Optimal Design of Shielding for a Stepper Motor in an Energy Degrader System

A cyclotron-based proton therapy facility (PTF) is being developed at Huazhong University of Science and Technology (HUST). An energy degrader driven by a stepper motor is used to regulate the proton beam energy to the range of 70-240 MeV for clinical requirements. However, the interactions between protons and materials in the energy degrader lead to severe ionizing radiation, which will damage the stepper motor nearby, consequently deteriorating the beam quality. Shielding made of composite materials must be used to block the gamma rays and neutron effectively. For a given shield design, the stepper motor's maximum radiation exposure does not always correspond to the severest beam loss. FLUKA simulation results show that the stepper motor will suffer the maximum radiation dose rate of 5.62 Gy/h at 190 MeV of output beam energy. A composite shielding material of Pb and polyethylene is chosen to shield ionizing radiation. Because of the installation space restriction, the total shielding thickness is 14 cm. When the Pb thickness is 5.5 cm and the polyethylene thickness is 8.5 cm, the motor's suffered dose rate achieves the minimum, which is only 1/4 times the case without shielding.