the

assumption that the energy at any point x of fragment born anywhere in
the coating with an average energy E0 can be described by a semi-empiri-

cal slowing down law similar to Eq. (4) where n = 1.5. This resulted in

an equation of the form
1.5
fx_ pg(x-x )
E(x) = Eo0 Pfxf R g (11)'
f g
where E = the average born energy of a fragment

f = the density of the foil

x = the distance the fragment travels in the foil

Rf = the total range of the fragment in the foil in mg/cm2

Pg = density of the gas
x-x = distance the fragment travels in the gas

R = the total range of the fragment in the gas in mg/cm2









The final solution resulted in an integral which can not be

simply evaluated except for two asymptotic cases. For high gas pressures

where the fragment can just reach the cylinder axis results in the form

E = 2E f (1 -P1 )2 (12)
dfR 8p 2 R g

where f = the fission density

r = the tube radius

and all other.terms are as defined above.

For low pressures where the range of the particle is larger than the

dimensions of the cavity, the solution is of the form, using the same

notation,

E = 2E f R A (13a)


where
2 2
p 9 r 2.691R 1 1.075i per
A = 1 2.357 p9- + 0.375 pgr In 1691R + 1.5
Rg g R pro 075

+ 0.0378 R( o (13b)
Y9

Thus the energy deposition is of the form

Ed = Constant*p*A (14)

where the attenuation factor,which is a function of Pgr /Rg, can be

pretty well determined from the two limiting cases. These values of A

are shown in Table 2. A plot of Eq. (14) is shown in Figures 12 for
2 2
argon of different tube radius where R = 10.8 mg/cm and R = 4.1 mg/cm2

It should be emphasized that these results are for the energy

deposition per unit volume along the cylindrical axis. Approximately

five times as much energy is deposited in the gas volume for a radius

of 1.85 cm versus a radius of 0.4 cm at 500 torr. However, for laser







r + 0.6 cm
0


2.0





S1.5 r = 1.0 cm
4 1.5 o









r 1.85 cm

0.5






0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

PRESSURE (atms)


Energy Deposition at the Center Line for Various Radius Tubes from Equation 14.


Figure 12:










applications the quantity of primary interest is not total volume deposi-

tion but energy deposition per unit volume. For the above two cases

approximately 4.6 times as much energy is deposited per unit volume for

the smaller radius tube. Another advantage of a smaller radius is the

wider range of pressures for which the energy deposition remains fairly

constant as can be seen in Figure 12.


pr
Rg

0

.1

.2

.3

.4

.5

.6

.7

.8

.9

1.0


Table 2
Attenuation Factor, A, versus p ro/R

A A
Low-pressure High-pressure


1.0

.786

.604

.448

.317

.207

.118


.408

.3

.208

.135

.075

.0333

.0083

0


Argon

General Considerations

The gases helium, neon, argon, krypton and xenon are the favored

materials in gas lasers. Not only are they used as the active lasing

medium but also frequently are used to transfer excitation to a secondary






S34


gas which produces coherent light. The common characteristics of the

electronic structure of the noble gases is that for neon, argon, and

zenon, the highest p-shells are filled and there are no electrons outside

of these shells when the atom is in its ground state.

The electronic configuration of argon is ls 2s 3p 3s 3p with

a S ground state. The excited states of atomic argon have 3p5 cores
5 5
with one electron excited, eg, 3p 4s, 3p 4p..., and the spectrum of argon

exhibits considerable fine structure, Its structure is reviewed in

several standard books (such as 30). This complexity arises from the

variety of possibilities which occur when the angular momentum of the

core J (either 1/2 or 3/2) is coupled to the angular momentum of the

excited electron j to give a total angular momentum J. Hence, argon

has s,p,d,f and s', p', d', f' states based on the J = 3/2 and J = 1/2

core of Ar+, respectively.

Thus the coupling scheme in argon is neither LS or j-j type but

instead appears to be of an intermediate type called j-j where A is
c
the orbital angular momentum of the excited electron and jc is the core

or hole angular momentum. Such coupling is called pair coupling. The

terms of an atom in which pair coupling prevails are designed by Racah's

symbols, which consist of the symbol of the outer electron configuration

followed by [K] where K = a + jc. Racah's notation is complicated but

provides a model of the situation. However, the most commonly used

notation'is the Paschen notation, which is a system of shorthand symbols.

Although the letters s,p, and d are used, one cannot infer that a Paschen

symbol with the letter s always refers to an outer electron in an s or-

bit. Figure 13 gives a chart of the lowest excited states of argon with

Paschen symbols on the right, electron configuration of the Racah symbols



















S-States


R odd P



- 6
s
6
s


P-States


R even P

6 = 4 --
P 4
6 r- 6 1 P
P

5p' 4 ~
P 3
5 6 1p
p


5
5 s 2
5 s
s


d-States


R odd P




4 d'C 4- *


4d 8 4d



3d'l 8 4--- 3 *

3-= 8 3d
d


f-States


R even P



4f'=4 J*
4f --8 -=*


p = 4 -
2
-6- P


4 '
s 1
4 s s
s


Figure 13: Chart of the Lowest Excited States of Ar Atoms. Paschen Symbols
on Right, Electron Configuration of the Racah Symbols on Left,
Number of States in Center. (*indicates miscellaneous or improvised
symbols)









on the left and the number of states in the center. The ionization energy

of argon is 15,76 eV for the Ar (3p5 2P32) and 15.74 for the Ar+(3p5

Pi/2)'

The ease with which non-equilibrium conditions can be generated
in the neutral inert gases is the result of the basic excitation processes.

For all of the noble gases, with the exception of helium, it is possible

to preferentially populate entire series of excited levels. This effect

is clearly shown in Figure 14 using Paschen notation where n = 2,3,4

and 5 for Ne, Ar, Kr, and Xe respectively. The interaction of an elec-

tron flux with a noble gas will inherently lead to the creation of an

inversion between s- and d- states and excited p- states.31,32 Over 300

.noble gas neutral state laser transitions have been reported.33 For

argon only two transitions produce significant gain:

Wavelength Transition(Pachah) (Paschen)

1.6961 microns 3d1I/2] 4p [3/2] 3d3 2p

2.0616 microns 3d [3/2]2 4p' [3/22] 3d3 2p3


For the present study the excited levels of argon are divi-

ded into 6 groups. It is not the purpose of this study to investigate

the excitation of each individual level but attempt to investigate the

amount of energy stored in the gas at different energies. Certain ex-

cited groups of atoms exhibit preferential collision phenomenon with

ground state atoms and also with nitrogen. Each group is treated

as a single excited level at the average energy for the levels it repre

sents. Each level (with one exception) is assigned a transition probe

ability that is an average over the gA values of the levels of the
34
group. Of the four states that comprise the first group, Ar(1), two

are metastable and two are in resonance with the ground state. The


e_
a-


















(n+3)s



(n+2)s



Spontaneous
Emission




(n+l)s


Laser


- (n+l)d


(n+2)p


nd
,12 levels


Electron
7 Collisions


j4 levels \


I \/
1 \ /

SWall or
3-Body
Collisions \ /
I \ /


Kr,Ar,Xe


Noble Gas (np6)


Figure 14:


Preferential Electron Excitation of the Noble
Gases Kr,Ar and Xe.


Laser





38


atoms excited to a resonance state emit photons which are imprisoned

sufficiently that one may consider them of long lifetime, Therefore, the

resonance states are treated as metastable.

