Investigation of the Neutron Stray Radiation Field Produced by Irradiating a Water Phantom with 200-MeV Protons

Proton therapy is a widely used method of cancer treatment. Undesired secondary particles such as neutrons accompany the irradiation. Depending on the measurement position, the expected main dose contribution due to undesired secondary particles arises from neutrons with energies >20 MeV. Conventionally used Andersson and Braun-type survey instruments may underestimate the ambient dose equivalent up to a factor of 2 due to their limited response for high-energy neutrons. Therefore, it is desirable to investigate the neutron stray field in conditions comparable to therapy treatment, in particular the resulting dose to equipment placed in the treatment vault to estimate possible consequences to its operation. The irradiation of a water phantom with 200-MeV protons adequately reproduces these conditions.