Design considerations of the gas-filled beam pipe in a scanning nozzle for HUST-PTF

An active scanning nozzle is under developing in Huazhong University of Science and Technology Proton Therapy Facility(HUST-PTF), which has a 300∗300mm2 radiation area at the iso-center. In order to reduce the energy loss and transverse scattering of 70-250 MeV proton beam in the nozzle, a gas-filled beam pipe in the nozzle is designed to take the beam through the scanning magnets. The effects of different kinds and different densities of the gases on the proton beam are analyzed with Monte Carlo simulation by using Geant4, at the same time, the influence of the sealing film of pipe is considered. The results show that compared with the air , helium-filled pipe can significantly reduce the beam scattering. The impact of gas purity on the proton beam is significantly increased when the percent of air entering the beam pipe is more than 5%. Slightly increasing helium pressure is beneficial to maintain the gas purity of the pipe and produce little energy loss of proton beam. According to the analytic result, keep the helium gas pipe 1.1 times atmospheric pressure, while the air concentration is lower than 5%, the energy loss of the proton beam passing through the nozzle is less than 3%, and the transverse dimension Sigma value of beam size is less than 8.5 mm, meet the requirements of the treatment system.