Neutron leakage calculation of Varian medical linear accelerator with 115In foil activation and CR-39 detector by Monte Carlo simulation

High-energy medical accelerators, such as 15 MeV electron beam Varian accelerators, are widely used in Iran and worldwide for treatment and imaging. Due to photon-neutron interactions in the head of accelerators, neutrons with high energy enter the treatment room. In this article calculation of the energy spectrum of neutron leakage using the Monte Carlo N Particle X version (MCNPX) code was investigated to determine the possibility of neutron leakage in the treatment room. The neutron leakage was measured by activating a115In foil and tracking the neutron density in the CR-39 detector on the patient's bed in the treatment room. Finally, by placing an adult MIRD phantom on the patient's bed in the treatment room, the neutron dose distribution per unit time in sensitive organs due to leakage neutrons has been calculated. The Monte Carlo results show that the number of neutrons entering the treatment room from the accelerator head per second is 2.22E+12 (n/s). The activity of the indium foil per gram and the Track density rate of the CR-39 detector for neutron leakage of the Vital Varian accelerator at 15 MeV in the treatment room in 10 min are 116 Bq/g and 3.9E7 tracks/(cm2. s), respectively. By adjusting the field size of the accelerator for liver cancer treatment to a 5 x 5 cm2 field, the effective dose rate in MIRD phantom-sensitive organs by neutron leakage is reported to be 1.62 mSv/s (97.2 mSv/min). Therefore, 15 MeV electron beam Varian accelerators have neutron leakage to the treatment room. If the principles of neutron protection are not applied to them, the leaking neutrons due to the diffusion process will have a high radiation risk for patients and treatment room technicians.