In these times of increased terrorist threats, the possibility of some type of nuclear incident exists. By using a few simple components, it's possible to build a low-cost Geiger detector. Detecting α radiation requires a special Geiger tube. But for detecting only β and γ radiation, a neon glow lamp can be used.
Operation is as follows (see the figure): The alternating current is rectified by diode D1. A simple Zener-diode stabilization network circuit using D2 supplies a voltage of 100 V dc. The value of R1 depends on the source voltage and must be calculated with the equation, R1 = (V ac −s 100 V)/5 mA. Potentiometer R2 is used to set the detector voltage very close to, but slightly below, the neon lamp's ignition voltage. The lamp must not ignite. If a radioactive particle then ionizes the gas in the lamp, it ignites.
At that point, resistor R3 drops most of the voltage, so the lamp voltage is lower than the holding voltage. No additional current flows through the lamp until the next radioactive particle ignites it. During the small moments of current flow, a short and quiet clock noise comes out of the loudspeaker.
More circuits like this one can be found in my book about experiments with radiation sources (written in German), Experimente mit Strahlenquellen im Haushalt (www.peterlay.de).
Note: As with all circuits that can be connected directly to an ac power source, suitable insulation, grounding, and polarization precautions should be taken to avoid the risk of potentially lethal electric shock.