Photoneutron spectra around an 18 MV LINAC

Photoneutron spectra around an 18 MV LINAC were calculated in order to observe the effect produced by media around the accelerator. Calculations were carried out with MCNP 4C code, three different cases were analyzed: Head model, Head and phantom model, and Head, air, phantom and wall model. The spectra were calculated in five detectors located at the irradiation room at different distances from the isocentre. A sixth detector, located near the entrance door was included to analyze how the maze change the neutron spectrum. Neutrons are mainly pro- duced in the LINAC head change the shape of evaporation neutrons from the source term, some of these neutrons leak out the head with lesser energy, another neutrons goes with the treatment beam. At any site near the isocentre neutron spectrum has evaporation and thermal neutrons joined by a set of epithermal neutrons. As the distance from the iso- centre increases evaporation neutrons tend to decrease while, epithermal and thermal neutrons tend to remain constant regardless de distance due to room return pro- duced by the walls. The maze contributes to reduce the neutron fluence, reducing the evaporation neutrons; resulting spectrum is mainly the contribution of thermal and epithermal neutrons. Near the door these neutrons can produce activation and prompt gamma rays.