The frist device that springs to mind when thinking about measuring radioactivity is the
Geger-Muller tube. However these counter tubes are getting hard to find and expensive ,and even if you do
Manage to get hold of one, you will still need to find a way to generate its operating voltage of several hundred volts. It is less well known that even a humble photodiode such as the BPW34 can be used to detect gamma radiation