[jorgem]

http://www.amazon.com/RADStickerTM-radiation-exposure-determ...

[kaybe]

Or you could just get an app (and some black tape), depending on the energy range you want to look at:

http://www.xinexus.ch/nuclear-radiation-dosimeter/index.html

edit: I think I got the wrong thing there.. better go for this:

http://www.hotray-info.de/html/radioactivity.html