Photoneutron Shielding Design for an 18 MV Saturne 20 Medical Linear Accelerator

Abstract

Background: Photoneutrons are produced by high energy photons and electrons interaction with various high-Z nuclei of the materials in the linac head components. These neutrons cause a patient to receive an extra absorbed dose. In this research, we investigated the effect of shielding with various materials and different thicknesses at different distances from the linac collimator to reduce the neutron dose from an 18 MV Saturne 20 medical linear accelerator. Methods: Monte Carlo codes namely, Monte Carlo N-Particle eXtended (MCNPX) and Monte Carlo N-particle code (MCNP4C) were used for the simulations. A photoneutron shielding was modeled in the Monte Carlo codes using plexiglass, polyethylene and graphite materials. Photoneutron dose was calculated at the presence of these shields and was compared with that which was at the presence of no shields. Neutron flux to dose rate conversion factors of International commission on radiological protection-21 (ICRP-21) and National council on radiation protection and measurements-38 (NCRP-38) were used for the calculations. Findings: There was a good agreement (less than 5% discrepancy) between the simulations and experimental measurements. Using a polyethylene shield, photoneutron dose was decreased by up to 13%, compared to no shield. for plexiglas and graphite, decrease in photoneutron dose was 12% and 6%, respectively. Conclusion: Photoneutron shielding using plexiglass, polyethylene and graphite materials can be effective to reduce the Saturne 20 neutron dose. Among the used materials for neutron shielding, polyethylene showed the maximum reduction and graphite gives the lowest reduction of photoneutron dose.

Keywords

• Shielding design
• Photoneutron dose
• Saturne 20 medical linear accelerator