Introduction: One of the concerns about using of high energy photons in radiotherapy is neutron contamination production. The threshold energy of photoneutron production in most materials with high atomic number is about 7MeV. Therefore, it seems to be logical that the main components with high atomic number elements used in the LINAC head have a large contribution to the neutron contamination production. In this study, we investigated the neutron contamination flux of Varian Clinac 2100 machine and design a secondary shield to reduce the neutron contamination dose and flux.
Materials and Methods: The Varian Clinac 2100 linear accelerator with 18MV performance energy was simulated by the FLUKA Monte Carlo code. Percent depth dose (PDD) and dose profile curves was measured using a PTW thimble chamber to validate our mode. In a device with photon mode and 18MV performance energy, materials with low and moderate atomic number, such as water and polyethylene, play an effective role in slowing down the fast neutrons. Slow neutrons are easily absorbed by the certain materials, such as boron. In this research, four different materials of water, polyethylene, borated polyethylene and zirconium borohydride which have been considered as suitable neutron shield in previous studies, have been design as a second shield. The shield is designed with a cylindrical shape and with a thickness of 5 cm around the LINAC head elements.
Results: Because of hydrogen and zirconium atoms, the zirconium borohydride is more effective in slowing down the fast neutrons. Also, the boron absorbs the slowed down neutrons. Furthermore, the process of absorbing neutrons does not produce a significant secondary gamma. Using zirconium borohydride, the neutron flux around the LINAC is reduced more than 70%. Also, the neutron contamination dose in out-of-field areas in water phantom are reduced by 81%, which increases the safety of linear accelerators for patients and personnel significantly.
Conclusion: We can conclude that among the four materials used for the neutron shielding In this study, the zirconium borohydride has a good shielding properties against the neutron contamination.