Correlation between radioactivity induced inside the treatment room and the undesirable thermal/resonance neutron radiation produced by linac

High-energy therapeutic beams used in the radiotherapy induce photonuclear and electronuclear reactions which are accompanied by generation of undesirable radioisotopes and neutrons inside the treatment room. These neutrons at thermal and resonance energies induce nuclear reactions through the whole accelerator bunker. In consequence various radioisotopes emitting high-energy photons appear. In this paper the correlation between radioactivity induced inside the treatment room and the undesirable thermal and resonance neutron radiation generated by the therapeutic accelerator X-rays was studied. The thermal and resonance neutron fluence determined in chosen places inside the bunkers was 1.0 × 105–3.4 × 105 cm−2 Gy−1 and 1.0 × 105–1.6 × 106 cm−2 Gy−1 at thermal energies (<0.1 eV) and 3.9 × 104–1.3 × 105 cm−2 Gy−1 and 1.0 × 105–1.1 × 106 cm−2 Gy−1 at epithermal energies (0.1 eV–10 keV), for the 15 MV and 20 MV beams, respectively. The gamma energy spectra measured inside the accelerator bunker depended on the neutron radiation level. The net count rates of the gamma peaks from the decays of the excited state 56Fe* and 28Si*, the result of the simple capture of the neutron, for the 20 MV beam were almost one order of magnitude greater than those for the 15 MV beam. Moreover, it turned out that the activation of the wedge – the main accelerator accessory was caused by neutrons.