To BPE or not to BPE: neutron tenth-value layers in polyethylene with variable boron content for LINAC shielding

Borated polyethylene (PE) is an effective neutron moderator and absorber in medical linear accelerator shielding; however, there is limited data regarding the required material thickness for adequate neutron attenuation. To address the gap in shielding data, our study systematically quantifies first and equilibrium tenth-value layers (TVLs1 and TVLe) for PE containing 0%, 5% and 30% natural boron by weight from thermal to fast neutrons. Comprehensive Monte Carlo simulations (n= 3504) were performed to estimate TVL thicknesses from thermal to 20 MeV neutrons. A current tally was used to count neutrons exiting the shield and determine thicknesses corresponding to 10% and 1% transmission. Sixteen energies and 73 thicknesses of materials were modelled with statistical uncertainties below 3%. TVL thicknesses were independently validated with particle and heavy ion transport code system using identical simulation parameters. We found that TVL values ranged from 1.3 mm for thermal neutrons in borated polyethylene (BPE), to 50 cm for 20 MeV neutrons in pure PE. In all cases, adding boron to PE reduced the TVL, with the greatest effect at thermal energies, and a smallest effect at 12 MeV. Here we provide the first comprehensive characterisation of BPE's ability to attenuate neutrons, supporting shielding design for medical linear accelerators.