Abstract: A gamma ray detector and measurement device having a generally elongate housing (11); a scintillator module (12) movably mounted in the housing (11) and arranged to respond to the presence of gamma rays in the local environment in which the device is located and which rays pass through the wall (13) of the housing and into the scintillator module, the module having an optical output (14) for transmitting an optical signal based on converted incoming gamma rays and an optical transducer module (15) also movably mounted in the housing (11) and having an input (16) arranged to receive the optical output from the scintillator module, the transducer module being able to convert the instantaneous optical output into an electrical output corresponding to the energy and flux of the incoming gamma radiation as detected in the scintillator module; in which the output (14) of the scintillator module (12) and the input of the transducer module (15) are closely spaced to each other, and define a space (17) therebetween to