Beam monitoring system, particle therapy system, and beam monitoring method

A beam monitoring system that detects an immediate gamma ray generated along an irradiation axis of a particle beam with which an irradiated object is irradiated and monitors an arrival position of the particle beam, the beam monitoring system comprising:

• a detector including a plurality of detection elements arranged along a traveling direction of the particle beam and configured to detect gamma rays;
• a gamma ray shield disposed between an irradiation axis of the particle beam and the detector and formed with a transmission portion configured to transmit gamma rays;
• and
• a calculation unit configured to estimate a Bragg peak at which energy of the particle beam is maximized using a count value of the gamma rays respectively detected by the plurality of detection elements, wherein
• the gamma ray shield includes a plurality of the transmission portions along the irradiation axis, and
• the calculation unit integrates a count value of gamma rays injected at different times through the plurality of transmission portions into one detection element, creates a profile obtained by plotting an integrated value of gamma ray count values for the respective detection elements, and estimates the Bragg peak using the profile.