AN APPARATUS FOR MEASUREMENT OF ELECTRON

70 Pages·2005·7.08 MB·English

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ORNL-TM- 1444
-
Contract No. W 7405-eng-2 6
HEALTH PHYSICS DIVISION
AN APPARATUS FOR MEASUREMENT OF ELECTRON ATTACHMENT AND
ELECTRON SWARM DRIFT VELOCITIES AT HIGH TEMPERATURES
W. G. Hendrick, L. G. Christophorou, and G. S. Wurst
Submitted by W. G. Hendrick a thesis to the
a5
Graduate School of the University of Tennessee
in partial fulfillment of the requirements
for the degree of Master of Science
FEBRUARY 1 968
OAK FtIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee
operated by
UNION CARBIDE CORPORATION
for the
U. S. ATOMIC ENERGY COMlMISSION
3
4456 0549314 5
ACKNOWLEDGEM EN TS
The authors wish acknowledge the $. A. Har-i-er,
to assistant; of
D. Pittman, and J. T. Cox.
TABLE OF CONTENTS
CHAPTER PAGE
I. GENERAL INTRODUCTION 1
11. EXPERIMENTAL EQUIPMENT 11
Chambers 11
Vacuum, Gases, Purification 18
Electronics and Readout 22
Data
28
Hea thg Sys tern
111. FUNCTIONING OF THE APPARATUS AND
EXPERIMENTALPROCEDURE 36
RESULTS AND DISCUSSION 41
Iv.
Electron Attachment to SF 41
6
Temperature Dependence of the Attachment
Rates
in SF 43
6
Electron Drift Velockties in Nitrogen and Ethylene
a5 a Function of Temperature 47
Drif-t velocities nitrogen 47
in
52
Tkift velocities in ethylene
Velocity Dependence and Magnitude. of the Cross
52
Section for Momentum Transfer Ethylene
in
V. CONCLUSIONS .5 7
58
BIB LIOG PHY
RA
...
111
LIST OF TABTJES
TABLE PAGE
I.
I. A ttachnient Cross Sections for SF 46
6
11. Electron Swarm Velocities in Nitrogen at
Drift
Various Temperatures 48
111. Electron Swarm Drift Velocities Ethyl,o ne
in at
Various Tempera tures a9
rv. Temperature Dependence of w(T) for Various Values of N 54
iv
LIST OF FIGURES
FIGURE PAGE
1. Schematic of Electron Attachment a Uniform Electric
in
Fie Id 5
2. Calculated Dependence of Pulse Height on T /t for
0 1
Various Values of f 7
3. Ionization Chamber for Measurement of Attachment
Coefficients and Electron Drift Velocities at
High Temperatures 12
High Temperature Electron Attachment Apparatus 13
4.
5. Photograph of Equipment 14
6. Circuit of Preamplifier Used Electron Attachment
GI
Studies 24
7. a) Ultraviolet Xenon Light Source Schematic; b) Photodiode
Schematic 25
8. Diagram of System 32
Heating
9. Diagram of Heater Wiring 34
10. Measured Response of the Linear Amplifier 37
11. Temperature Dependence of the Electron Attachment
Rates in SF6 44
vi
FIGURE PAGE
12. Electron Drift Velocities Nitrogcri versus E/P for
in
298" and 473" 50
M.
13. Electron Drift Velocities Ethylene versus E/P for
in
si
298" and 473" K.
14. Experimental and Calculated Values of w (for o! 7 0) as
a Function of Temperature for Various E/P 56
CHAPTER
I
GENERAL INTRODUCTION
Free electrons of low energies (< 10 eV. ) interact readily with coni-
plex molecules. [See, example, Christophorcu and Compton (1967). ] Free
for
electrons a t these very low energies can be produced as an electron swarm of
knwn electron energy distribution or as a monoenergetic electron beam.
The nature of the electron-molecule interaction is not yet fully
understood in spite of its importance in the fields of radiation physics,
radiation chemistry, and radiation biology. Although the behavior of free
electrons in a gas may be different from that in a biological system, know-
ledge of the electronic properties of biologically important molecules in the
gas phase is fundamental and basic.
In electron attachment studies it is extremely important to distin-
guish between the various ways by which an electron can be attached to a
molecule. Three such ways can be distinguished in the gas phase.
1. Temporary nondissociative electron attachment,
-*
AX-te 2 AX (1)
The lifetime of the temporary negative ion is determined
internal rearrangement and by molecular collision. If
by
an equilibrium is reached, the ion density, ni/ne, is
1
2
determined by the molecular electron affinity, Eo, i. e. ,
E /kT
0
n./ne = const e
1
2. Permanent nondissocia tive electron attachment,
-*
AX 4 e +AX -+AX- t energy. (3)
__
The excited negative ion, AX :t. , has to be stabilized by
radiation or by collision.
3. Dissociative electron attachment,
No collisional stabilization is requk-ed for this process.
Frequently, the electron attachment processes are studied by two
experimental methods: the electxon swarm and the electron beam. In the
electron swarm method electrons are set free in a gas by various means,
such as ionization by energetic charged particles, ultraviolet radiation, or
thermionic emission. These free electrons make many collisions with the
atoms OK the molecules of the gas through which they pass in the presence
of an electric field, and they have a wide distribution in their kinetic
energies. Regardless of their energy of liberation they come into an equi-
librium energy distribution which is characteristic of the gas through which
they travel and the ratio E/P, the pressure reduced electric field
Li
-1 -1 .
volts Cm. torr This equilibrium energy distribution results