When radioactive elements decay energy is released, some in the form of gamma radiation. The amount of energy is dependent on the element. This allows us to determine the composition of the radioactive element by measuring the energy of the gamma ray. This is can be done using gamma scintillators, materials that when struck by gamma rays emit visible light, the amount of which is proportional to the energy of the beginning gamma ray. The amount of light is relatively small and must be amplified, in this case using a photomultiplier tube, which does pretty much what it says on the can. This signal is then sent to multichannel analyzer where it's measured and stored.