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Monte Carlo model of Siemens Oncor medical linear accelerator

This study was conducted in order determine neutron flux around Siemens Oncor medical linear accelerator 18 MV photon beam. We built the model of the accelerator using Monte Carlo N-Particle transport code (MCNP611) and verified it by comparing the characteristics of the photon beam with experimental measurements. Modeling of the accelerator head cover was of the particular interest, since it is very often omitted from the simulations. Reconstruction of this part is not straight forward, since it has bending magnet and non-symmetric geometry. Also the manufacturer did not give any geometry specifications of the accelerator head cover in the primer, so besides quantities that can be measured, lot of parts should be assumed. From this reason three different compositions were simulated, so the final geometry can be interpolated form them to fit the experimental data. Detectors were set at three planes in the rectangular box with 2 m edge, where target is placed in box center, so the patient plane, plane above accelerator and plane on a side of the accelerator head were of our special interest. In the simulations we had altogether 61 detectors that enabled us to determine the neutron spectra, mean energies and neutron fluxes at points around accelerator head. Besides measured quantities, simulations also gave us the information of neutron place of origin (i.e. target, primary collimator, flattening filter, head cover…) and neutron flux energy spectra in detectors. Variance of Monte-Carlo simulations was reduced using DXTRAN spheres for each detector, which enabled us to calculate the contribution of any collision in the simulation, to the detectors. The computational results were compared with the experimental measurements that were performed at the same points (as in the computer simulations). Experimental results were obtained using passive detectors (CR-39) covered with boron (10B) foil as a converter. The measurements were corrected for energy dependence of detectors using the neutron spectra determined by MC simulations and boron crossection. The results of Monte Carlo simulations and experimental results are in good agreement, although the accelerator table and vault were omitted in the simulation. Unlike the photon beam that is forwarded in one direction, neutron flux is much more uniform around the accelerator.

Monte Carlo, Medical linear accelerator, MCNP

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