Optimum shielding structure for the wall of medical LINAC facility

When considering the space limit of high energy LINAC facility in a hospital, the design of shielding wall is strongly required to optimize the wall thickness. Shielding wall usually consists of concrete, and iron plate is sometimes added in concrete to decrease the wall thickness. We investigated the optimum design of the shielding wall in medical LINAC facility by simulation. The calculations were carried out by the three-dimensional Monte Carlo code, MCNP5. The electron energy used in this investigation is 28 MeV. In the calculations, the wall thickness was fixed to 100 cm from the viewpoint of durability and reliability and the cost to build the shielding wall. In the shielding wall consisting of iron and concrete, the suitable configuration was found to have 40-50 cm-thick iron at the front side. But it was insufficient to achieve the required attenuation rates of 2 x 10 and 5 x 10 for effective neutron and photon doses, respectively. We further searched the optimum combination of shielding materials to get the most efficient attenuation profile and finally obtained the optimum arrangement of 100 cm thick shielding wall having the multi-layered structure of iron 40 cm, polyethylene 10 cm, iron 20 cm, lead 10 cm, polyethylene 10 cm, and concrete 10 cm in this order. In order to investigate the shielding capability of this multi-layered shielding wall, the neutron and photon dose rates were measured using the 45 MeV electron linear accelerator of Hokkaido University to verify the MCNP5 calculated results. The MCNP5 calculation was found to give good accuracy within 30% for the multi-layered shielding structure.