Over at the Arduino X10 Collection site there is a two part series in the making of a DIY Geiger Counter Project which is built with an Arduino Microcontroller. Part 2 can be found here. I can’t think of a time I was looking for a geiger counter but you never know. The project is built around a Russian Military SBM-20 Hard Beta and Gamma GM Detector. If you are thinking about making one keep in mind that the circuit has some very high voltage involved so be careful!
“There are 2 types of tubes. Those that have a mica window are the most sensitive. They will detect alpha and beta particles as well a gamma rays. Because of the mica, they are more fragile and generally are more expensive. The other type, like the SSBM-20 which I used, have only the metal jacket. They will detect gamma rays (the most penetrating) and some beta particles (more easily stopped). Considering the SSBM-20 is all metal, it does a good job with beta – as long as you put the sample right on the tube. Uranium is a big beta emitter, so some sensitivity to beta is a good thing.”