Ionization gage behavior in various gas mixtures and atmospheric composition

Coad, D. E.

Culton, J. W.

Peacock, R. N.

Propst, F. M.

Tibbetts, G. G.

English

NASA Technical Reports Server

REPORT 
Required under the terms of NASA Research Grant NsG 376 
"Theoretical and Experimental Studies 
of the Underlying Processes and Techniques 
of Low Pressure Measurement" 
June 1, 1967 to November 30, 1967 
Personnel: 
R. N. Peacock D. E. Coad 
F. M. Propst G. G. Tibbetts 
J. W. Culton 
December 1, 1967 to May 31, 1968 
Per sonne 1 : 
R. N. Peacock D. E. Coad 
J. W. Culton 
'3 
u. 
1 
1 
I (THRU) I 
The research in this letter report was supported in major part by NASA Research 
Grant NsG 376 and in part by the Joint Services Electronics Program (U.S.Army 
Electronics Laboratories and U.S.Army Research Office, Office of Naval Research 
and the Air Force Office of Scientific Research) under Contract DAAB 07-67-C-0199 
until the end of 1967 and during the remainder of the period, solely by the 
Joint Services contract. 
https://ntrs.nasa.gov/search.jsp?R=19680023820 2020-03-12T06:33:35+00:00Z
1 
IONIZATION GAUGE STUDIES I N  LOW PRESSURE ATMOSPHERES 
S tud ie s  repor ted  he re  were suggested by George Newton of NASA t o  
confirm t h e  behaviour of NASA t o t a l  p re s su re  gauges while  i n  f l i g h t .  The 
problem concerned gauge errors caused by the d i f f e r e n t  atmospheric composi- 
t i o n s  encountered i n  f l i g h t  and t h e  e f f e c t  of t hese  compositions on gauge 
su r faces .  The r e sea rch  e f f o r t  p rev ious ly  performed under NASA g r a n t  NSG 376 
w a s  a l t e r e d  t o  approach those  a spec t s  of t h i s  problem t h a t  were wi th in  t h e  
scope of p re sen t  l abora to ry  f a c i l i t i e s .  B r i e f l y ,  t hese  a s p e c t s  are: 
1. gauge p res su re  i n d i c a t i o n  i n  gas  mixtures;  
2. su r face  i o n i z a t i o n  e f f e c t s  i n  t h e  f l i g h t  modulated 
B-A gauge; 
3. re la t ive  s e n e s i t i v i t i e s  of gauges i n  d i f f e r e n t  gases.  
A vacuum system capable  of low p res su res  and accommodation of t he  f l i g h t  
gauges,  commercial B-A gauges, and Scheumann type gauges w a s  cons t ruc t ed  
t o  do t h e  above tes ts .  
It w a s  suggested by CSL t h a t  the omegatron-Klopfer i o n i z a t i o n  gauge 
p rope r ly  d i sp layed  t h e  p r o p e r t i e s  of a monoenergetic i o n i z i n g  device.  With 
r e f e r e n c e  t o  a n  abso lu te  p re s su re  sca l e  and acceptance of publ i shed  ion iza-  
t i o n  c ros s - sec t ion  d a t a ,  t h e  omegatron could be used i n  a t o t a l  p re s su re  mode 
as a r e l a t ive  s tandard  between d i f f e r e n t  gases  and mixtures  of gases.  F l i g h t  
models of a modulated B-A gauge and a Redhead type gauge t h a t  had previous ly  
been extensively ralihrated i n  n i t rogen  were obtained from NASA through George 
Newton and David Pelz.  These gauges provide the  r e fe rence  t o  abso lu te  pres-  
s u r e  on t h e  n i t rogen  scale and are t h e  p r i n c i p a l  s u b j e c t s  of t h e  proposed 
s tudy  i n  t h e  atmospheres and pressure  ranges agreed upon wi th  NASA. 
With b i a s  t o  achieve t o t a l  ion  c o l l e c t i o n  and a monoenergetic e l e c t r o n  
beam, t h e  omegatron i n  a t o t a l  p ressure  mode should d i s p l a y  s e n s i t i v i t i e s  t o  
2 
gases that are in the same ratio as the ionization cross sections for those 
gases. To illustrate this, an I+/I- versus electron beam energy plot (hold- 
ing constant pressure in a pure atmosphere with constant ionization path length 
and constant ion collection efficiency) would be a relative ionization cross- 
section curve. Such a curve is perturbed by secondary electrons which are 
emitted from metal surfaces-- e.g., the electron collector-- and increase the 
ion yield, shifting the electron energy scale, and thus distort the cross- 
section (sensitivity) curve. Figure 1 illustrates data taken at constant 
filament emission using 200 V electrons in argon. Ideally, I+/I- is a 
constant. However, as the electron collector potential approaches that of 
the preceding element, secondary electrons proceed back along the beam path 
