Poster — Thur Eve — 24: Commissioning and preliminary measurements using an Attix‐style free air ionization chamber for air kerma measurements on the BioMedical Imaging and Therapy beamlines at the Canadian Light Source

Abstract

Synchrotron facilities, including the Canadian Light Source (CLS), provide opportunities for the development of novel imaging and therapy applications. A vital step progressing these applications toward clinical trials is the availability of accurate dosimetry. In this study, a refurbished Attix‐style (cylindrical) free air chamber (FAC) is tested and used for preliminary air kerma measurements on the two BioMedical Imaging and Therapy (BMIT) beamlines at the CLS. The FAC consists of a telescoping chamber that relies on a difference measurement of collected charge in expanded and collapsed configurations. At the National Research Council's X‐ray facility, a Victoreen Model 480 FAC was benchmarked against two primary standard FACs. The results indicated an absolute accuracy at the 0.5% level for energies between 60 and 150 kVp. A series of measurements were conducted on the small, non‐uniform X‐ray beams of the 05B1‐1 (∼8 – 100 keV) and 05ID‐2 (∼20 – 200 keV) beamlines for a variety of energies, filtrations and beam sizes. For the 05B1‐1 beam with 1.1 mm of Cu filtration, recombination corrections of less than 5 % could only be achieved for field sizes no greater than 0.5 mm × 0.6 mm (corresponding to an air kerma rate of ∼ 57 Gy/min). Ionic recombination thus presents a significant challenge to obtaining accurate air kerma rate measurements using this FAC in these high intensity beams. Future work includes measurements using a smaller aperture to sample a smaller and thus more uniform beam area, as well as experimental and Monte Carlo‐based investigation of correction factors.