Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room

Radiotherapy using high-energy photon beams (10–20 MV) is accompanied by the production of secondary neutron radiation via ( γ /X,n) reactions. These interactions as well as subsequent neutron capture are the source of induced gamma radioactivity. When studied with standard range of spectrometric systems, only decay gamma radiation is usually registered, whereas a significant part of radiation— prompt gammas—is omitted, what might result in a significant underestimation of occupational risk for therapists in the vicinity of the door to the treatment room during therapeutic beam emission. Presented study has shown the main components of gamma radiation field in this localization investigated with the use of high-purity germanium spectrometry. Among them, prompt gamma radiation in light elements of concrete and in metal construction of the door, as well as 477.6 and 2224.6 keV photons emitted by neutron absorbing layers, contributes the most. Effective dose values depend on thickness of the door as well as on neutron production by particular linac and are within the range of 1.8–56.2 μ Sv/h. Standard environmental radiometry could underestimate these values by about 60% due to low efficiency for high-energy photon counting.