T1-Ferroxcube, Primary: 40 turns tapped at 14, Secondary: 850 turns 
Speaker- Brush BB-134 (Crystal) 
FIG. 2. Oval case makes this model of 
G-M counter easier to hold 
FIG. 3. Circuit for portable G-M counter has transistor oscillator and voltage tripler. 
It is powered by three penlight batteries 
for Portable Detectors 
work on this subject has been described 
previously (3), but much further de- 
velopment has taken place since. 
Because of the high ambient back- 
ground radiation, good directionality is 
essential. On the other hand weight 
and size must be kept down to make 
the unit acceptable to mine staff; 
pocket size is desirable. One possible 
approach is a very small, insensitive 
G-M tube to reduce background. 
However, this means insufficient sensi- 
tivity to low-grade ores, and for this 
reason we chose a different approach. 
We use the highly localized sensitivity 
of an end-window G-M tube run in the 
“corona-spark region” (3, 4); with this 
system shielding is not excessive. 
Field tests on earlier units (‘‘spot- 
ters’) have shown that cables for 
separate detector probes and earphones 
are objectionable. Therefore all re- 
2N 131 
U75SHP(2) 
cent units have been designed on the 
flashlight pattern as_ self-contained, 
tubular detectors. 
Three different models have been 
made: (a) a simple, highly collimated 
detector with loud aural indication, (b) 
a similar unit with meter indication 
and interchangeable shields and (c) a 
less directional counter for the detec- 
tion of radioactive dynamite (6). 
These instruments differ only in minor 
details; most of their mechanical 
features are the same. They use 
halogen-quenched end-window G-M 
tubes with a concentric lead shield at 
one end. The indicator is mounted 
onthe otherend. The unit is switched 
on and off by a knurled ring on the case. 
Figure 4 is an exploded view of the 
directional counter with speaker indi- 
cation. The tube is run at 1,200 volts; 
that is, above the G-M plateau. The 
|Meg |Meg 
: 100ua -4 
T1-Ferroxcube, Primary: 45 turns tapped at 20, Secondary: 2000 turns. 
T2-2Hammond 142J’s in series 
speaker is a miniature, waterproof, 
permanent-magnet type made by Can- 
adian Benaudi Ltd.* Power is sup- 
plied by a 15-volt hearing-aid battery 
The poor shelf life of these batteries is a 
weakness of the present unit, and 
alternatives are being investigated. 
The shield is a lead-filled brass con- 
tainer. Figure 5 shows the circuit. 
Different insets can be screwed into 
the shield to provide different colli- 
mation angles. The tube window is 
protected from rock particles by a 
small screen. 
Meter model. The directional 
counter with meter indication uses the 
same shell with the addition of a metal 
adapter ring to mount the meter in 
place of the speaker. Its circuit is 
shown in Fig. 6. The meter used 
* 287 Metcalfe St., Ottawa, Ont. 
FIG. 6. Meter model uses half-wave rectifier supply and replaces speaker in circuit with FIG. 7. Heavy shield adapts directional 
transformer-coupled meter-and-rectifler system 
counter for high-grade-uranium mines 
51 
