New aspect determination of photoneutron contamination in 18Â MV medical linear accelerator

In this study, the neutron contamination due to the 2300C/D Varian medical linac has been calculated by using the long counter. In doing so, the energy response of the long counter in wide energy ranges (1keV-7MeV) of the neutron has been flattened and its effective center and directional sensitivity has been determined. All calculations have been performed by using the simulation of the measurable arrangement with the Monte Carlo N-Particle MCNPX code at isocenter for different photon field sizes. The obtained results showed that the inverse square formula for determining the neutron fluence needs to be corrected so that in the power value of d, the distance from the source to the point of interest, the value 1.88 ± 0.08 is substituted with 2 at inverse square law in the field size 10 × 10 cm2. Therefore, the values of the neutron strength, neutron fluence and the ambient dose equivalent, H*(10) have been obtained by using the above correction. Finally, the obtained results have been compared with other published results. On the other hand, it is found that the flattening filter caused to sharp the energy fluence at the center of the beam by using the point detector tally and consequently the high dose delivered to the patient.