The Measurement of Neutron Contamination in High Energy X-Ray Radiotherapy Using a He-۳ Gas Dosimeter

Introduction: Neutron contamination is likely caused by the collision of high-energy photon interactions (γ, ṉ) with heavy metals used in the construction of the accelerator. This study is essential to quantify the excess dose from neutron contamination by Elekta Precis. The main object is to assess the neutron contamination with a gas dosimeter containing He-۳.   Material and Methods: In this study, neutron contamination was estimated at different points in and out of the treatment room. We used two types of dosimeters: a He-۳ gas dosimeter (CRAMAL ۳۱) as a neutron dosimeter and Farmer ionization chambers, and PC-electrometer (Sun Nuclear, USA) which is sensitive to photons measured photo-neutron doses. Both the neutron and Farmer dosimeters were applied in the presence and absence of acrylic plates at the same point. In this study, Monte Carlo (MC) code was utilized to prepare the correct proportion of neutron dose. Results: At different points in and out of the treatment room, neutron contamination was approximately in the range of background dose (D = ۰.۰۰۱ µSv). The neutron dosimeter displayed ۴۸.۷۹۲ µSv, ۲۵.۴۵۶ µSv and ۲۸.۷۵۶ µSv for ۶, ۱۰ and ۱۵ MV photon energy, respectively. He-۳ gas dosimeter showed that the neutron dose net was negligible under the treatment field. Conclusion: He-۳ gas dosimeter detected more than the usual neutron dose in ۶ MV photon energy than we expected. Due to the high photon flux under the radiation fields, a He-۳ neutron dosimeter reported photo-neutron dose. Nevertheless, the photo-neutron dose was in the range of micro Sievert (µSv). He-۳ gas dosimeter was not suitable for neutron dosimetry in places with high photon fluence because of the low energy peak in the detection of neutrons.

Keywords:

Linear Accelerator , Dosimetry , Radiation , Neutron Activation Analysis