The invention concerns an evaluation apparatus (50) for evaluating gamma radiation events detected by a gamma camera (10) and identifying valid gamma radiation events, said (gamma camera (10) including a scintillator (12) for emitting scintillation photons (42) at photo conversion positions (44) in the scintillator (12) in response to incident gamma rays (22) and resulting gamma radiation events and a position-sensitive photodetector(14) for detecting emitted scintillation photons (42) and obtaining therefrom a spatial signal distribution (24). The invention further concerns a calibration apparatus (56) for in-situ calibrating a position-sensitive photodetector (14) of a gamma camera (10) for the detection of gamma radiation events.