Dose Rate Measurements Behind Different Shielding for 250 Mev Protons on A Thick Copper Target

A dedicated cyclotron for proton therapy will be installed at the Paul Scherrer Institut, feeding two treatment rooms and one experimental area. It will produce 250 MeV protons at a maximum current of 500 nA. The shielding calculations for these facilities are based on published parameters. To support the calculations, an experiment has been carried out in an existing experimental area with 250 MeV protons stopped in a thick copper target. Neutron and photon dose rates were measured (a) at 90degrees and 78degrees on the roof, for concrete shielding thicknesses of 0.5-3 m and (b) at 20degrees behind a shielding wall of concrete and iron. The dose rate component due to neutrons above 20 MeV was determined using a C-12(n,2n)-monitoring system; a measurement with this monitor was possible only for 0.5-1.5 m roof thickness. Comparisons with the calculational method show good agreement in case of the forward shielding (b); for the lateral shielding (a) the calculations in general underestimate the dose rate.