Humans are only able to sense ionizing radiation indirectly, meaning
it's not detected until one starts loosing hair and teeth. Fortunately
there are ways to measure radiation directly which is how humans can
stay safe around ionizing radiation. The heart of such a
device is an evacuated tube, charged with a voltage so high the tube is
just below breakdown. When an ionizing particle hits the tube it will
pass through it, and as the name suggests, ionize particles along the
way. Even if a particle only ionizes a few gas molecules the tube will
avalanche, or arc over internally for a brief moment. This
avalanche can be detected by an electronic circuit and emitted as a
tick, flash or sent further to a digital counter.
at the right, detection range 0.04 - 0.4 r/H. FHZ74 on the left, 0 - 50
The circuit presented below consists of
a high voltage supply, and means of converting tube avalanches to
audible ticks. A small, regulated flyback supplies the high
voltage needed. Each Geiger Mueller tube will have it's own "plateau"
which is the working voltage range. Voltages higher up the plateau
increase radiation sensitivity, voltages lower down do the opposite. I
chose 420V since it lays within the plateau of most tubes.
changing the zener diode
values one can adjust the output votlage to whatever is needed by the tube in question. For the main transformer a
disposable camera transformer is required, these vary little
from camera to camera and all will have the necessary windings. I
assume they have a common pin out, but if not experiment until you get
the circuit oscillating. The approximate inductance of the windings can
be determined by the winding size in the schematic (I know, very crude), so if you have an
L/C meter handy finding the pin out shouldn't be too hard. The
ingenious flyback supply isn't my design, I just ripped it off from here
Once the tube is energized and a particle hits it, it will short out.
This causes a negative potential at the diode side of the 100pF
capacitor which turns on the PNP Darlington transistor combo (the two
BD140s). Once on they flash the ultra-bright LED and chirp the buzzer.
The voltage at diode UF4007 is approx 5-6V. A real darlington
transistor could substitute the two BD140s, I only used them because I
had some available.
kindly donated some antique radium watch hands so I could test my
counter. With the SBM21 tube the counter goes mad when the watch hands
are held close, and at a few cm distance the count rate is over 100
counts per minute. So
the circuit is proven to be functional.