The plastic tray effect on the spectrum of photoneutrons prudused by 18 MV medical linac using the Monte Carlo method

Introduction: One of the important problems of using high energy linacs is the production of photoneutrons. High energy medical linacs produce secondary neutrons. These phtoneutrons increase patient dose
 
Materials and Methods: In this study, the effect of plastic trays on spectrum of photoneutrons produced by 18MV linac is investigated using the MCNP4C code. To determine the photoneutron spectrum a simplified linac head simulated. In order to contribute the all components of the spectra, an isotropic neutron source was considered with an energy spectrum derived from the relevant equation. Source defined at the center of a thick walled sphere of tungsten. Two kinds of trays, PMMA and Polyethylene, each with 1.5 cm thickness defined just under the aperture of simplified linac head. A typical treatment room simulated with the walls made of standard concrete. Neutron detectors defined as spheres of 10 cm diameter. One of the detectors defined just under the linac head and the other at the adjacent of the bucker door. Three situations were considered, one without tray and two others with trays. Simulation carried out by running the computer to produce 100 millions of particles for each case. F4 tally was used.
Results: The results show that spectrum of photoneutrons differs considerably for open field and shielded fields.