Document Type: Original Paper
Department of Physics, Faculty of Mathematics and Natural Sciences, Institut Teknologi Bandung
Department of Radiation Oncology, Concord International Hospital, Singapore/ Singapore
National Nuclear Energy Agency
Introduction: High energy linear accelerator (Linac) photon mode (E≥10 MV) can produce neutron as an unexpected contaminant. Although the numbers of neutrons produced are very small compared to photons or electrons in neutron radiation protection field, it is necessary to be evaluated because it has a high radiobiological effect (RBE) that can be more destructive compared to photons or electrons. This study aimed to measure the neutron contamination of 10 MV-FF and FFF photon beam Elekta InfinityTM accelerator.
Methods: The photo-neutron spectrum produced by the Linac head was evaluated using a Monte Carlo (MC) simulation. The geometry and composition of the head Linac material are modeled based on information from the manufacturer. MCNP6 is used to model the Linac head and the photon transport process to produce neutrons. The simulated field size was 10×10 cm2 in SSD=99 cm. Evaluation of neutron contamination was carried out for the Linac with the Flattening Filter (FF) and without FF or the Free Flattening Filter (FFF). The FFF beam was built by removing the FF from the Linac components. The scoring plane (the neutron spectra calculation area for FF and FFF beams) is placed 99 cm from the target.
Results: The neutron type produced by the head Linac Elekta InfinityTM 10 MV photon mode was mostly thermal and fast neutron. Although there were differences in the neutron intensity of FF and FFF, the type of neutron produced by the two modes have the same energy. Based on the neutron photo reaction energy threshold it can be concluded that the neutrons produced from the head Linac were the result of photo-neutron interactions between high energy photons with 96Mo and 184W isotopes.
Conclusion: The photo-neutron quantity does not change for FF and FFF beams, but a larger quantity of neutrons is produced in the FFF beam.