Monte Carlo simulation and unfolding of an extended Bonner sphere system for secondary neutron spectrometry in proton therapy facilities using conventional BSS experimental data

The extended-range Bonner Sphere Spectrometer (BSS) is a valuable tool for characterizing neutron fields in radiotherapy, including proton therapy facilities, where secondary neutrons contribute to the incidental dose received by the patient and staff. In this study we developed and validated a Monte Carlo (MC) model of an extended-range BSS by benchmarking against experimental measurements performed using a conventional BSS in a proton therapy facility. The extended-range system, which includes four additional spheres with aluminum and lead layers, was not available for this work; instead, simulations were validated against conventional BSS data and then applied to extrapolate the high-energy portion of the neutron spectrum. The MC simulations were performed using MCNP version 6.3 and include modelling of the BSS and using BSS response functions previously validated. The comparison in data trends between simulations and measurements confirms the reliability of the simulation model. The acquired neutron spectra show the characteristic shape, but also noticeable differences between the measurement locations in the intensity and width of the direct, evaporation, and thermal peaks. These differences are consistent with the changing measurement conditions in each case. The obtained spectra show similarity with other spectra available in literature. This work demonstrates that MC-modeled extended-range BSS can accurately characterize secondary neutron spectra in proton therapy facilities, providing a foundation for improved radiation protection and dose optimization studies.