Scintillation detectors uses a photo luminescent source (such as ZnS) which interacts with radiation.  When a radioactive particle decays and strikes the photo luminescent material a photon is released.  This photon is multiplied in a photomultiplier tube which converts light into an electrical signal.  This signal is then processed and converted into a channel.  By comparing the number of counts to the energy level (typically in keV or MeV) the type of decay can be determined.