The GM tube and the transformer modem are from Sparkfun. I have slightly modified the original schematic, taking the impulse from the cathode of the tube, not from the anode, and added a Schmitt trigger and a RC filter (R9/C4) to get a clear TTL output signal for counting the impulses with a micro controller.
The Geiger tube has a mica window for alpha radiation. I used americium-241 from a dismantled ionization smoke detector to test the Geiger counter. Americium-241 is a so called alpha emitter and has a half-life of 432 years. About one percent of the emitted radioactive energy of 241Am is gamma radiation.