Study of an energy degrader made of B₄C/graphite composites

The HUST-PTF (proton therapy facility in Huazhong University of Science and Technology) is now under construction based on an isochronous superconducting cyclotron at Huazhong University of Science and Technology. Because the energy extracted by the cyclotron is fixed, an energy degrader is necessary to modulate the beam energy after the cyclotron for clinical requirements. However, the multiple Coulomb scattering that occurs in the degrader increases the beam emittance. Therefore, collimators are designed after the degrader to decrease the beam emittance. Because of the severe beam losses in the aforementioned energy degradation and emittance restriction, the main design consideration is improving the beam transmission efficiency. In this note, a novel energy degrader made of B₄C/graphite composite (BGC) is studied regarding its material fabrication, improved beam transmission efficiency, energy degradation, emittance growth, and secondary-neutron yield using the Geant4 Monte Carlo software. Moreover, the thermal properties of the BGC degrader are analyzed. Finally, for a degrader with 40% boron content, the emittance after the degrader decreases by 5.5% and the transmission efficiency after the final collimator increases by 15.9% at 70 MeV compared with a pure-graphite degrader.