One group, for the excited levels above 15 eV (Ar(6)), is

assigned a transition probability consistent with the excited levels

which contribute to the Hornbech-Molnar process

Ar* + Ar Ar2 + e (15)

The rate k and the lifetime of the excited states contributing to the

above process was determined in an experiment by Lampe and Hess.35 They

used a pulsed electron beam followed after a variable delay time by a

pulsed ion-extraction field. Their results compare favorably to the

independent work of Kaul and Dahler et al. The data for the six

groups to be used in the subsequent analysis is given in Table 3.

A reasonably complete set of discrete and ionization cross sec-

tions for argon is given by Peterson and Allen.38 They used combinations

of data and theoretically extrapolations of the generalized oscillator

strengths. Their calculations result in the final population for each

excited state as a function of incident electron energy. These popula-

tions are as a result of the degradation of the primary electrons and

all generations of secondary electrons and is presented as an efficiency

where

e = W. N/E (16)

where Wi = threshold energy of the level i

N number of excitations

E = energy of the incident electron.

The efficiency versus incident electron energy is relatively constant

for incident above approximately 80 eV. In Table 3 are the results of







Table 3


Properties of the Six Groups of Argon


Group Level Represented Average Average e # Excited/lOOeV
(Paschen notation) Energy gA (x10-8) Absorbed
(eV)

Ar(1) s 11.68 .08 .685

Ar(2) 2p 13.21 1.3 0.04 .303

Ar(3) 2s,3s',3d 14.1 0.5 0.04 .284

Ar(4) 3p 14.59 0.055 0.005 ,034

Ar(5) 4d 14.77 0.1 0.008 .054

Ar(6) Higher Levels 15.2 0.021 0.019 .125


Ar 2p5 2P3/2 15.7 0.56 3.44

Ar+ 15,7 0.594 3.79
(Adjusted)









Peterson and Allen that correspond to the six groups of argon and the ion

for an incident electron energy of 100 eV.

From Platzman,39 the energy loss per ion pair, W, can be related

to the equation
Nex(g)
W = E. + Eex(g) N + (17)
g i
where Ei is the energy necessary to produce an ion

Eex is the energy for the excited atom in state g

Fse is the energy of the subexcitation electrons

N ex/N is the ratio of number of excited atoms in state g to the
ex 1 number of ions.

For the noble gases the quantity E. exceeds the ionization energy

I because of the energy used in producing excited ions and multiply

charged ions. The ratio of Ei/I for the noble gases is approximately

1.06. This results in the adjusted value in Table 3 for the e of Ar.

Using Eq. (17) and the values in Table 3 approximately 60% of the deposi-

ted energy is used in the formation of ions, 20% in the formation of

excited states and 20% of the energy is left in subexcitation electrons

with an average energy of 5.4 eV. Thus, since a large portion of the

deposited energy is used in the formation of ions, the resultant spectrum

will depend on recombination processes.

Recombination

When an electron and a positive ion recombine, energy must be

released in some form. The means with which this excess energy is given

off give rise to the possible processes of recombination.

1. Radiative recombination. The excess energy is released as radiation

Ar + e Ar* + hv (18)









2. Three-body recombination. The excess energy is taken up by a third

body

Ar+ + e + e Ar* + e + kinetic energy

Ar + e + Ar Ar* + Ar + kinetic energy (19)

3. Dissociative recombination. This is possible only if the positive

ion is molecular

Ar + e Ar* + Ar + kinetic energy (20)

In each case the products leave an argon atom in an excited state.

In general, for process 2, the electron is captured into a

highly excited state so that this process cannot be taken as contributing

directly to the effective rate of recombination in the plasma. This is

because the reverse of the process will usually occursbefore spontaneous

emission or collisional deexcitation can occur or the excited state un-

dergoes a collision of the form

Ar* + Ar Ar + e. (21)

In either case the recombination due to this process is small and,

therefore, is ignored in the present study.

Recombination in the noble gases has been quite baffling to

experimenters with different experiments yielding different results.

This can be seen more clearly in Table 4.

Only recently have adequate explanations been offered.47'48'49

Radiative recombination (process 1) should have a recombination coeffi-
-12 3 49
cient on the order of 1 x 10 cm /sec. Recent experiments have shown

that the predominant loss mechanism for the ion for pressures above 1

or 2 torr is the Holt-Biondi process

Ar + Ar + Ar Ar + Ar. (22)







Table 4

Measured Recombination Coefficients in Argon


Observers

Kenty

Biondi & Brown

Biondi

Holt et al.

Olsen & Huxford

Kaiser

Mehr & Biondi


Electron Temp.

31000k

3000k

300k

3000k

83000k

300k

300k


Pressure in mm Hg

.8

20-30

20-30

2-30

50-70

760

20


n
e
1013

1010

1011

1010 to 1011

1016


100 to 11
10 to 10


ae

2 x 10-10

3.7 x 10-7

8.8 x 10-7

5x107 1.1x10-6

2.5 x 10-13

3 x 10-6
8.5x10-6
8.5x10


Ref.

40

41

42

43

44

45

46









The thus formed molecular ions have very large recombination coefficients.

Therefore, experiments performed at a few torr or lower did not measure

the recombination coefficient of the ion but the formation rate of Ar2

by three body collisions. Another source of experimental error is the

electron loss due to attachment of electrons to atoms or molecules to

make negative ions. Therefore, extreme precautions must be taken to in-

sure the purity of the gas and the absence of negative ion forming impuri-

ties.

In addition to these effects, when studying recombination due to

heavy charged particles the effects due to columnar recombination must be

considered. As the charged particle slows down, its initial ioniza-

tionlies close to its path and, thus, the electron density will be much

higher in this region than the average for the medium. Therefore, the

rate of recombination is higher immediately after the passage of

the particle and will decrease in time as the electrons diffuse away.

The measured recombination coefficient based on the average electron

density of the medium 'is then higher than for a uniformly ionized medium.

In argon, for pressures above a few torr, recombination proceeds

predominately through the molecular ion and not the ion. The formation

rate of the molecular ion is very pressure dependent, from 5 nsec at one

atmosphere to greater than 50 micro-seconds at pressures less than 100

torr. Thus, the effects of columnar recombination are a function

of pressure not only due to the diffusion rate of the electrons, but

also the formation rate of the recombining species. The inclusion of

these effects are beyond the scope of the present research. In addition,

it is not clear what effectscolumnar recombination has on the

excited species formed due to recombination (which is of interest here).






44












Ar+ + Ar + e

Ar







Ar + Ar




Inter r ion
e 1: D unstable
Molecule



2

= 0






Internuclear Separation

Figure 15: Dissociative Recombination Process in Argon










For the present analysis, the radiative recombination rate (process 1)
-11 3
is taken to be 1 x 10 cm /sec as a maximum and dissociative recombi-

nation as 3 x 10-6 cm3/sec.

As mentioned, for argon, recombination proceeds primarily through

the molecular ion and, therefore, it is of interest here to evaluate the

excited products of dissociative recombination. The formation of these

excited atoms is better understood by referring to Figure 15 and Eq. 23


Ar (v) + e -(Ar ) Ar* + Ar + K.E. (23)
2 2 unstable

where v indicates vibrational excitation of the molecular ion. The fig-

ure shows a single stable potential curve of the Ar2 ion and a single

repulsive branch of the unstable excited molecule, which is assumed to

cross the ion curve at its minimum. This situation gives the maximum

rate of recombination. Mehr and Biondi46 have found that for a con-

stant ion temperature that recombination is proportional to the electron
-0.67
temperature by the dependence a ~ T When the electron and ion
e
temperatures were equal, Fox and Hobson found a temperature dependence
a ~ T-1.3

Thus, it is concluded by Biondi that higher vibrational levels

of Ar~ have smaller recombination coefficients. This same result was
49 40
concluded by Loeb4 in his evaluation of Kenty's data.40 Kenty also

investigated the afterglow spectrum of Argon and compared it with an

arc spectrum.of argon. His comparison indicates that in the afterglow,

lines involving transitions from high s and d states to the 2p states

were much stronger with respect to ls-3p and 1s-2p lines in the arc.

