A gamma ray detector for determining the position of gamma ray interactions has at least one module. Each module has a converter for converting gamma rays to charged particles, a scintillator for emitting light in response to charged particles produced by the converter, a photodetector to determine when light has been emitted from the scintillator, a two-coordinate position detector for determining the X and Y coordinates of charged particles interacting with the position detector, and a signal device for signaling the presence of emitted light in the photodetector and for activating the position detector.