While searching for darlington arrays I found an interesting german page about a gamma ray detector using a photodiode and an darlington circuit. I still have one channel free on my LED driving IC and the used diode is not expensive.

http://www.b-kainka.de/bastel131.html

http://www.elektronik-labor.de/Projekte/Alpha.html

https://www.sparkfun.com/datasheets/Prototyping/Solar/bpw34.pdf