(confined by a magnetic field) causing additional ionization. The bottom 
curve shows the effective decrease in electron current to the electron collec- 
tor. This is accompanied by increases in electron current to elements further 
back down the path. The top curve, in effect, shows the variation in sensi- 
tivity as a function of electron collector voltage. This could account for 
omegatron sensitivity variation with electron collector potential and reported 
sensitivity ratios differing from ionization cross-section ratios. 
A device similar to an omegatron, but with additional features for total 
ion collection and entrapment of secondary electrons, was constructed to 
permit ionization by a monoenergetic electron beam. It displays sensitivi- 
ties to gases in the same ratio as the ionization cross section for those 
gases. 
Figures 2 and 3 along with normalized total ionization cross-section curves. 
Normalized I+/I" versus electron beam energy curves are shown in 
1 
The curves were taken at constant pressure and are for electron beam confining 
fields of 600 and 3500 gauss. The higher magnetic field crossed with the ion 
drawout electric field effectively changes the collected ion current. To 
3 
overcome this problem an electromagnet was constructed, permitting the 
device to be used either as a definitive total pressure apparatus or as an 
crmegatron type of partial pressure analyzer. The latter mode of operation 
requires the higher magnetic field for resolution. 
Although there is no attempt to relate the CSL device to any absolute 
pressure scale, Figures 4 and 5 show the device to be linear in argon and 
heli.um, respectively, in comparison to an ordinary B-A gauge. 
at approximately ~ x ~ O - ~ O  Torr in this system indicates that x-ray currents 
Data taken 
caused by electrons striking ionizing region apertures will be well below 
the ion currents in pressure regions of interest. In the partial pressure 
mode, no attempt was made to achieve high resolution since simple gas mix- 
tures are involved. 
At this time: 
1. The system has been constructed and functions well. 
2. All gauges have been installed. 
3.  The measuring instruments are properly calibrated. 
4.  The CSL device functions in its partial pressure and 
total pressure modes. 
5. Preliminary sensitivity runs have been made in noble 
gas atmospheres. 
6. A program is being written for the CDC 1604 computer 
at CSL to handle the data reduction. 
Surface Physics 
S t u d i e s  on adsorption and dzscrption of I$, N2, szd co fro= pdycrys ta l l fne ,  
(Ill), and (110) tungsten have been completed. 
thesis by G. G. Tibbetts. 
This work has been written as a 
Rapp, Englander-Golden, J. Chem. Phys. 43 ,  1464 (1965). 1 -
4 
Voltage - - 
t 
" Box " 
Voltoge Voltage 
t 
GI 
2 -  - 
I I I I I I 10-6 - 
I+/ I- 
KR-231 
Figure 1. The effect upon omegatron behaviour of secondary electron 
emission from the electron collector. 
CSL device with 600 gauss 
electron beam containing field 
CSL device with 3500 gauss 
electron beam containing f ield 
data in  ref. 1 
_._._ 
---- Ionization cross section from 
I I I I I I I I 1 I I 1 I 
-I 100 200 
Electron volts KA-250 
Figure 2. Argon ions per ionizing electron (normalized) versus ionizing 
electron energy. 
6 
a 
1 .o 
0.9 
0.8 
0.7 
1 0 . E  
0. : 
0. 
C 
CSL Device with 600 gauss 
electron beam containing f ield 
-.-.- CSL Device with 3500 gauss 
electron beam contoining field 
---- Ionization cross section from 
data in ref. 1. 
I I I I I I I I I I I I I 
, 100 200 300 
Electron volts K R - 2 5 1  
Figure 3. Helium ions per ionizing electron (normalized) versus ionizing 
electron energy. 
c 
7 
Argon Atmosphere - 
4 
m 
+ 
H 
10-16 I I ! I ! ! ! !  ! ! ! I  I I 1 I I I l l  I I I I I ~- I 
10-4 10-5 10-6 10- 7 
I+ Omegatron-Klopfer gauge LR- 236 
Figure 4. B-A gauge ion current versus omegatron-Klopfer gauge ion 
current in argon using constant emission currents. 
8 
Helium A t m o s p a  
I + Omeqatron -Klopfer gauge u-235 
Figure 5, B-A gauge ion current versus omegatron-Klopfer gauge ion 
current in helium using constant emission currents. 


Theoretical and experimental studies of the underlying processes and techniques of low pressure measurement Letter report, 1 Jul. 1967 - 31 May 1968

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