This is a Geiger Counter made using components from the vintage Radionic electronics construction system.
It’s powered by a 12 volt battery and includes no integrated circuits.
Two transistors form the basis of an oscillator which runs at around 50 Hz.
The oscillator drives a 9 volt mains transformer in reverse, so that the voltage is stepped up rather than down as usual.
The 250 volt AC output from the transformer is rectified and raised further using a voltage doubler formed of 2 diodes and 2 capacitors.
The high voltage DC is regulated down to 400 volts using a string of 3 zener diodes and 2 neon bulbs.
The regulated 400 volt DC output is fed to the Geiger-Muller tube. When radiation causes ionisation in the Geiger-Muller tube,
an electron avalanche effect causes a discharge pulse.
This pulse is amplified by a transistor and is used to trigger a 2 transistor monostable, which passes on the pulse which is now a square wave of fixed width.
The reshaped pulse is used to charge a capacitor, which is simultaneously being discharged by a resistor and meter in series.
The component values are chosen so that the meter reading corresponds to the average counts per minute over the recent past (over around 10 seconds).
The background radiation is typically 20-30 counts per minute at this location.
Background radiation sources include radon gas, the earth below us and cosmic rays from outer space.
The radioactivity of various everyday objects is measured:
Low sodium salt from a supermarket product uses potassium chloride as a substitute for sodium chloride.
Most potassium atoms are potassium39, (a stable isotope) but around 1 in 10,000 of them are potassium40, an unstable radioactive isotope.
Consequently, low-sodium salt is mildly radioactive.
