Neutron spectra and H*(10) of photoneutrons inside the vault room of an 18 MV LINAC

Abstract

Neutron spectra and the ambient dose equivalent were estimated inside the radiotherapy hall with an 18 MV linac. Estimations were carried out using Monte Carlo methods where a realist hall was modeled including a phantom made of equivalent tissue. The source term for photoneutrons was calculated using the Tosi et al. function that account for evaporation and knock-on neutrons. The spectra were estimated using two different energy distributions. Detectors were located in several sites inside the hall including the maze and outside the hall door, all detectors were located at the plane were the isocenter is located. In the treatment hall, as the distance respect to the isocenter is increased, the amount of neutrons and the H*(10) are reduced, neutrons in the high-energy region are shifted to lower region peaking around 0.1 MeV, however the epithermal and thermal neutrons remain constant due to the room-return effect. In the maze the spectra are dominated by epithermal and thermal neutrons that contributes to produce activation and the production of prompt gamma-rays.