Abstract
BACKGROUND: In ion beam radiotherapy, treatment radiation fields are inevitably contaminated with secondary neutrons. The energies of these neutrons can reach several hundreds of MeV. Fluorescent nuclear track detectors (FNTDs) offer a promising solution for dosimetry of fast and high-energy neutrons, particularly given their low linear energy transfer in water (LET) detection threshold. PURPOSE: This study presents an experimental FNTD sensitivity analysis in six fast mono-energetic neutron fields, comparing the response to poly allyl diglycol carbonate (PADC) neutron detectors, and investigates the feasibility of estimating ambient dose equivalent for neutrons, H∗ (10). Moreover, it investigates the impact of converter thickness on the detector signal for both fast and high-energy neutrons and analyzes the resulting differences in signal. METHODS: FNTDs and PADCs were exposed to mono-energetic neutron fields with energies of 1.2 MeV, 2.5 MeV, 5 MeV, 6.5 MeV, 14.8 MeV, and 19 MeV and evaluated based on the track density. The H∗ (10) values for FNTDs were determined by applying energy calibration factors, k(E) , which were determined through Monte Carlo (MC) simulations. The benchmarked MC model is employed to investigate the sensitivity of FNTDs to high-energy neutrons up to 200 MeV for various polyethylene (PE) converter thicknesses and to analyze the detector signal, including the particle type and the recoil proton LET. RESULTS: The sensitivity values revealed an energy dependence for FNTDs, with variations by a factor of up to 23, whereas PADC detectors showed a smaller variation, ranging from 3 to 12. Accurate H∗ (10) estimation can be achieved employing MC-derived k(E) factors, with deviations not exceeding 10 % . The sensitivity values increased almost continuously up to 200 MeV for PE converter thicknesses above 2 mm , whereas plateaued for thinner PE converters above 10 MeV to 15 MeV. For neutrons above 20 MeV , the generated fragments are deuterons, tritons and 4He , which constitutes up to 15 % or more of the total fluence in a 150 MeV neutron field. The recoil proton LET dropped from approximately 47 keV μm-1 to nearly one order of magnitude less between 1.2 MeV and 19 MeV, with an average LET of approximately 2 keV μm-1 at 150 MeV . CONCLUSIONS: This study compares FNTD and PADC detector sensitivities, demonstrating a notable energy and converter thickness dependence for FNTDs, which is essential for precise dosimetry. Accurate H∗ (10) values for fast mono-energetic neutrons up to 19 MeV were determined utilizing MC simulations. A benchmarked MC model for fast neutrons was then applied to analyze the FNTD signal for high-energy neutrons.