He further found that by rearranging the 2p-md and 2p-ms lines into

their respective series that the higher respective series member was









enhanced in the afterglow. Further support is given by a spectrographic

study of the recombination in Helium by Biondi and Holstein.50 They

conclude that dissociative recombination of the higher vibrational levels

of He~ give rise to higher excited states of the atom and that these

vibrational states can suffer on the order of 106 collisions without

losing their energy of vibration.

The above discussion indicates that higher vibrational levels

of the molecular ion have smaller recombination coefficients and dis-

sociate upon recombination into higher excited states of the atom. The

problem is further complicated by the possible existence ofmore than

one stable potential curve of the molecular ion which may contribute to
51,52
the recombination process.,

Dissociative recombination excitation of the 2p levels has been

studied in most detail for Ne and to a lesser extent for argon.53 The

approach in these studies was to measure the Doppler broadening of the

afterglow lines caused by the kinetic energy of the excited atoms pro-

duced by the dissociative process. In the afterglow of Neon they found

that the line broadening indicated many dissociative shoulders. These

shoulders would correspond to different vibrational levels in the molecu-

lar ion. Similar profiles were found for argon but the structure was

narrower indicating that cascading plays a more important role in the
54
argon than neon. Hanle, Kugler and Schmillen,54 using filters, studied
the decay spectra of noble gases excited by a pulsed electron beam. They

found that the observed spectra could be divided into a fast and a slow

decaying component. In argon, the ratio of the amplitude of the slowly

decaying component to the amplitude of the emission during the pulse over

the region 7000-7200R was approximately 0.1. The origins of the slow









decaying component cannot be definitely established, but clearly it arises

from a combination of direct dissociative recombination and cascade from

higher levels.

In conclusion, to completely describe the contribution of dis-

sociative recombination to the excited state densities of the atom re-

quires knowledge of;

1. Stable configurations of the molecular ion and possible vi-

brational states,

2. Recombination coefficients (which are T dependent) for each

state,

3. Electron energy distribution,

4. Distribution of vibration states upon formation which may

depend upon gas temperature,

5. Collisional relaxation and radiative process of the vibra-

tional and excited states of the molecular ion,

6. Excited species formed upon recombination,

7. Rate of reverse processes,

In the present analysis, in an attempt to account for the above

effects, the molecular ion is divided into two groups, Ar2' and Ar ".
2 2
The Ar2' comprises the lowest excited levels of the molecular ion which

dissociate upon recombination into Ar(1), Ar(2) and Ar(3). The Ar"

comprises all higher states of the molecular ion and dissociates upon

recombination into Ar(2), Ar(3), Ar(4), Ar(5) and Ar(6) and also has

collisional losses to form Ar2 upon collisions with argon or impurity

gases.









Rare-Gas Molecules

Continuum emission in the vacuum ultra-violet (VUV) range from

excited inert gases at high pressures has been known for over 30 years

and used extensively to provide light sources in the VUV. Reference 55

contains a good bibliography of the basic papers. The emission arises

from stable excited molecular states that radiate to a repulsive ground

state producing a continuum. Recently, these molecular states have

found application as upper levels for VUV lasers.56'57 These continue

are centered at 1100 and 1250R in argon.

It has been established by Mulliken, Tanaka and Yoshina58'59 that

diatomic rare gas molecules have a large number of bound and repulsive

electronically excited states. The high resolution work of Tanaka of

the absorption spectrum of diatomic argon was investigated between 780-

1080o region. Nine discrete band systems were identified. Three sys-

tems were shown to have dissociative limits at the argon metastable levels

(Is), three to the 2p.levels, two to the 3d and 2s levels and ten very

diffuse but relatively strong bands of one system at the 3p levels.

In addition to the VUV continue, continue of molecular origins
3,60,61
has also been observed in argon at 1800R and 2250R. However, the

continuum at 1800R may be impurity related.61 A continuum has also

been observed in the positive column in an argon glow discharge centered

at 3100o which may be an extension of the 2250R continuum.62 A continuum

at 98808 has also been observed in an electron beam experiment at high
63
pressure. More recently, continuous emission has been observed at

4450R by alpha particle excitation,64 This last group also measured

the decay of the 2250A continuum versus pressure and adequately explained

their results with a model. These results have been confirmed by









by Tom Maguire and this author and conform to the proposed model of

argon (see Chapter IV).

For the present analysis, two levels of the argon molecule are

considered. The analysis of Birot et al. indicates that the upper

level of the 2250R continuum, Ar2", is formed by three-body collisions

with the 3p levels of argon, Ar(4). Their results indicate a transi-

tion probability of the Ar2" of 2.1 x 109 1/sec. The lowest excited

states of argon, Ar(1), will be rapidly converted by 3-body collisions

to a stable argon excimer, Ar21. Transitions from these eximers to the

repulsive ground state gives rise to the 1100R and 1250d continuum of

argon. The determination of a definite value for the radiative life-

time of the argon eximer is complicated by the existence of two low-lying

molecular states with different radiative transition probabilities. These

states can be mixed by collisions with electrons and other atoms to give

a lifetime that depends on the excitation conditions. The variation of

this lifetime has been treated by Lorents et a.65 For this study, a

single average value is used which should be adequate for the pressures

and excitation used in the present experiments, 2.5 x 107 I/sec.


Nitrogen


Considerable interest has developed in the excitation and ioni-

zation effects produced in nitrogen by electron impact. This is due

partly to the success of molecular nitrogen and Ar-N2 lasers and for the

interpretation of optical and ionization phenomena in the upper atmo-

sphere. 67, 68, 69 The pertinent energy levels of nitrogen and their

relation to argon is shown in Figure 16. The ground state of N2 is a

'E3 state further distinguished by the presymbol X. Previous work on
3







































10 a'll

o8 -
V3113


6
A3 +
u
4


2
1 1 1 +

0.0 0.4 0.8 1.2 1.6
Internuclear


Figure 16:


2.0 2.4 2.8 3.2
Distance (R)


3.6


Potential Energy Curves for Some Electronic
States of N2 and N2+









energy transfer from metastable argon to N has indicated that the trip-

let N2(C3u ) and N (B31 ).are the principal products, while the lowest

triplet N2(A3E+ ) results from subsequent cascade.70,71

The A3 lies 6.2 eV above the ground state and are referred

to as N2A in subsequent discussions. Transitions to the ground state are

very weak as they are of intercombination type. However, they are observed

under low pressure conditions such as the upper atmosphere, and are re-

ferred to as the Vegard-Kaplan bands.

Two strong band systems arise from transitions between triplet

states. They are known as positive bands because they show up prominently

in the spectrum emitted from the positive column of a glow discharge in

N2. The first positive bands arise from transitions between the B3

state, hereafter referred to as N2B, and the N2A that lies 1.15 eV below.

