Evaluation of the photoneutron field produced in a medical linear accelerator.

The doses and spectra of photoneutrons produced in a medical linear accelerator with photon energies of 10 and 15 MV were evaluated. The Monte Carlo code, MCNPX, was used to simulate the transport of these photoneutrons around the head for 10 and 15 MV photons. The fully-described geometry of the accelerator head was used in this calculation. The photoneutron energy spectra and doses for various photon field sizes were calculated at each of 20 positions. The results indicate that the maximum dose equivalents are observed in 20 x 20 cm(2) case among photon fields. It was found the neutron average energy at isocenter for a 0 x 0 cm(2) field is 0.38 MeV for 10 MV and is 0.45 MeV for 15 MV. The neutron doses at 10 positions around the head in the treatment room of the operation facility at 10 and 15 MV were measured using the bubble detectors. Measurements were compared with the calculations under the same geometry in the experiment. It was found that the majority of the calculated results agreed to within the standard deviations of the measurements. These above results can be applied in the verification of maximum allowed neutron leakage percentage of treatment dose defined in the IEC. We have been employing them to derive the empirical formula for neutron dose equivalent level at the maze entrance of medical accelerator treatment rooms in a study that is still underway.