Medical physics · 2025
Sensitivity analysis of fluorescent nuclear track detectors for fast and high-energy mono-energetic neutron dosimetry.
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Schmidt Stefan, Christensen Jeppe B, Lutz Benjamin, Stabilini Alberto, Yukihara Eduardo G, Vedelago José
Аннотация
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 ∗ $H^{*}$ (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 ∗ $H^{*}$ (10) values for FNTDs were determined by applying energy calibration factors, k ( E ) $k(E)$ , 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 ∗ $H^{*}$ (10) estimation can be achieved employing MC-derived k ( E ) $k(E)$ factors, with deviations not exceeding 10 % $10\, \%$ . The sensitivity values increased almost continuously up to 200 MeV $200 \,\mathrm{MeV}$ for PE converter thicknesses above 2 mm $2 \,\mathrm{mm}$ , whereas plateaued for thinner PE converters above 10 MeV to 15 MeV. For neutrons above 20 MeV $20 \,\mathrm{MeV}$ , the generated fragments are deuterons, tritons and 4 He $^{4}{\rm He}$ , which constitutes up to 15 % $15 \,\%$ or more of the total fluence in a 150 MeV $150 \,\mathrm{MeV}$ neutron field. The recoil proton LET dropped from approximately 47 keV μ m - 1 $47\, \mathrm{keV}\,{\umu}{\mathrm{m}}^{-1}$ to nearly one order of magnitude less between 1.2 MeV and 19 MeV, with an average LET of approximately 2 keV μ m - 1 $2\, \mathrm{keV}\,{\umu}{\mathrm{m}}^{-1}$ at 150 MeV $150 \,\mathrm{MeV}$ .
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 ∗ $H^{*}$ (10) values for fast mono-energetic neutrons up to 19 MeV $19 \,\mathrm{MeV}$ were determined utilizing MC simulations. A benchmarked MC model for fast neutrons was then applied to analyze the FNTD signal for high-energy neutrons.
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 ∗ $H^{*}$ (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 ∗ $H^{*}$ (10) values for FNTDs were determined by applying energy calibration factors, k ( E ) $k(E)$ , 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 ∗ $H^{*}$ (10) estimation can be achieved employing MC-derived k ( E ) $k(E)$ factors, with deviations not exceeding 10 % $10\, \%$ . The sensitivity values increased almost continuously up to 200 MeV $200 \,\mathrm{MeV}$ for PE converter thicknesses above 2 mm $2 \,\mathrm{mm}$ , whereas plateaued for thinner PE converters above 10 MeV to 15 MeV. For neutrons above 20 MeV $20 \,\mathrm{MeV}$ , the generated fragments are deuterons, tritons and 4 He $^{4}{\rm He}$ , which constitutes up to 15 % $15 \,\%$ or more of the total fluence in a 150 MeV $150 \,\mathrm{MeV}$ neutron field. The recoil proton LET dropped from approximately 47 keV μ m - 1 $47\, \mathrm{keV}\,{\umu}{\mathrm{m}}^{-1}$ to nearly one order of magnitude less between 1.2 MeV and 19 MeV, with an average LET of approximately 2 keV μ m - 1 $2\, \mathrm{keV}\,{\umu}{\mathrm{m}}^{-1}$ at 150 MeV $150 \,\mathrm{MeV}$ .
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 ∗ $H^{*}$ (10) values for fast mono-energetic neutrons up to 19 MeV $19 \,\mathrm{MeV}$ were determined utilizing MC simulations. A benchmarked MC model for fast neutrons was then applied to analyze the FNTD signal for high-energy neutrons.