A gamma ray detector is described. The detector comprises a plastic scintillation body for receiving gamma rays and generating photons in response thereto. The scintillation body is in the form of a truncated cone defined by a base surface and an end surface separated along an axis of extent of the scintillation body with a lateral surface extending therebetween. A photodetector is optically coupled to the base surface of the scintillation body so as to detect photons generated by gamma ray interaction events within the scintillation body. A specular reflector is provided adjacent, but separated from, the lateral surface of the scintillation body so as to reflect photons that leave the scintillation body through the lateral surface back into the scintillation body.