Neutron Field Outside the Room of Linear Medical Accelerator

摘要

Neutron radiation field in radiotherapeutic facility is mainly generated as a result of photonuclear reactions (γ,n), occurring when high-energy beam from linear medical accelerators is emitted. The intensity of undesired neutron production is strongly dependent on an accelerator model, i.e. beam type, end-point energy and head materials. Outside the treatment room neutron field is also dependent on the topology of the facility (room surface area, wall composition, maze shape and length, position of the control panel). Four linacs working in 20 MV photon mode (600 MU/min), installed in rooms of different size were considered in terms of the occupational neutron dose. The influence of the direction of X-ray beam emission as well as the size of irradiation field and wedge usage on the neutron field behind the treatment room door were investigated. Prompt gamma neutron activation analysis (PGNAA) has been used for determination of photoneutron fluence outside the treatment room. In situ portable spectrometry system based on semiconductor high purity germanium (HPGe) detector was used. Neutron capture reactions as well as inelastic scattering processes on germanium crystal were observed. The analysis was based on photopeaks count rates selected from registered spectra. Application of neutron energy moderators made of paraffin, allowed for roughly estimation of photoneutron spectra, distinguishing the slow, intermediate and fast components of neutron flux. Ambient dose equivalent H*(10) was estimated with the use of fluence-to-dose conversion coefficients. Additionally neutron dose rate meter LB123 was used. Dose rates 50 cm away from the bunker door were found to be within the range 0.531 - 2.500 μSv/h depending on the linac and direction of 20 MV beam emission. At the operator's console H*(10) was of the order of 0.027 μSv/h. Obtained results and their agreement with dosimeter indication have shown that PGNAA of HPGe spectrometer could be used in characterization of low rate neutron radiation field.