Hybrid Phy-X/PSD–Geant4 Assessment of Gamma and Neutron Shielding in Lead-Free HDPE Composites Reinforced with High-Z Oxides

This study evaluates lead-free high-density polyethylene (HDPE) composites reinforced with high-Z oxides (Bi2O3, WO3, Gd2O3, TeO2, and a Bi2O3/WO3 hybrid) as lightweight materials for gamma-ray and fast-neutron shielding. A hybrid computational framework combining Phy-X/PSD with Geant4 Monte Carlo simulations was used to obtain key shielding parameters, including the linear and mass attenuation coefficients (μ, μ/ρ), half-value layer (HVL), mean free path (MFP), effective atomic number (Zeff), effective electron density (Neff), exposure and energy-absorption buildup factors (EBF, EABF), and fast-neutron removal cross section (ΣR). The incorporation of heavy oxides produced a pronounced improvement in gamma-ray attenuation, particularly at low energies, where the linear attenuation coefficient increased from below 1 cm−1 for neat HDPE to values exceeding 130–150 cm−1 for Bi- and W-rich composites. In the intermediate Compton-scattering region (≈0.3–1 MeV), all oxide-reinforced systems maintained a clear attenuation advantage, with μ values around 0.12–0.13 cm−1 compared with ≈0.07 cm−1 for pure HDPE. At higher photon energies, the dense composites continued to outperform the polymer matrix, yielding μ values of approximately 0.07–0.09 cm−1 versus ≈0.02 cm−1 for HDPE due to enhanced pair-production interactions. The Bi2O3/WO3 hybrid composite exhibited attenuation behavior comparable, and in some regions slightly exceeding, that of the single-oxide systems, indicating that mixed fillers can effectively balance density and shielding efficiency. Oxide addition significantly reduced exposure and energy-absorption buildup factors below 1 MeV, with a moderate increase at higher energies associated with secondary radiation processes. Fast-neutron removal cross sections were also modestly enhanced, with Gd2O3-containing composites showing the highest values due to the combined effects of hydrogen moderation and neutron capture. The close agreement between Phy-X/PSD and Geant4 results confirms the reliability of the dual-method approach. Overall, HDPE composites containing about 60 wt.% oxide filler offer a practical compromise between shielding performance, manufacturability, and environmental safety, making them promising candidates for medical, nuclear, and aerospace radiation-protection applications.