Alan over at vk2zay.net posted an article describing how he designed and built his gamma ray detector using a photodiode:
Gamma photons interacting with cheap photodiodes produce small current pulses which are easily amplified and allow detection of individual photon events. This offers the possibility of cheap, small and rugged radiation detectors of reasonable sensitivity. While not as sensitive as larger GM-tube detectors, this solid state device is still quite useful for determining if something is radioactive enough to be interesting/concerning.
Via Electronics Lab.