The invention is related to a method for monitoring a range of a particle beam in a target. The method is using gamma detectors for detecting prompt gammas produced in the target. The time differences between the time of detecting a gamma quantum and a time of emission of a particle or a bunch of particles from the radiation device are determined. A statistical distribution of those time difference is used to deduce information related to the range of the beam. The invention is also related to an apparatus for monitoring a range based on measured time profiles of detected prompt gammas.