The second positive bands arise from transitions to the N2B(B 3 state)

from the C3u hereafter referred to as N C, that lies 3.7 eV above.

Weak emissions have been observed from electron-impact excitation of
nitrogen from the metastable E3+ state, hereafter referred to as N2E.7
g 2
This state, which lies at 11.87 eV above the ground state has been ob-

served in three forbidden electronic transitions, N2E N2A, N2B, N2C.

In the subsequent analysis of Ar-N2 mixtures, only the above

electronic states of N2 :are considered. The first negative bands of
2+ +
N2 arising.from transitions between the B Eu state of N2 and the ground

X 2+ state of the ion are not considered separate in the present

analysis, but may perhaps be included in later studies. Intense First

Negative Group Emissions are usually not observed.except at.low pressures

because they are easily quenched due to high collision cross sections

for ion-molecular.reactions. In the present analysis, we will only









consider the ground state of the ion which has an appearance potential of

16.6 eV.

Unlike most molecules, no stable negative ions are formed by

electron impact in nitrogen--neither N2 nor N ions are energetically

stable. This should simplify the recombination process for N2. However,

in early experiments on recombination in nitrogen, it was not possible

to identify the positive ions involved. An afterglow experiment in pure

N2 indicates that the main ions are N3 and N4 in the pressure range 0.1

to 7 torr and only at pressures less than 10-2 torr that N2 becomes the

only significant ion. The recombination coefficient of these ions has

not been measured and therefore, in the subsequent analysis, it is assumed

that recombination proceeds through the N2 ion.

Experiments by Mehr and Biondi73 have indicated that this is in-

deed the case. They measured the recombination coefficient, c, of N2 at
o+ -7 3
300 K to be 2.7- 0.3 x 10 cm /sec, and practically independent of gas

temperature. The measured variation of a with electron temperature was
39 +
proportional to Te .3 The recombination of N2 is again a dissociative

process similar to Ar2. The excited species produced

N2 + e N* + N (24)

has been investigated by Warke74 but the results are not of interest in

the present analysis. Here the recombination process is simplified by

assuming that dissociative recombination produces a single excited state

of atomic N. It is assumed that this excited state is the 2p2(3P)3s4p

level of N with a transition probability to ground state of 6.5x109sec-1 74,75

The energy deposition of protons in molecular nitrogen has been

evaluated by Edgar, Miles and Green.75 Their analysis uses the continu-

ous slowing-down approximation for an incident beam of protons of energy E.











Table 5

Properties of Nitrogen Excited States


Excited Average Transition e # Excited/lOOeV
Energy Probability Absorbed
(eV) (1/sec)


N2A 6.17 1.5 .034 0.55

N2B 7.35 1.1x105 .032 0.44

N2C 10.1 2.2x107 .025 0.25

N2E 11.9 1.3x103 .0036 0.03

Other Excited 11.5 .116 1.01
States

N2+ 13.2 .34 2.58
1






54


It is questionable as to whether their results are applicable in the

present situation, however, it is felt that they give a reasonable

approximation. The results for the states pertinent in the present

study are presented in Table 5.

The W value (eV/ion pair) from Table 5 is 38.76 which is in

good agreement for experimental values of 36.676 The Nex/Ni was cal-

culated to be 0.88 which is also in close agreement with other work

using alphas of 0.9.39














CHAPTER IV

DEVELOPMENT OF THE KINETIC MODEL


General Considerations


In order to evaluate the present experiment and so that these

results could be applied to other rare gas-secondary gas systems, a

kinetic model of the argon-nitrogen system is developed. From Chapter

III it is noted that as a fission fragment slows down in the gas, its

energy is lost by direct ionization and the production of delta rays

and that it is the delta rays and their secondary electrons that are

primarily responsible for excitation of the gas.

To evaluate the energy flow in the system, the interaction of

fission fragmentsis divided into three categories. First, is the exci-

tation of the primary gas, Ar, by the incident fragment and secondary

electrons. For the present analysis, the argon ion, Ar+, and six

groups of excited levels of the atom, Ar(1)-Ar(6) are considered. For

a more complete description of these quantities, the reader is referred

to Chapter III and Table 3. The second category to be considered is

effects taking place in the argon after the excitation is produced. An

excited level of the atom may be populated by cascading into the level,

collisional excitation by electrons and atoms, and dissociative recombina-

tion into the level. Deexcitation is by collisions with electrons or

atoms, spontaneous emission and collisional processes leading to forma-

tion of molecules dimerss). The formation of molecular ions and dissoci-

ative recombination processes was discussed previously and in the present









analysis two states of the molecular ion Ar2' and Ar2" are included.

Also, two levels of the molecular argon are considered. First, the

3p levels of argon, Ar(4), will rapidly undergo 3-body collisions to

produce a molecule Ar2", the upper level of the 2250A continuum observed

in the experimental studies. Secondly, the argon metastab les, Ar(1),

are also converted by 3-body collisions-to form a stable argon eximer,

Ar2. Transitions from these dimers to the repulsive ground state will

give rise to the 1250R continuum of argon.

The third category to describe the energy flow in the system is

due to the subsequent events caused by the addition of a second gas, in

this case nitrogen. The addition of nitrogen provides a path for

collisional transfer of excitation from the argon to the nitrogen. The

nitrogen has two additional effects on the kinetics. First, it may

change the electron energy distribution and secondly, it may collision-

ally relax the vibrational levels of the molecular ion. Both these

latter two effects undoubtedly change the excited species produced upon

dissociative recombination.


Collision Phenomena


The reactions considered are listed in Table 6. The rate con-

stants have units of cm6 /sec, cm3/sec and 1/sec for 3-body, 2-body and

spontaneous emission respectively with the exception of reactions 1-7 and

66-70 which have units of l/sec-eV absorbed. No attempt has been made

to describe accurately the nitrogen kinetics but only its effects as a

secondary gas in argon.

Clearly, to describe the system requires the knowledge of a large

number of reactions, For the present analysis, 76 separate reactions are







TABLE 6

Reactions in Argon-Nitrogen Mixtures
Argon Rate Coeff. Ref.
(cm3/eV-sec)
Direct Excitation

1. Ar + FF Ar + e + FF 1. 3.79E-2 38
2. Ar + FF Ar(6) + FF 2. 0.125E-2 38
3. Ar + FF Ar(5) + FF 3. 0.054E-2 38
4. Ar + FF Ar(4) + FF 4. 0.034E-2 38
5. Ar + FF + Ar(3) + FF 5. 0.284E-2 38
6. Ar + FF Ar(2) + FF 6. 0.303E-2 38
7. Ar + FF Ar(1) + FF 7. 0.685E-2 38

Spontaneous Emission (1/sec)
8. Ar(6) Ar(2) + hv 8. 2E6 35
9. Ar(5) Ar(2) + hv 9. 1E7 34
10. Ar(4) Ar(1) + hv 10. 5.5E6 34
11. Ar(3) Ar(2) + hv 11. 5E7 34
12. Ar(2) Ar(1) + hv 12. 1.3E8 .34
13. Ar2 Ar2 + hv 13. 2E8 .64
14. Ar2 Ar + Ar + hv 14. 2.5E7 61,65










Recombination

Ar+ + e Ar(1)
+"11
Ar2 + e Ar(6)
Ar2" + e Ar(5)
Ar2" + e + Ar(4)
+,,
Ar2 + e+ Ar(3)

Ar2 + e Ar(2)
+"11 *
Ar2 + e Ar +

Ar' + e Ar(3)
Ar+ + e Ar(2)

Ar+ + e Ar(1)

Ar+ + e Ar +
2


TABLE 6 (Continued)

(cm3/sec)


+ hv
+ Ar
+ Ar
+ Ar
+ Ar

+ Ar

Ar

+ Ar

+ Ar

+ Ar
Ar


2-Body
+" +'
Ar + Ar Ar + Ar
2 2
Ar(6) + Ar-* Ar(5) + Ar

Ar(6) + Ar- Ar(4) + Ar
+'
Ar(6) + Ar- Ar2 + e

Ar(6) + Ar Products


1E-11
0.15E-6 (10%)
0.1125E-6 (7.5%)
0.1125E-6 (7.5%)

0.75 E-6 (50%)

0.375e-6 (25%)
1.5 E-6

0.45 E-6 (1.5%)

0.045 E-6 (1.5%)
2.91 E-6 (97%)
3 E-6


Ref.

