Measurement of Neutron Dose Equivalent and Penetration in Concrete for 230 MeV Proton Bombardment of Al, Fe, and Pb Targets

Abstract

Secondary neutron production from protons striking accelerator beam delivery components and the patient constitute the principal radiation hazard for 70–300 MeV accelerators used in proton radiation therapy. Because of the large mean free path of these high energy neutrons, neutron attenuation requires massive shields. To this end, we measured neutron dose as a function of emission angle and depth in concrete for the radiation environment produced by 230 MeV protons striking stopping targets of aluminium, iron, and lead. By using microdosimetric instrumentation, dose equivalent values were deduced. From these data, dose equivalent penetration as a function of depth in concrete and neutron emission angle were determined. Neutron production was found to vary rapidly with emission angle, while differences in dose equivalent values per incident proton as a function of depth and angle depended only slightly on target material.