I have played with silicon pin photodetector diodes for radiation 
detection (has to NOT have a window {I cut mine off} and only works in 
the dark). hooked up with a transimedance amplifier - it was sensitive 
to alphas only so you need a bare source like a smoke detector source 
(americium) for instance, or a gas light mantel (at least they used to 
be radioactive - thorium), or an OLD glow clock hand (radium and etc. -I 
had one seemed to be carbon 14 - no alphas from that), or uranium 
mineral, etcetc.
I don't know now where to get a bare silicon photo diode any more but 
this approach would be some $1000 less then some previous pointers - 
maybe < $20. Even Geiger counters can be in the $150 Or less range 
http://www.blackcatsystems.com/GM/geiger_counter.html
Geiger Counter Nuclear Radiation Detector Alpha Beta Gamma X-Ray

Looking forward,
Al Shinn



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