168 
sider a series of uses where the gage is 
adapted to be a thickness-by-transmis- 
sion gage, a thickness-by-reflection 
gage, and a concentration-by-reflection 
gage. 
Equipment. The gage detector is a 
cylindrical ionization chamber with a 
thin stainless-steel window for beta- 
particle entry. The ionization current 
from the chamber is measured by 
a stable, one-stage, bridge-type d-c 
amplifier. 
The very small ionization current 
(~2 X 10-*) is put through a large 
resistance (~5 X 10° ohms) so the 
signal produced by the total source is 
10 volts. This negative voltage is 
opposed by a positive voltage used for 
adjusting the operating point of the 
bridge circuit. The net sum of these 
voltages is applied to the grid of the 
electrometer tube as a signal in excess 
of its proper bias voltage. The elec- 
trometer tube forms one arm of a 
bridge (balanced when the proper bias 
voltage alone is applied to the elec- 
trometer tube). A variable sensitivity 
microammeter is set across the bridge 
to indicate any unbalance. The full- 
scale reading of the microammeter (at 
maximum sensitivity) is 20 wa, and 
the inherent ‘‘accuracy”’ of the meter, 
6M, is about 0.4 wa. The effective 
transconductance of the electrometer 
tube in the bridge circuit is about 
80 wa/volt. The time constant of 
the gage is 2.0 sec. 
The beta particles from strontium-90 
of maximum energy 0.6 Mev are 
almost completely absorbed by the 
combination of absorbers comprising 
the stainless-steel source cover and 
the window of the ionization chamber. 
Therefore, the effective beta radiation 
from strontium-90 is actually that 
from its daughter, yttrium-90, which 
emits beta particles with a maximum 
energy of 2.2 Mev. 
Coefficients. With this arrange- 
ment, it is found that for a 
Thickness-by-transmission gage 
Ma = 0.0045 (mg/em?)—! 
Thickness-by-reflection gage 
by = 0.012 (mg/cm?)"! 
Concentration-by-reflection gage 
K=4xX 10 
Calculation. The table shows the 
characteristics calculated for three 
types of beta gages: 
Thickness-by-transmission gage. 
Thickness measured is 20 mg/cm? and 
Io = 2 X10-°amps. Source ~ 10 me. 
Thickness-by-reflection gage. A 10 mg/ 
cm? piece of plastic (Za ~ 5) laid ona 
thick iron plate (Zs = 26) is measured 
and Jz = 2 X 10-® amps. Source ~ 
10 me. 
Concentration-by-reflection gage. A 1.0 
weight per cent solution of lead 
(Z = 82) in water (Z, ~ 7) is meas- 
ured. J) = 2 X10-® amps. Source 
= 20 mc. 
A value of Qavy = 1.6 X 10716 
coulombs is estimated from a consid- 
eration of the ionization chamber 
geometry and data on the specific 
ionization of beta particles. It is 
assumed that +/(q?)a,, is approxi- 
mately equal to qavg. 
Certainty. From the table it can 
be seen that in each case, the ‘‘accu- 
racy’’ of the meter, rather than the 
fluctuations due to the radioactive 
source, is the limiting factor in the 
measurements’ uncertainty. Assum- 
ing that we do not to change the load 
resistor and thus the response char- 
acteristic of the gage, and assuming 
that it is impractical to use a more 
accurate microammeter, there remain 
two means for improving the certainty 
of measurement. 
The first alternative involves in- 
creasing g, by changing the electronic 
circuitry of the beta gage to form a 
more elaborate amplifier. An increase 
in amplification will not affect o./z 
or g./c. Thus the increase of gm will 
be effective in decreasing the uncer- 
tainty only up to the point that 
g:/x or o,/c becomes larger than 
6z/z or 6c/c. 
The second possibility entails in- 
creasing J) or Js by increasing the 
strength of the source of beta radiation. 
The value of o./z or o,/c will decrease 
in this case along with 62z/z or 6c/c, 
but less rapidly because of their 
inverse square root and first power 
dependence, respectively, on ionization 
current. 
* * * 
The subject matter of this article con- 
stituted a portion of the lecture on Reflection 
Beta Gages presented on April 21, 1958, for 
the course ‘‘ Advanced Radioisotope Tech- 
niques in Industry’’ sponsored by the Oak 
Ridge Institute of Nuclear Studies. 
BIBLIOGRAPHY 
1. J. R. Carlin, Electronics 22, 110 (1949) 
2. C. W. Clapp and S. Berstein, Gen. Elec. Rev. 
68, 31 (1950); Trans. Am. Inst. Elec. Engrs. 
69, 488 (1950) 
J. R. Carlin, Rubber Age and Synthetics 66, 
173 (1949) 
W. R. Dixon, NRC-2358 (National Research 
Council, Chalk River, Canada, 1951) 
J. Kohl, Petroleum Refiner $1, 117 (1952) 
L. I. Schiff, R. D. Evans, Rev. Sct. Instr. 7, 
456 (1936) 
Soe 
