Estimation of photoneutron yield in linear accelerator with different collimation systems by Geant4 and MCNPX simulation codes.

At present, the bremsstrahlung photon beams produced by linear accelerators are the most commonly employed method of radiotherapy for tumor treatments. A photoneutron source based on three different energies (6, 10 and 15 MeV) of a linac electron beam was designed by means of Geant4 and Monte Carlo N-Particle eXtended (MCNPX) simulation codes. To obtain maximum neutron yield, two arrangements for the photo neutron convertor were studied: (a) without a collimator, and (b) placement of the convertor after the collimator. The maximum photon intensities in tungsten were 0.73, 1.24 and 2.07 photon/e at 6, 10 and 15 MeV, respectively. There was no considerable increase in the photon fluence spectra from 6 to 15 MeV at the optimum thickness between 0.8 mm and 2 mm of tungsten. The optimum dimensions of the collimator were determined to be a length of 140 mm with an aperture of 5 mm  ×  70 mm for iron in a slit shape. According to the neutron yield, the best thickness obtained for the studied materials was 30 mm. The number of neutrons generated in BeO achieved the maximum value at 6 MeV, unlike that in Be, where the highest number of neutrons was observed at 15 MeV. Statistical uncertainty in all simulations was less than 0.3% and 0.05% for MCNPX and the standard electromagnetic (EM) physics packages of Geant4, respectively. Differences among spectra in various regions are due to various cross-section and stopping power data and different simulations of the physics processes.