Publishing this one in a hurry, in case it is useful to our friends in Japan. This is a simple Geiger counter circuit. It does require some specialized parts, in particular a Geiger tube and some high voltage Zener diodes, but the rest of the components are readily available. If you're feeling like experimenting, try making your own Geiger tube... see the last step of this instructable for some comments on that. This is the second electronic circuit I ever made, so it's pretty basic, suitable for beginners.