241Am-Be based Simulated Workplace Neutron Reference Fields using Graphite and Polyethylene Moderators: Spectral and Dosimetric Characterization

The accurate characterization of simulated workplace neutron fields is a prerequisite for the meaningful calibration of radiation protection instruments under realistic spectral conditions. This work presents Monte Carlo simulation data (including: neutron fluence rate spectrum, neutron ambient-dose-equivalent-averaged energy, as well as total neutron fluence and ambient dose equivalent rate) of the 241Am-Be based simulated workplace neutron calibration facility at the Institute for Nuclear Science and Technology (INST, Hanoi, Vietnam) using the Particle and Heavy Ion Transport code System (PHITS, version 3.36). A 241Am-Be neutron source of X14 capsule type was modelled at the centre of a concrete-walled calibration room (7.0 m × 7.0 m × 7.0 m), surrounded by spherical moderating assemblies of high-density polyethylene (HDPE, ρ = 0.95 g cm−3) and graphite (pure carbon, 12C) with wall thicknesses ranging from 15 to 35 cm. Volumetric track-length estimators (“T-Track” tally) were implemented along the diagonal calibration axis at source-to-detector distances of 100 to 250 cm to resolve both the thermal and fast spectral components with the statistical uncertainty less than ≤ 1.0%. Neutron fluence spectra due to HDPE configurations were benchmarked against published experimental data obtained at the same facility using a Bonner sphere spectrometer, showing excellent agreement. Total neutron fluence rates and ambient dose equivalent rates, H*(10), were derived by scaling T-Track outputs by the calibrated source emission rate and by applying ICRP Publication 74 fluence-to-dose conversion coefficients, respectively. Neutron ambient-dose-equivalent-averaged energies due to HDPE and graphite configurations were also calculated and compared with published data. The comparative analysis demonstrates that HDPE efficiently thermalizes and absorbs fast neutrons, whereas graphite thermalizes the field while preserving a significantly higher total neutron population. Bigger-thickness configurations (60 cm and 100 cm-diameter graphite spheres; 60 cm-diameter HDPE sphere) further illustrate the unique capability of carbon-based moderators for generating intense thermal neutron fields. Both moderator types are confirmed to produce simulated workplace neutron fields compliant with ISO 12789.