title: Monte Carlo characterizations mapping of the (γ,n) and (n,γ) photonuclear reactions in the high energy X-ray radiation therapy.
abstract: OBJECTIVE
  The aim of this work was to map the characteristics of (n,γ) and (γ,n) reactions in a high energy photon radiation therapy.
  
  
  BACKGROUND
  Photoneutrons produced in the high energy X-Ray radiation therapy may damage patients and staff. It is due to high RBE of the produced neutrons according to their energy and isotropic emission. Characterization of the photoneutrons can help us in appropriate shielding.
  
  
  METHODS
  This study focused on the photoneutron and capture gamma ray phenomena. Characteristics such as dose value, fluence and spectra of both the neutrons and the by produced prompt gamma ray were described.
  
  
  CONCLUSIONS
  Neutron and prompt gamma spectra in different points showed the neutrons to be thermalized when increasing the distance from the linac. Energy of the neutrons changed from about 0.6 MeV at the isocentre to around 10(-08) MeV at the outer door position. Although the neutrons were found as fast neutrons, their spectra showed they were thermal neutrons at the outer door position. Additionally, it was seen that the energy of the gamma rays is higher than the scattered X-ray energy. The energy of gamma rays was seen to be up to 10 MeV while the linac photons had energy lower than 1 MeV. Neutron source strength obtained in this work was in good agreement with the published data, which may be a confirmation of our simulation accuracy.
  
  
  CONCLUSIONS
  The study showed that the Monte Carlo simulation can be applied in the radiotherapy and industrial radiation works as a useful and precise estimator. We also concluded that the dose from the prompt gamma ray at the outer door location is higher than the scattered radiation from the linac and should be considered in the shielding.
authors.affiliation: University of Medical Sciences, Department of Medical Physics, Iran.
authors.forename: Hosein
authors.initials: H
authors.lastname: Ghiasi
keywords: Capture gamma ray; Monte Carlo; Photoneutron; Shielding