49
Estimated
Estimated
Data Fit

Data Fit

Data Fit
45 and Text

Data Fit

Data Fit

Data Fit
45




41

77

77

35,36,37,77

35,36,37,77


(cm3/sec)

1E-15

3E-11

2E-11

1E-10

1.5 E-10









2-Body (Continued)

Ar(5) + Ar Ar(4) + Ar

Ar(5) + Ar Ar2
+'
Ar(5) + Ar Ar2 + e

Ar(5) + Ar Products

Ar(4) + Ar Ar(3) + Ar

Ar(3) + Ar Ar(2) + Ar

Ar(2) + Ar Ar(1) + Ar

Ar(1) + Ar(1) Ar+ + e + Ar
I I +'
Ar2 + Ar2 Ar2 + e + Ar + Ar
2 2 2


TABLE 6 (Continued)

(cm3/sec)

31. 5E-11

32.

33. 5E-11

34. 1E-10

35. 5E-13

36. 5E-13

37. 5E-13

38. 5E-10

39. 5E-10


Ref.

77

Proposed

35,36,37,77

35,36,37,77

Estimated from Measurements

Measured

Estimated from Measurements

71,78

78


3-Body
Ar + Ar + Ar

Ar+ + Ar + Ar

Ar + Ar + Ar

Ar(4) + Ar + I

Ar(2) + Ar + J

Ar(1) + Ar + )


+"
Ar + Ar
2
Ar2 + Ar
2
Products

Ar2 + Ar

Ar2 + Ar

SAr2 + Ar
I


31.

32.

33.

34.

35.

36.

37.

38.

39.


(cm /sec)

2.5E-31



2.5E-31

1.2E-30

1E-32

1E-32


Data Fit

Data Fit

79

64

Estimated

71,78,80







TABLE 6 (Continued)


Ref.


Arqon-Nitrogen

46. Ar+ + N2 Ar + N2
S+ N + N
47. Ar + N Ar + N
2 2 2 2


Ar(6)

Ar(5)

Ar(5)

Ar(5)

Ar(4)

Ar(4)

Ar(4)

Ar(3)

Ar(3)

Ar(3)

Ar(2)

Ar(2)

Ar(2)

Ar(1)


+Ar(5) + N2

-Ar(4) + N2

+ Ar + N2E

- Products

-Ar(3) + N2

SAr + N2E

- Products

- Ar(2) + N2

+ Ar + N2E

+ Products

- Ar(l) + N2

-*Ar + N2E

- Products

SAr + N2C


46.

47.

48.

49.

50.

51.

52.

53.

54.

55.

56.

57.

58.

59.

60.

61.


(cm3/sec)

6E-12

1E-10


4E-10

4E-10


81

Data Fit

Estimated

Estimated

Proposed


48.

49.

50.

51.

52.

53.

54.

55.

56.

57.

58.

59.

60.

61.


+ N2

+ N2

+ N2

+ N2

+ N2

+ N2

+ N2

+ N2

+ N2

+ N2

+ N2

+ N2

+ N2

+ N2


4E-10

1.2E-10

3E-11

1.5E-10

1.2E-11

3E-11

1.5E-11

1.2E-10

3E-11

1.5E-10

5E-12


Data Fit

Proposed and 78

Data Fit

Inferred from Data Fit to Ref. 16

.Proposed and 78

Inferred from Data Fit to Ref. 16

Inferred from Data Fit in Ref. 16

Proposed

Inferred from Data Fit to Ref. 16

70








TABLE 6 (Continued)


Argon-Nitrogen (Continued)

Ar(1) + N2 Ar + N2

Ar(1) + N2 Ar + N2*
II
Ar2 + N2 Ar + N2B


Nitrogen

Direct Excitation

N + FF N2 + e + FF

N2 + FF N2E + FF

N2 + FF N2C + FF

N2 + FF N2B + FF

N2 + FF N2A + FF


Spontaneous Emission

N2E N2C + hv
N2E N2B + hv

N E N N2A + hv
2E Total
N E + Total


(cm3/sec)

2.5E-11

3E-11

lE-1l


(cm /eV-sec)

2.58E-2

.03E-2

.25E-2

.44E-2

.55E-2


(1/sec)

1.3E3

3.5E2

2E3

3.65E3


Ref.


70

78,82,83,84,85

73,78


72

72

72

72 o








TABLE 6 (Continued)


Ref.


Spontaneous Emission (Continued)

N2C N N2B + hv

2 2
N2B N2A+ hv

N2A N2(Ground)
N* N


2-Body


(1/sec)

2.2E7

1 .1E5

1.5

6.5E9

(cm3/sec)


N2E

N2C

N2C

N2B

N2B


N2A

N2A

NA

N2B


+ Ar

+ Ar

+ N2

+ Ar

+ N2

+ N2A
+ N2A

+ N2A
+ NA
+ N2A

+ N2B


89. N2 + e


- N2C = Ar

N2B + Ar
+ N2B + Ar

+ N A + Ar

+ N2A + Ar

N N2E + N2

\ N2C + N2

- N2B + N2

\ Total

SN2C + N2

- N + N


79. 3E-12

80. 8E-13

81. 1.5E-11

82. 4E-15

83. 2E-12

84. 1E-11

85. 1E-11

86. 8E-11

87. 1E-10

88. 1E-13

89. 3E-7


78

70,78

78,85

78,86

78,87,88

78

78,89,90

91

78,89,90,91

87
73,74
73,74


85

86

69

74,75









considered. The reactions.shown in the table neglect diffusion losses

which must be included for very low pressure studies. However, for

the range of pressures studied, diffusion losses are small for all of the

excited states of interest. Also neglected are electron deexcitation

collisions as well as electron excitation of an excited level from a

lower level.

Reactions 16-25 are certainly the most difficult to model accur-

ately. As discussed previously, no experiments have been performed to

measure the ratio of excited states produced in the dissociative recom-

bination process. It is established that dissociative recombination

populates all of the excited levels of the atom. Figure 17 shows a single

stable potential curve of the molecular ion and several repulsive branches

of the unstable excited molecule along with the pertinent excited levels

of N2. The rate coefficients for these reactions is assigned to agree

qualitatively with the arguments presented in Chapter III, and to agree

quantitatively with the observed experimental results.

Almost all of the excited states produced in argon are rapidly

converted to metastable atoms, Ar(1), either by recombination, collisions,

or spontaneous emission. Therefore, it is expected that these states will

dominate the energy transfer reactions.

