High-resolution Monte Carlo investigation of the spatial distribution of neutron and photon absorbed doses in a polyethylene shuttle for soil sample irradiation

The accurate characterization of mixed neutron–photon radiation fields generated during neutron irradiation of environmental samples is essential for dosimetric optimization and radiological protection in activation analysis facilities. In the present work, a Monte Carlo simulation study was performed using the Particle and Heavy Ion Transport code System (PHITS) version 3.35 to quantify the spatial distribution and attenuation behavior of neutron and photon absorbed doses in a polyethylene shuttle designed for soil sample irradiation. The three-dimensional geometry of the irradiation capsule, moderator structure, and surrounding air environment was modeled in detail, and absorbed doses were evaluated at radial distances ranging from direct contact up to 1 m under zero-cooling-time conditions, corresponding to prompt radiation fields during irradiation. This study provides a strong neutron dose dominance in the near-field region, with absorbed doses approaching 60 Gy, followed by a rapid attenuation beyond 5 cm driven by geometric dispersion and neutron leakage. The photon component, although lower in magnitude, exhibits measurable buildup effects at intermediate distances due to prompt gamma production from neutron interactions. All Monte Carlo statistical uncertainties were maintained below 2%. This work provides a high-resolution dosimetric database describing the coupled neutron–photon radiation field around a realistic polyethylene irradiation shuttle, offering quantitative information that is currently scarce in the literature and supporting dosimetric optimization and radiation protection in neutron activation analysis facilities.