Sylvain sent over these instructions for buiding a USB-able radiation detector –

The geiger counter is a pretty simple device to detect radiations. Many forms of radiations from radio elements (alpha, beta, gamma) can be detected but it is more sensitive to beta and gamma radiations. The Geiger-Muller tube is a simple device, it’s a tube filled with a gas with two electrodes. A high potential is applied betweens electrodes. When a ionizing particle arrived, it create a temporary conductive path between electrodes. The resulting current can be detected by an electronic amplifier.

Yup, each temporary conduction creates one of the Geiger counters signitaure ‘tick-tick’ noises. – How to build a geiger counter with USB interface