The flow chart in Figure 18 summarizes the reactions transfer-

ring energy out of the excited argon into the N2 excited states. Several

excited atom transfer processes have been studied in low density condi-

tions and the rate constants determined. The rate constant for the 1s3
3
and Is5 state quenching by N2 seems well established by measurements in

several laboratories.70,82,83,84,85 The quenching rates of the Is2 and














A 2 3/2 2 1/2 N

igh X2 +
evels g

- 4 p
: 4 s

4- 4d
3p


SFig. 17. Potential Curves
for Art, Ar+,
and N
2


15







14







13


0)



z


2nd
PG


B3H
9










K




I -
2;













e


(n
cvn


Figure 18: Energy Flow Diagram for Ar-N2










Is4 states have recently been measured by Hurst and co-workers.92 All

of these states are produced in the excited argon and are treated here

as one state, Ar(1). For higher pressures, these states are most likely

in statistical equilibrium, but at low pressures this can no longer be

expected to be true.

The disposition of this energy among the states of N2 is much

less clear. Only for the 1s3 and 1s5 states has there been any detailed

investigation of the excited states produced,70 and these results indi-

cate that six N2B states are produced for each N2C. This same ratio is

used here as indicated by reactions 61 and 62. Previous modeling

attempts using these reactions could not adequately account for the ob-

served N2C excited state population.66'67'78 The difference was made up

by either increasing the value of reaction 61 or to allow some transfer

into the N2E state by metastable collisions. To more accurately account

for this difference, some transfer is allowed from the Ar(2) and Ar(3)

levels into the N2E state which is quenched rapidly, reaction 79, to

the C state.

Transfer from Ar2 to N2 has not been studied experimentally;

however, other researchers have given an estimate.7678 This rate con-

stant to produce N2B is indicated by reaction 65. It is likely that

some N2A is produced directly, but it would not be distinguishable from

that produced by cascade from N28. In any case, except at very high

pressures, little transfer from the excimer actually takes place.

Collisional quenching by argon and nitrogen dominates the cas-

cade from N2C to N2B. The C state quenching by N2 has a well established

rate constant,85 and the argon quenching rate was recently derived from

experimental measurements.78 The light produced in the N2C 2+ system








is certainly intense, but only at quite.low pressures does the radiative

decay by spontaneous emission compete favorably with collisional relaxa-

tion.

A similar situation occurs for the relaxation of N2B into N2A

but here the details of vibrational population are important for laser

application. The electronic quenching of the lower vibrational states

of N2B(v<4) is very much slower than that for the higher vibrational

states. The quenching rate given by Dryer and Perrer for N2B(v=0) by

N2 is 1.6 x 1012 cm3/sec.87 This value is an order of magnitude smaller

than that quoted by Young et al.88 which was from studies of higher vi-

brational levels(v>4). For the present analysis, the value measured

by Hill et a.78 for the quenching of N2B by nitrogen and argon is used.

The cascade of these higher N2 states produces in the end a large

population of N2A, which serves as an energy reservoir. The radiation,

decay and the quenching by ground state N2 and by argon are essentially

negligible. The only significant decay mechanisms are mutual destruc-

tion in collisions of N2A with another N2A, reactions 84-87. This pool-

ing reaction has been the subject of study by several investigators and

each gives quite different results.78'89'90'91 This reaction is diffi-

cult to follow experimentally since the N2A state cannot be easily ob-

served directly. The situation is further complicated by the possible

dependence of the reaction rate on the vibrational quantum number. This

indicates that the products of the pooling reaction is dependent upon

the vibrational relaxation rate of the N2A, which is pressure dependent.

Many of the.previous measurements of A-state pooling were made at low

pressures and the possible incomplete vibrational relaxation may explain

the differences in the measurements of the pooling rate coefficients.









The coefficients used in the present analysis were measured for pressures

of interest here, approximately one atmosphere.78


Computer Technique

The population of the species present can be found by solving a

system of nonlinear simultaneous equations. Initially, each species

equation is formed as a differential equation in time using all the

source and sink reactions for that species. Then the differential equa-

tion is set to zero for the steady state case. This results in a system

of equations of the form

F (NIN2'...Nn) = 0 i = l,n (25)

For example, the equation to solve for the Ar density utilizes reac-

tions 1, 15, 38, 42, and 46 and is of the form

dAr- 0 = F (N) = k(1)*Ed*Ar k(15)*Ar+*n + k(38)*[Ar(1)]2

k(42)*Ar *[Ar]2 k(46)*Ar *N2 (26)

In the above equation, k(1) is the number of Ar+ formed per eV

absorbed in the gas and is, therefore, independent of pressure. The

quantity Ed is the energy deposited per total number of particles found

by solution of equations (9), (10) or (14). Thus the quantity Ed*Ar

represents the energy deposited in eV in the argon where Ar is the ground

state density. This equation represents one of the simplest in the

Ar-N2 system and has 5 unknowns.

The Ar-N2 system is thus described by 20 equations, represent-

ing 20 species, and 89 rate coefficients. The equations are solved

using the method of a Newton-Raphson iteration method for nonlinear

simultaneous equations. Defining the vector N = (N1,N2,...N ), we can









write F.(N) = (F (N),F (N),...F (N) If a new variable is now defined


8F.(N)
fi (N) (27)
1

then an n x n matric, P(N), results, where

P(N) = (f i(N)) 1
Thus, the det(P(N)) is the Jacobian of the matrix Fi(N) evaluated for

the vector N. Starting with N where the superscript indicates an initial

guess

N+l = + dNk. (29)

The quantity dn is the solution vector for the set of simultaneous linear

equations,

P(N)dNk = 1 F(N ). (30)
-k -k
As k-*c, F(N ) approaches zero and thus N approaches the vector (N)so1

where F(N) = 0. The initial problem is reduced to the more trivial case

of solving a set of linear simultaneous equations.

The next problem that arises is when to stop the iteration process.

There are three criteria:
K+1 k k+l
1. When (N K+-N )/Nk+ = ESPI, where ESPI is some allowable error

term. The difference between two successive iterations must go to zero if

the process is to converge. If convergence is very slow or round off

error is very severe, it is possible that the difference between two succes-

sive terms is very small even though not close to the right answer.

2. In addition, therefore, F(N ) must be less than some ESP2.

3. Finally, a limit must be established on the number of itera-

tions in case of no convergence because of time and money.






70


The resultant computer code is listed in the Appendix and solu-

tions were obtained for a variety of mixture ratios and pressures.














CHAPTER V.

EXPERIMENTAL RESULTS


The primary interest in the present studies is the interaction

processes between the primary gas argon and various concentrations of a

secondary gas nitrogen. Each MCFIG investigated was filled to 760 torr

with the desired research grade gas at ambient temperature. The gases

investigated were argon, nitrogen, and the mixtures 10%, 0.1% and 0.01%

nitrogen in argon. In addition, one MCFIG was investigated with an un-

known fill of nitrogen and possibly other impurities due to errors in

the gas-handling procedure. The subsequent data analysis indicated that

this MCFIG contained approximately 20 to 50 ppm nitrogen in 760 torr of

argon.

Argon


The predominant emission of argon .is in the region between
0
6900A to 8500A and a large continuum centered around 2250R. In addition,

a large number of lines, mostly ArII, are observed from 2900A to 5200A.

The only impurity emission that can be identified is the (0,0) band

of OH at 3064A (A2 + X2). The impurity analysis supplied with the

gas indicates less than 0.15 ppm water. It requires 5.09 eV to disso-

ciate HOH and 4.05 eV to excite the OH to the A2E+ level. The first

excited level of argon is at 11.7 eV (Ar(1)) and it is probable that

this relatively strong OH emission at 3064A is excited either by colli-

sions or by resonance radiation from these levels. However, one must not









preclude the possibility of effects from subexcitation electrons.

