I bought a few Geiger tubes about 10 years ago. The larger one is more sensitive, but doesn't detect beta radiation. The smaller one does alpha/beta/gamma, but it's less sensitive. When you buy the tubes, you don't get the benefit of a calibrated measurement device, so I also bought a few Cesium 137 calibration sources. (No idea what sort of government watch lists I ended up on after placing that order.)

Next, I built a simple Geiger counter based upon a Raspberry Pi (Model B) and a Piface LCD display. I wanted to increase dynamic range of the measurements, so I did some circuit modifications to recharge the Geiger tube faster after an event. This allowed for higher counts, but consumes more power and lowers sensitivity. Also, getting the Pi to interrupt on each event for efficient counting has its limitations. A separate digital counter that is reset upon each read sample is better.

Anyway, it's a rabbit hole that ended up taking a lot more time and effort than I expected, but I was happy with the results.