my work required
handling radioactive sources, my
rule of thumb was
never to touch one
bigger than 10 µCi
with my fingers. I
used tweezers or
tongs instead. Bear
in mind that radiation damage is
more or less cumulative. (Exactly how
much so is still
hotly debated.) A
source that is harmless in normal use
should not be kept
in your desk drawer
FIGURE 1. Take advantage of that pesky inductive turn-off pulse to power a Geiger-Müller tube.
A dual timer makes a beep for every beta or gamma ray detected.
rapid quench. As the tube's own capacitance is about 1
pF, C4 defines the charge delivered by the tube.
Each particle detected generates a 5 µS pulse up to
100 V high at the G-M tube's cathode. Two 100K resistors
between the cathode and ground protect the counting
circuit and provide a protected test point. Q3 turns on for
about 3 µS for each pulse. The output pulses drive a 556
dual timer. One half lengthens the pulse to 10 mS and
gates the other half, a 3. 5 kHz oscillator.
This drives a piezo-electric buzzer, making a beep for
each particle. (My buzzer came from a broken calculator
but Digi-Key part 102-1126-ND should do.) The pulses
on Q3 can also drive a frequency counter to measure the
mean radiation level. In my area natural background radiation produces about seven beeps per minute; Colorado
residents may get twice that rate.
NUTS & VOLTS
E January 2003
The absolute calibration of the detector depends on
the tube size and type, and on the particle type and energy. I calibrated the G-M tube in my original detector with
a known Cs137 gamma source. At 100 µRad/H, its count
rate was 68 per minute. Since the tube is much more sensitive to beta particles, a given count rate corresponds to
a much lower beta dose rate. A high count rate indicates
either that something is badly wrong in your neighborhood or that a small source is very close to the tube. The
latter, of course, is how you test that everything works.
Distance is important. A source can generate a horrendous dose rate on contact yet, because of the inverse
square law, be relatively harmless a few feet away. When
The HV generator uses two fairly rare components. One is a dust-cored,
wave-wound 10 mH RF choke. The other is an MPSA42,
a high-voltage transistor in a TO- 92 package. Digi-Key
stocks them both as well as the 1N4937 600 V fast recovery rectifier. Don't be tempted to substitute a 1N4005, it's
far too slow.
Also stick with the wave-wound inductor, more conventional inductors could have too high a self-capaci-tance and might break down when generating around
500 V. I used Digi-Key part M7103-ND. HV capacitors are
harder to find. If you can't find 630 V parts, 500 V ones
should be safe enough. (A 0.5-inch square of double-sided PC board makes an adequate 5 pF HV capacitor.)
The MPSA42 transistor is rated at 300 V but during
the HV pulse its base is grounded and the higher, collector-base, breakdown voltage applies.
I don't have the maker's figure for this but testing a
sample showed no detectable current below 480 V and
only a microamp at 500 V. (Allied Electronics sells the
400 V MPSA44 but I haven't tried it.) In any case the
worst that can happen is that the maximum available output voltage will be limited to the transistor's breakdown
voltage. You can get a good feel for just how high a pulse
voltage you're getting by putting a times-ten scope probe
on the collector of Q1. The 15 pF added by the probe
reduces the output voltage significantly but its 10 Meg
resistance has no effect at all. (Scope probes are usually
rated for 600 V input but it would be as well to check.) If
you want to measure the output voltage directly you'll
need a meter with a better than 100 Meg input resistance.
I've no idea who made my G-M tubes. In 1979 my