The observed argon spectrum was thoroughly investigated by

Davie15 and will not be repeated here. Over 20 Arl lines and over 44

ArII lines were reported in the region between 2200 to 8400A. The data

used in the excited state population calculations was taken at a point

3.9 mm from the source where the spectral intensities were highest.

The calculated energy deposition rate at this point for a flux of 1.6 x
12 2 17 3
10 n/cm -sec is -1.6 x 1017 eV/Cm3-sec. However, this does not take

into account the flux depression due to the presence of the stainless

steel MCFIG and its fissioning source. These effects are estimated to

reduce the energy deposition by a factor of approximately four.93 The

calculated populations of the 3-dand 2-plevels is approximately 2 x 104
5 3
and 5.1 x 10 atoms/cm3, respectively. These populations are accurate

within a factor of ten. The population of the Ar" which gives rise to

the 2250 continuum could only be estimated, since the system calibration

was accurate only above 3000R. Using a transition probability of 2 x

10 /sec., the Ar" population are approximately 1.5 x 106/cm Most of

the energy of excitation results in the production of an Ar(1) atoms of

~11.7eV. These excited argon atoms form, by three body collisions with

two ground state atoms, an argon excimer, Ar2, which gives rise to the

1250A continuum of argon. Approximately 50% of the energy deposited is

emitted at 1250A, whereas only 3% is emitted at 2250A and less than 0.3%

from the argon 2p levels.


Nitrogen


The emission spectra of nitrogen is. familiar to all spectro-

scopist and fission fragment excited nitrogen produced no surprises.









The most prominent emission is from the nitrogen second positive band

system. Although a considerable amount of excited ions are formed by

the passage of the fission fragment only the (0,0) and (0,1) bands of
the N2+ first negative band system are observed with only fair inten-

sity. No emissions from any other excited ion or from the first posi-

tive group (N2B N2A) are observed. Table 7 lists the observed spectrum

and the relative intensity of the emission bands. A relative intensity


of 1000 corresponds to 2.21 x

The dominant inpurity

CN violet system as noted in

and it is well known to serve

tion by the reaction

N2A(~6.2eV) + NO

NO(A2 +)
where k is 8 x 10-11 cm3/sec

lation is estimated to be 7 x


1012 transitions/sec.

emission is from the NO y.-bands and the

Table 7. The NO emission is very intense

as a good tracer of the N2A state popula-


k1 NO*(X5.5eV) + N2 (31a)

k2 NO(X2 ) + hv (31b)

and k2 is 5 x 106/sec. The NO(A2 +) popu-

105/cm3. From the impurity analysis sup-


plied with the gas, a NO concentration of 0.01 ppm is estimated. Thus,

the N2A population is on the order.of 2 x 10 /cm3


Argon-Nitrogen

As the concentration of nitrogen is increased in argon several

phenomenons can be noted. The nitrogen second positive band system

rapidly increases in intensity while the argon spectrum decreases in
intensity. Figure 19 indicates the population of the 2p level of argon,

(Ar(2)), and the N2C state versus nitrogen concentration. Error bars
indicate accuracy of relative populations. Absolute populations are













Table 7: Observed Spectra of 760 torr Nitrogen


Wave-
Length (A)


V' V"


Relative
Intensity


Wave-
Length (A)


N2-Second

3371.3 0
3576.9 0
3804.9 0
4059.4 0
4353.6 0
4667.3 0
3159.3 1
3339 1
3536.7 1
3755.4 1
3998.4 1
4269.7 1
2976.8 2
3136.0 2
3309 2
3500.5 2
3710.5 2
3943.0 2
4200.5 2
2962.0 3
3116.7 3
3285.3 3
3
3671.9 3


N2 1st
B2 +
u


3914.4
4278.1


Positive Group


1000
516
174
43
8
2
248
Obscured
64
50
17
7
17
49
2
5
10
6
2
14
13
2

2


Negative Group
SX2E +
g


0-0
0-1


NO system
A2 ++ X2
A -* X fl


2262.8
2269.4
2363.3
2370.2

2471.1 >
2478.7

2587.5
2595.7

2713.2
2722.2


0 0

0-1


0-2


0-3


0-4


CN Violet System

B 2 A 2


3850.9
3854.7
3861.9
3871.4
3883.4
4152.4
4158.1
4167.8
4181.0
4197.2
4216.0


VI VII


Relative
Intensity


468

692


340


160


































































Nitrogen Concentration (ppm)


Figure 19:


Measured Populations of Argon 2p Level and N C State
Versus N2 Concentration. Populations accurate within
a factor of ten.


h









accurate to within only a factor of ten. The intensity of the OH ,3064R

band system observed in pure argon rapidly decreases in intensity with

the addition of nitrogen. Since both water and nitrogen are competing

for excitation by the same mechanism, transfer from Ar(1), and since the

water concentration is small, the OH spectrum intensity does increase.

The CN violet system shows the same intensity variation with

nitrogen concentration as the nitrogen second positive system. The

excited levels lie 4.31 eV above the ground state and the first excited

state of nitrogen is at 6.3 eV and metastable. Therefore, it is reason-

able to assume that the N2A state collisionally transfers to the CN

molecule similar to NO in reaction (31). The NO-ybands are only observed

in the pure nitrogen indicating that the NO concentration is a direct

function of the nitrogen number density. Therefore, for 10% nitrogen

in argon the number density of NO is down a factor of 10 from the pure

nitrogen and the NO emission should not be observed.

Figures 20 and-21 illustrate the populations of the different

2p levels versus the nitrogen concentration. Unfortunately, emissions

from the 2p10 level cannot be monitored with the present experimental

apparatus and the populations can only be estimated.

For low concentrations of nitrogen, the 2p2 level of argon seems

to be enhanced relative to the other 2p levels. However, as the nitrogen

concentration is increased, this level is quenched more rapidly. Thus,

the relative magnitude of the observed argon 2p emission lines changes

rapidly with the addition of nitrogen. This indicates that a substantial

fraction of the 2p2 level is populated by dissociative recombination.

Table 8 indicates the relative intensity of the nitrogen bands

arising from the v'=O vibrational level of the N2C state. The good
























STotal 2p
-- 2D_


0
I _II.I I'


102 103 104
Nitrogen Concentration (ppm)


Figure 20:


Measured Populations of Argon 2p Levels versus
N2 Concentration. Populations accurate within
a factor of ten. Relative Populations accurate
within 30%.


106


5



2


105


100
























Total 2p


2p5


I I I I I I I
100 101 102 103 104 105 106
Nitrogen Concentration (ppm)
Figure 21: Measured Populations of Argon 2p Levels versus
N2 Concentration. Populations accurate within
a factor of ten. Relative populations accurate
within 30%.















