A Monte Carlo simulation of gamma-ray backscattering from concrete shields coated with nanoparticle layers

In recent years, nanocomposite shields have emerged as a promising alternative to conventional lead ones, with rapidly growing applications in medical and industrial sectors. Although the scattering characteristics of nanocomposite shielding materials have been widely investigated, their backscattering behavior remains unexplored in scientific studies. Therefore, obtaining accurate measurements of gamma photon backscattering is essential for evaluating the effectiveness of nanomaterials in radiation shielding. This study investigates the influence of nanoparticles on gamma photon backscattering in low-density polyethylene (LDPE)-based composite radiation shields, using Monte Carlo simulations. Following the validation with available experimental and simulation results, the photon backscattering of LDPE composites doped with W, Ti, Zn, Pb, and Bi particles layered over concrete were analyzed at weight percentages (wt%) of 5%, 10%, 15%, and 20%, and particle sizes ranging from bulk and microscale (10 µm) to nanoscale (100 nm) via the MCNPX code. For comparison purposes, the reflection coefficient (RC) was calculated for 0.1-15 MeV gamma-rays at reflection angles of 92° to 165^° and the photon energy spectra were elevated for 0.25, 0.5, and 1 MeV gamma-rays at 135^° reflection angle through simulating the ring dosimeters in the proper positions. The results indicate that RC value decreases with increasing the incident gamma energy, reaching its minimum at 15 MeV in all configurations. Besides, the maximum RC (40.18) is observed in gamma energy of 0.1 MeV, in the case of a sample doped with 20% wt of Bi nanoparticles. Moreover, numerical comparisons reveal that reducing particle size enhances the RC, so that nanoscale composites exhibiting a 15-30% higher RC than bulk counterparts at lower energies. Notably, the highest improvement in shielding performance was achieved with 20% Bi nanoparticles at low energies (0.1 MeV), where RC increased by 15% compared to standard concrete. Furthermore, the findings demonstrate that increasing the wt% from 5 to 20 at nanoscale dimensions reduces the number of gamma-photons detected by 25%. Additionally, reducing particle size from bulk case and 10 µm to 100 nm leads to a slight decline in gamma counts (about 8%), attributed to a higher surface-to-volume ratio and increased scattering events. In overall, these findings confirm the effectiveness of nanoparticle composites in enhancing radiation shielding performance and highlight their potential as advanced, lead-free alternatives for medical and industrial applications.