TABLE 1. The output pulse shape varies at the Geiger
threshold. A pulse height about twice the threshold level is
a good guide to the best operating voltage.
370 V (threshold)
390 V (above threshold)
440 V (operating point)
U1 LMC555 timer
U2 LMC556 dual timer
Q1 MPSA42 high-voltage
Q2 2N3904 or similar
Q3 2N3904 or similar
D1 1N4448 or equivalent
D2 1N4937 HV,
of the "plateau" is the optimum operating point. Unfortunately, hanging a
voltmeter on the converter takes far
more current than the G-M tube.
The alternative is to look at the
output pulse (TP in Figure 1) on a
scope. If you see variable height pulses around 10 V high you are below
the plateau. There the pulse height is
proportional to particle energy.
The onset of avalanche is easy to
spot. Some pulses will start off small,
then suddenly burst out into a much
Adjust the trimmer until all the
pulses show a consistent height; with
my tubes this was about 12 V. Note
this new peak height then adjust the
high voltage until the pulses are twice
L1 10 mH wave-wound
C1 15 µF 20 V tantalum
C2 0.01 µF plastic foil
C3 0.01 µF 630 V
C4 4. 7 pF 630 V
C5 0.1 µF ceramic
C6 1000 pF ceramic
as big, say 25 to
30 V. Table 1
shows the results
One thing to
watch is that the
quite slowly, particularly when
adjusting it downwards. Make small trimmer adjustments and wait ten seconds or so for
the pulse height to stabilize.
R1 330K (All resistors
0.8-inch Piezo buzzer
I Name My Sources
Testing G-M tubes is easier if the
count rate is higher than background. Small (1 µCi) radioactive
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sources are available commercially
but cost around $75.00 each.
I've used a Wyoming dinosaur
bone as a source — they absorbed
uranium as they fossilized. My two
inch piece of rib generates about four
counts per second when placed next
to the tube.
If you want to create a stir you
could try prowling around your local
science store with a Geiger counter
checking for "hot" fossils. Other common home-made test sources are
objects made from yellow-tinted
glass or china-ware with a bright red
If more than twenty years old
these may contain enough uranium
to drive most detectors off-scale. Try
your local thrift store. My most powerful source is a thirty-year-old packet of colored glaze left over from an
attempt at jewelry making.
One interesting experiment is to
put a beta source a fixed distance
from the G-M tube. Check how the
count rate changes as you interpose
different thicknesses of aluminum or
A thick enough sheet, about half
an inch, will filter out all the betas,
leaving only gamma rays. NV