Relative


Table 8

Intensity of Second Positive Group of N2


wavelength v' v" N? Concentration (ppm)
50 102 103 105 106

3371.3 0 0 10 10 10 10 10

3576.9 0 1 5.9 5.2 5.2 5.0 5.16

3804.9 0 2 2.1 1.9 1.9 1.7 1.74

4059.4 0 3 0.55 0.43 0.42 0.43 0.43

4343.6 0 4 0.11 0.092 0.083 0.082

4667.3 0 5 -- 0.016 0.014 -







Table 9

Relative Populations of Second Positive Group of N2

N2 Concentration Population v'
(ppm) (cm-3) 0 1 2 3
v' = 0

50 1.69x104 1.0 .12 0.07

102 1.15x105 1.0 .115 0.041

103 4.87x105 1.0 .114 0.032 -

105 1.52x106 1.0 .129 0.027 0.0035

106 3.95x105 1.0 .221 0.049 0.0016









agreement between data runs supports the validity of the data and ab-

sence of additional radiation damage to the optical system. The rela-

tive N2C vibrational state populations for different concentrations of

nitrogen is tabulated in Table 8 and represented graphically in Figure

22 for 10% nitrogen in Argon and 100% nitrogen. Clearly indicated is

a relative enhancement of the v'=0 populations which results from the

collisional transfer process from Ar(1) or cascades from the N2E state.

Figure 23 indicates the N2C vibrational state populations versus nitro-

gen concentration. The error bars indicate accuracy of relative popula-

tions.

Temperature Effects


As indicated in Chapter 2 and Figure 8 the MCFIG heats up during

reactor operation. The ambient gas temperature reaches a steady state

temperature of about 1000C. Figure 24 shows the relative intensity of

the 3371A band of nitrogen, 6965A Arl, the 2250A continuum of Argon and

the 3093A OH impurity emission versus temperature. For collision in-

duced phenomena the temperature variation is explained by the change in

the collision frequency which is a function of the form

v(collision frequency) ( (#density) x (relative velocity). (32)

From the perfect gas law and by assuming that the gas particles are in

a Maxwell-Boltzmann velocity distribution, the collision frequency is

proportional to
1 3
S T 2 T 2
v x or -x (33)
T T T
0 0 0
o o o

Thus, as the capsule heats up the relative intensity of the 33712 band

of nitrogen and the OH impurity emission should increase by the factor





















































11.0



Figure 22:


11.5 12..0

Energy (eV)

The Two Curves Represent the Relative
C3 u Vibrational State Populations (Nv')

for 100% N2 and 10% N2 in Argon at 760
torr.


1.0


.5



.2


0.1


0.05




0.02


0.01


0.005


0.002


0.001
























































Nitrogen Concentration (ppm)


Figure 23:


Measured N2C State Vibrational State Popula-
tions versus N2 Concentration. Populations
accurate within a factor of 10.









1.5 -

1.4 -
1.3 3371 Nitrogen


1.2

1.1

" 1.0

-" -6965A Arl
S2250 Ar
- .8

.'7 3093A OH
.6 0o

.5

.4

.3


20 30 40 50 60 70 80 90 100 110

Temperature (oC)

Figure 24: Temperature Dependence of Spectral Emissions for 103 ppm N2 in Argon









3
400k 2 = 1.54 (34)
3000k
The agreement is excellent for the nitrogen emission but apparently

other effects must be considered to explain the OH emission.

The 6965R ArI line and the 2250A argon continuum care not as

dependent upon collision phenomena with ground state atoms. However,

if recombination effects are considered, the rate of recombination

changes with the gas temperature. From Chapter III the recombination

coefficient, a, is proportional to T- If the 6965A line and

the 2250 continuum intensities are determined by recombination effects,

their intensities should be reduced due to the rise in temperature

by the factor


400 067= 0.825 (35)

in excellent agreement with the observed data.

Spatial Effects

As indicated earlier, all data sets were taken at a point 3.9mm

from the source where the spectral intensities for pure argon were

highest. This assures that the energy deposition by the fission frag-

ments for a given pressure was a constant. Unfortunately, detailed

spatial information was obtained for only pure argon, pure nitrogen and

0.1% nitrogen in argon. Figure 25 indicates the relative intensity of

selected argon emissions and the nitrogen second positive group emission

versus distance from the source where all intensities have been normal-

ized to 1.0 at a distance of 3.9mm. Also indicated is the spatial vari-

ation of the volumetric energy deposition rate.










6965R (Ar2p2)




0 (Ar2pl)
- 7504A

- 2250R (Ar2")



6965R (Ar2p2)


0
3371A
N2


/ dEd(x)
I dV


a~1 a I I I g


I -I~


Figure 25:


Spatial Variation of Relative Intensities.
3.9 mm, Dashed at 5.8 mm)


12 14 16
for 103 ppm N2 (Solid Lines Normalized to 1.0 at


1 _I I I I I i -' '- -' -


=-









For distances closer than 3.9mm all the observed intensities

decrease rapidly, indicating that the region sampled by the monochro-

mator is being reduced by the source itself. For distances greater

than 3.9mm the argon atomic emissions are enhanced relative to the nitro-

gen.

Emissions from the argon 2p2 level at 5mm from the source were

approximately 25% more intense than at 3.9mm even though the volumetric

energy deposition rate was down approximately 20%. All the other 2p

levels (except the 2pl0 which is not observed) behaved similar to the

2pl level plotted. Referring to Figures 20 and 21, note that with in-

creasing nitrogen concentration that the 2p2 level is quenched more rapid-

ly than the other 2p levels. Thus, not only does the relative magnitude

of the observed argon 2p emission lines change rapidly with the addition

of nitrogen .but there is also a spatial variation.

Above approximately 6mm from the source, the argon emission be-

gins to decrease at approximately the same rate and effectively follows

the volumetric energy deposition rate. This does not appear to be the

case for the nitrogen emission. Figure 26 indicates the relative spatial

intensity of the nitrogen emission for 0.1% nitrogen in argon and 100%

nitrogen. Clearly, the nitrogen emission decreases more rapidly for

0.1% nitrogen in argon than for the 100% nitrogen case, where again the

relative intensities in the latter case are effectively following the

volumetric energy deposition rate.

Referring again to Figure 25, the dashed line is the relative

intensity of the 2p2 level normalized to 1.0 at 5.8mm from the source.

Clearly, this level is being effectively quenched relative to the other




















3371 N2 )
o 1N2 100% N2
3914A2 2


0
3371A
N

10 3ppm








S I I I I I I I


1 2 3 4 5 6 7 8 9 10 11 12 13 1

Distance from Source (mm)
Figure 26: Relative Spatial Dependence of N2 Emissions


4 15 16 17


- - --


, .I









2p levels as the volume sampled becomes closer to the source. The ob-

served spectra close to the source is influenced by the electron energy

distribution which is skewed to lower energies and, thus, affecting

the dissociative recombination rate. In addition, the gas temperature

changes rapidly close to the source and this spatial distribution is a

function of the gas mixture. Temperature effects on observed intensi-

ties is discussed in the last section. Clearly, the 2p2 level intensity

variation with nitrogen concentration as shown in Figure 20 and the

spatial variations are coupled and dependent upon recombination and

collision phenomena. The spatial variation of gas temperature may also

affect the nitrogen emission as observed in the argon-nitrogen mixtures

since the emissions are entirely dependent on collision processes and not

direct excitation.


Measurements of Rate Coefficients


During the past year, a photon counting system has been developed

to study the excitation of various gases by fission fragments emitting

from a Cf252 source.94 By observation of the afterglow decay emissions

of the excited states produced, it is possible to determine

1. lifetimes of excited states,

2. rate coefficients for vibrational and electronic de-excita-
tion,

3. collisional transfer coefficients, and

4. electron-ion recombination coefficients.

In the argon-nitrogen mixtures, the photon.counting system

could observe the stronger emissions from the 2p levels of atomic argon,

the 2250R continuum of argon and the nitrogen second positive bands.




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