Sensitivity analysis of fluorescent nuclear track detectors for fast and high-energy mono-energetic neutron dosimetry.

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, $H^{\ast }$ (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 $H^{\ast }$ (10) values for FNTDs were determined by applying energy calibration factors, $k\left(E\right)$ , 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 $H^{\ast }$ (10) estimation can be achieved employing MC-derived $k\left(E\right)$ factors, with deviations not exceeding $10\%$ . The sensitivity values increased almost continuously up to $200\mathrm{MeV}$ for PE converter thicknesses above $2\mathrm{mm}$ , whereas plateaued for thinner PE converters above 10 MeV to 15 MeV. For neutrons above $20\mathrm{MeV}$ , the generated fragments are deuterons, tritons and ${}^4\mathrm{He}$ , which constitutes up to $15\%$ or more of the total fluence in a $150\mathrm{MeV}$ neutron field. The recoil proton LET dropped from approximately $47\mathrm{keV}\mu m^{-1}$ to nearly one order of magnitude less between 1.2 MeV and 19 MeV, with an average LET of approximately $2\mathrm{keV}\mu m^{-1}$ at $150\mathrm{MeV}$ .

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 $H^{\ast }$ (10) values for fast mono-energetic neutrons up to $19\mathrm{MeV}$ were determined utilizing MC simulations. A benchmarked MC model for fast neutrons was then applied to analyze the FNTD signal for high-energy neutrons.