FIG. 8. Semidirectional counter locates 
dynamite that fails to explode 
is International Instruments’ Model 
150W microammeter, 100 ya full scale. 
This has proved rugged enough and 
easy tomount. A Benaudi rubber ring 
fits the adapter ring to the counter 
ease. For greater simplicity no zero- 
setting control is provided, and the 
sensitivity must be checked periodi- 
cally with a suitable standard source. 
In order to adapt the counter for 
use in high-grade-uranium mines a 
special heavy shield was designed that 
is interchangeable with the standard 
shield. For easier carrying it is pro- 
vided with a shoulder strap, and its use 
has proved feasible in field tests under- 
ground. Use of the heavy shield does 
not often justify its extra inconvenience, 
but it has proved valuable where it is 
needed. It is shown in Fig. 7. 
Semidirectional Counter 
In connection with the work on 
location of radioactive dynamite (see 
p. 170) (5) it became clear that the 
O 
Ring switch 
K3/15 zt 
T1-Ferroxcube, Primary: 45 turns tapped at 17, Secondary, 2000 turns 
T2- Output: 500 ohms, Input 50.000 (2 Hammond 142 U transformers) 
Vi-Victoreen5950 corona regulator 
FIG. 9. Circuit for semidirectional counter is similar to that of Fig. 5, but it has higher 
sensitivity. Hearing-aid battery supplies 15 volts 
directional counter could be too direc- 
tional. A semidirectional counter was 
designed (Fig. 8). For this a slightly 
larger end-window tube was chosen 
with a larger shield opening. The cir- 
cuit is shown in Fig. 9 and is seen to 
follow that of Fig. 5 in essentials, 
though its sensitivity is higher. Me- 
chanical details had to be especially 
designed as the EW3H G-M tube re- 
quired a longer case than the other 
model. 
Scintillation Detector 
The design of a gun-type portable 
scintillation counter (Fig. 10) was 
undertaken as it was felt that existing 
counters were unduly heavy and re- 
quired too many expensive batteries 
(6). This detector uses three 935 
batteries for power. The meter has 
five overlapping ranges and a zero-set 
control. The complete circuit dia- 
gram is presented in Fig. 11. 
The sensitivity of the detector on its 
most sensitive range is such that a 
Summary of Counter Characteristics 
Simple Direc- Semidirec- ‘Scintilla- 
Counter Geiger tional tional tion gun Carborne 
Sensitive element 20th Cen- Amperex 20th Cen- 114 X 119 Nal(Tl) & 
tury G1OH 230N tube tury in. Nal(Tl) 6292 
tube EW3H & 6199 
Operating voltage 370 1,200 600 
Weight (total) (Ib) 13 3.0 5.8 2.9 7.8 
Weight (without 1.4 1.8 3.4 (top 
shield) part only) 
Approx. sensitivity 0.03-12 0.03-0.35 0.02-0.2 0.008-10 0.01-10 
range (mr/hr) (meter type) 
Sensitive cone — 40° 60° — — 
Estimated component $55 $70 $75 $170 $200 
cost 
Battery voltage (volts) 4.5 15 15 4.5 6.3 
Current drain (ma) 5 4 8 6 
Expected life (days at 21 5 > 30 (buzzer 
4 hr/day) extra) 
52 
100-gm sample of ore containing 0.5% 
U;0s will give a deflection of 6 wa at a 
distance of 6 in. The effective battery 
life is governed mainly by the require- 
ments of the oscillator, which must 
provide enough voltage through the 
quadrupler circuit to keep the corona 
regulator struck. 
Carborne Counter 
In mobile prospecting by jeep or 
car it is not usually possible for the 
driver to watch a meter or dial while 
navigating across country over poor 
roads. For that reason a scintillation 
unit was designed to be mounted in a 
car and trip an alarm whenever a pre- 
determined radiation level is exceeded. 
The detector unit is self-contained and 
demountable and can be used as a 
portable counter to investigate mineral 
occurrences. The buzzer is driven 
from the car battery, and the counter 
has batteries that can be trickle- 
charged from the car battery. 
The complete circuit diagram is pre- 
FIG. 10. Gun-style scintillation detector 
has zero set and 5 meter ranges 
