SU‐FF‐T‐319: Measurement of Neutron Background in Electron Beams From a Dedicated IORT Linear Accelerator and a Conventional Linear Accelerator

Abstract

Purpose: To measure the neutron background for an electron‐only (IORT) and a conventional linear accelerator. Methods & Materials: A Siemens Mevatron ME [6 (not used), 9, 12, 15 and 18 MeV] and several Varian Clinacs [6 (not used), 9, 12, 16 and 20 MeV] were used in this study. Bubble (Type BD‐PND; BTI Bubble Technology Industries) and track‐etch detectors (TE) (Luxel+, Ja; Landauer) were used in these experiments. The detectors were placed at 1m from the target at azimuthal angles of 0°, 45°, 90°, 135° and 180°. Results: For conventional electrons at 0° the neutron leakages (Sv/Gy) are: 2×10−5 at 12 MeV, 1.0×10−4 at 16 MeV and 4×10−4 at 20 MeV. For angles >0°, the leakage is almost angle independent [2×10−6 at 12 MeV; (0.5–2)×10−5 at 16 MeV and (2–4)×10−5 at 20 MeV]. For the ME, the neutron leakage was lower than for the conventional linac and also independent of angle for angles >0° {[0°: 6×10−6 at 12 MeV; 2×10−5 at 15 MeV; 5×10−5 at 18 MeV]; [other angles: (2–5×10−7 at 12 MeV; (0.7–1.0)×10−6 at 15 MeV; (2–4)×10−6 at 18 MeV]}. Using the upper limit of 5×10−7 Sv/Gy at 12 MeV for angles >0° and assuming a workload of 200Gy/wk and an inverse square factor of 10, the neutron dose is 0.01 mSv/wk. For the primary beam at 12 MeV, the 10x higher dose is compensated by the attenuation in the primary beamstopper†. Conclusions: Measurements have been made of the neutron leakage from an IORT machine and a conventional linear accelerator. The results show that the IORT has a leakage well below that of the conventional machine and that at 12 MeV for the IORT machine, the leakage is sufficiently low at all angles as not to be a regulatory problem.