Calculation of photoneutrons produced in the targets of electron linear accelerators for radiography and radiotherapy applications

Photoneutron spectra produced by bremsstrahlung in electron linear accelerators for radiography and radiotherapy applications have been investigated. Calculations with the Monte Carlo code FLUKA were performed for electron beams of different energy values striking the same tungsten target, and a 15 MeV electron beam bombarding targets of different materials as well as different thicknesses to study the photoneutron spectra and the neutron dose equivalent at the isocenter. This work presents neutron yields and neutron energy spectra for targets under different conditions and compares these with the corresponding published results. The results show that the photoneutron average energy is below 1.0 MeV under the above mentioned conditions, which is much lower than the one published in NCRP Report No. 79. For a 4 mm tungsten target and incident electron energies of 9, 10, 15, 18 and 20 MeV, the photoneutron dose equivalents at the isocenter are 0.006, 0.016, 0.147, 0.231 and 0.261 mSv/GyX-ray, respectively. For copper, tantalum, tungsten and gold targets with different thicknesses, the photon doses at the isocenter are of the same order (∼10−15 Gy/e), while photoneutron yields of copper targets are one order smaller than those of the other three materials, and so are the neutron dose equivalents of copper targets. The medium atomic number materials, such as copper with a thickness of about 0.56 radiation length, which is sufficient to prevent electron leakage, appeared to be good choices for sufficient photon production and lower neutron contamination.