WE‐E‐141‐08: Measurements of Secondary Neutron Spectrum and Dose Equivalent From a 250 MeV Passively Scattered Proton Beam Using An Extended Range Bonner Sphere Spectrometer

Purpose: Secondary neutrons from proton therapy have a wide energy range, from thermal to the maximum proton beam energy, which can be as high as ∼250 MeV. Most measurements reported in the literature to date have either not included spectral information or were obtained using detectors with limited response to neutrons above 10 MeV. Therefore, the primary objective of the present study was to measure a secondary neutron spectrum from proton therapy using a spectrometer that was sensitive to neutron energies over the entire energy spectrum. Methods: We performed measurements using an extended‐range Bonner sphere spectrometer (BSS) with high‐atomic‐number shells placed within polyethylene spheres to increase high‐energy response compared to a standard BSS. Measurements were performed for a 250‐MeV passively scattered proton beam with a medium snout and closed aperture. All measurements were performed in air 100 cm from isocenter. Data were unfolded using the MXD_FC33 unfolding algorithm. The a priori spectrum for neutron unfolding was determined using the Bayesian statistical estimator methodology. We used Monte Carlo N‐Particle Code to calculate a response matrix specifically for this measurement system. Results: Total measured fluence was 8.36 ncm2Gy‐1 and included a low energy tail from thermal energies to approximately 1 eV (16% fluence), an evaporation peak from 1 eV to 10 MeV (69% fluence), and a direct neutron peak from 10 MeV to 250 MeV (15% fluence). The total ambient dose equivalent was 2.03 mSvGy‐1 with neutrons in the evaporation peak contributing the highest dose equivalent and neutrons below 1 eV contributing essentially no dose. Conclusion: We report both neutron fluence and dose equivalent spectra for a 250 MeV passively scattered proton beam. These data were measured with a spectrometer that is sensitive over the entire measurable neutron energy spectrum (thermal to 250 MeV) and thus could be a valuable tool for benchmarking Monte Carlo simulations. This research was supported in part, by the National Cancer Institute award 1R01CA131463‐01A1 (W.D. Newhauser, P.I.) and a subcontract of that award (R.M. Howell, P.I).