MEASUREMENTS OF THERMAL AND RESONANCE NEUTRON FLUENCE AND INDUCED RADIOACTIVITY INSIDE BUNKERS OF MEDICAL LINEAR ACCELERATORS IN THE CENTER OF ONCOLOGY IN OPOLE, POLAND

Emission of high-energy X-ray and electron therapeutic beams from medical linear accelerators is related to undesirable neutron production and to induction of radioactivity. In this work, measurements of thermal and resonance neutron fluence and induced radioactivity were performed inside two bunkers with medical linacs - Elekta in the Center of Oncology in Opole (Poland). The bunkers differ with a construction of their walls. The neutron measurements were performed by means of the induced activity method during emission of the 18 MV X-ray beam. The investigation of radioactivity induced by neutrons was based on the method of off-line gamma-ray spectroscopy measurements. This work has shown that the differences in the considered construction of bunkers do not influence significantly on the thermal and resonance neutron fluence as well as on the induced radioactivity. The greatest thermal neutron fluence (1.4 x 10(4) cm(-2) MU-1) as well as the resonance one (0.7 x 10(4) cm(-2) MU-1) was measured at the isocenter of a rotation of the accelerator head. The radioisotopes of W-187, Mn-56, Na-24 and Br-82 originating from the (n, gamma) reactions were identified in the spectral measurements.