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

Abstract 

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 1.0×10⁵–3.4×10⁵cm⁻²Gy⁻¹ and 1.0×10⁵–1.6×10⁶cm⁻²Gy⁻¹ at thermal energies (<0.1eV) and 3.9×10⁴–1.3×10⁵cm⁻²Gy⁻¹ and 1.0×10⁵–1.1×10⁶cm⁻²Gy⁻¹ at epithermal energies (0.1eV–10keV), for the 15MV and 20MV 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 20MV beam were almost one order of magnitude greater than those for the 15MV beam. Moreover, it turned out that the activation of the wedge – the main accelerator accessory was caused by neutrons.

Keywords: Thermal and resonance neutrons, Induced radioactivity, High-energy therapeutic X-rays, Medical accelerator, Neutron production, Radioisotopes