Radioactivity counter
There are apps (with Raspberry Pi and similar devices) for this. I'm not sure on their bad gap level, but I'd presume the very widely. 

First Sensor provides some high quality photo-diodes for this purpose.



first-sensor.com/cms/upload/datasheets/gamma-ray-detection.pdf



Aside, I don't think it's correct to say "Geiger counter" because it's not using a Geiger–Müller tube. "Rad detector" is more apt :)