The BPW34 (or BPW34F) seems to be the only useable PIN Photo Diode for a normal price.
It is not very sensitive, it can't be used to detect the background radiation.
It is a photo diode, so you have to place it in the dark.
Some schematics:
http://www.vk2zay.net/article/265
http://www.dgkelectronics.com/a-radiation-detector-with-a-solid-state-pin-diode-sensor/
http://www.mikrocontroller.net/topic/216426