Neutron and photon spectra in LINACs

A Monte Carlo calculation, using the MCNPX code, was carried out in order to estimate the photon and neutron spectra in two locations of two linacs operating at 15 and 18 MV. Detailed models of both linac heads were used in the calculations. Spectra were estimated below the flattening filter and at the isocenter. Neutron spectra show two components due to evaporation and knock-on neutrons. Lethargy spectra under the filter were compared to the spectra calculated from the function quoted by Tosi et al. that describes reasonably well neutron spectra beyond 1 MeV, though tends to underestimate the energy region between 10–6 and 1 MeV. Neutron and the Bremsstrahlung spectra show the same features regardless of the linac voltage.

► With MCNPX code realistic models of two LINACs were built.
► Photon and neutron spectra below the flattening filter and at the isocenter were calculated.
► Neutron spectrum at the flattening filter was compared against the Tosi et al. source-term model.
► Tosi et al. model underestimates the neutron contribution below 1 MeV.
► Photon spectra look alike to those published in literature.