A gamma-ray detector (42, 52, 72, 92) comprising a large-area plastic scintillation body (44, 64, 74, 94) and a photon detector (38, 58, 68, 78) optically coupled to the scintillation body to receive and detect photons (P.sub.1, P.sub.2, P.sub.3) generated by gamma-ray interactions. Selected portions of the scintillation body surface are provided with a reflective layer (46, 60, 80) in planar contact with the scintillation body. Other regions are not provided with a reflective layer. Thus specular reflection is promoted in at the surfaces provided with the reflective layer, while total internal reflection may occur in the regions which are not provided with a reflective layer, hi a scintillation body generally in the form of a plank, the photon detector is coupled to one end, and the regions provided with the reflective layer are the edges of the plank. The scintillation body may be shaped so that it reduced in cross section in a direction away from the photon detector. The spectrometer may include photon detectors coupled to both ends of the scintillation body.