Gamma ray spectroscopy has all sorts of fun applications. I wanted to build one to characterize radiation in food and other objects. Geiger counters are typically not sensitive enough to detect trace quantities of contaminants as they do not produce significant radiation above normal background. You are also unable to measure energies and identify radionuclides. It takes a fair amount of shielding if you want to make accurate measurements though since there is so much background noise in the typical environment.

I'm curious if you can also detect matter which has been activated from the high energy gamma rays. Photon activation begins around 6.2 MeV and really gets going above 10MeV. The gamma rays have sufficient energy to activate the nuclei of stable isotopes, causing them to become unstable, potentially decay and produce secondary radiation.

I was really hoping to put together a setup, but some jerk picked the lock on my storage unit and took all of my radioactive samples and measurement tools. FYI, the typical 'high security' circular locks are easily picked now and with most storage facilities allowing anyone with a storage unit unchecked access to your lock, it's very easy for the thief to go unnoticed.