See 
From filling system 
Proportional 
counter 
ap 
| 
ron. shield@ 
FIG. 3. Circuitry outside of shield begins with gain control, normally set at —12 db. 
tiator, is to control frequency response for optimum signal-to-noise. 
of their gains is accurately set at 16:1 by control in lower-gain amplifier. 
circuits arranged so that only highest discriminator triggered records. 
Energy loss in counter corresponding to each channel is shown. 
according to whether or not they are in coincidence with a pulse from the surrounding counter ring. 
Pulse stretcher in ring output insures coincidence pulse is present even if meson passes through G-M counter 
Parallel single-channel analyzer checks four-channel anticoincidence count 
higher two by low-gain amplifier. 
to cosmic-ray mesons. 
recovery. 
sample is removed from the chemical 
system as “dry ice,’ having been 
frozen with liquid oxygen. It is con- 
tained in a 100-cm? flask provided with 
a stopcock so that it can be removed 
from the chemical-purification system 
and joined to the counter-filling sys- 
tem without admission of air. At this 
stage the sample still contains traces of 
air trapped in the frozen CO, and traces 
of water, both of which would upset its 
counting characteristics. 
On the counter-filling system the 
CO; is distilled from one condensation 
trap toanotherandthen pumped. The 
process is repeated until the amount of 
gases, noncondensable in liquid oxygen 
(mainly air), is less than 0.1 ppm of the 
CO,. One distillation is usually 
sufficient. 
The frozen COz is now allowed to 
expand and pass into the counter 
through a P.O; drying tube that also 
acts as an efficient dust filter. It is 
essential that drying be carried out 
last; if drying precedes distillation, 
sufficient water vapor will be outgassed 
from the glass to affect counting 
characteristics. 
The counter is filled to the desired 
pressure (78-230 cm Hg) when the 
counter temperature is 18° C as meas- 
ured by a resistance thermometer with 
an accuracy of 34° C. At any other 
192 
channel 
= se 
High voltage High voltage 
counter temperature the filling pressure 
is changed so that the mass of gas in the 
counter is constant from one filling to 
the next to 1 part in 1,000. The work- 
ing voltage of the proportional counter 
as a function of filling pressure for a 
gas gain of 2,000 is shown in Fig. 4. 
Shield and Counter Ring 
To prevent gamma rays from reach- 
ing the counter, an 8-in.-thick iron shield 
is provided. This shield is built of 
mild steel plate and machined cast-iron 
blocks, the space provided for the pro- 
portional counter and ring of meson 
counters being 14 X 14 X 36 in. A 
layer of mercury 1 in. thick surrounds 
the counter to provide additional 
Gas gain= 2,000 
” 
2 
3 
> 
® 
a 
& 
re) 
> 
o 
Cc 
x 
= 
S 
= 
ie} 50 100 ISO 200 
Filling Pressure (cm Hg at !8° C) 
FIG. 4. Working voltage of the propor- 
tional counter as a function of CO; filling 
pressure for a gas gain of 2,000 
Scale 
of 1000 
of 1000 
of 100 
| 
height and{___] = 
oincidence ; 
analyzer Ch. 2:12-100 kev 
Anticoincidence 
display 
Ch3:100°400 kev 
Coincidence 
display 
Bandwidth control, normally a 40-yusec differen- 
Parallel main amplifiers are ring-of-3 feedback-stabilized; ratio 
Four-channel analyzer has four discriminators with cancelling 
Two lower-level discriminators are fed by high-grain amplifier, 
The analyzer also sorts pulses 
Coincidence events are largely due 
shielding and also absorb some of the 
gamma rays from radioactive contami- 
nants in the iron. The arrangement 
of these components is shown in Figs. 
1-3. 
Between the mercury shield and the 
iron shield is the ring of 18 G-M coun- 
ters, each 2 in. o.d. and 35 in. long. 
The ring of counters is divided horizon- 
tally into two segments. This division 
enables a faulty counter to be detected 
quickly by a comparison of the count- 
ing rates of the segments. Each seg- 
ment has its own preamplifier that pro- 
vides a 900-usec quenching pulse. 
Deadtime of the counters is 600 psec 
so that 900 usec is about the minimum 
quenching-pulse duration that can be 
used. 
The total counting rate of the ring of 
counters is 2,600 cpm, so a 4% loss of 
anticoincidence counting rate occurs 
due to the total deadtime of the ring of 
counters. The size of the ring of coun- 
ters is thus as large as is practicable 
unless the counting rate were to be re- 
duced by operating underground to 
reduce the meson flux. 
Associated Electronics 
The counter voltage supply, ampli- 
fiers, and pulse-height analyzers are 
described in Figs. 2 and 3. 
The output of the pulse-height analy- 
