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(Tuesday, 7/16/2019) 1:45 PM - 3:45 PM
Room: Stars at Night Ballroom 4
Purpose: To investigate the influence of magnetic field on the “stem effect” in fiber optic dosimeters as a function of fiber size, field size, and depth of measurement.
Methods: Two un-doped plastic transport fibers with core diameters of 2 mm and 1 mm were irradiated with 6 MV flattening filter free (FFF) photon using a TrueBeam™ Linac and an MRIdian™ MR-Linac with a 0.35 T magnetic field. The irradiation geometry was identical in both systems and the fibers received same amount of dose (2 Gy). The fiber was embedded in a solid water phantom placed at depths of 1 cm and 2 cm and was irradiated at two field sizes of 10×10 cm² and 15×15 cm². Spectroscopy was performed using a fiber coupled spectrometer in 450 to 650 nm range with 2 nm resolution.
Results: For the 1 mm fiber, the presence of the magnetic field increased the “stem effect” by 9% and 15% when the fiber was at 1 cm and 2 cm depths, respectively, irradiated by a 10×10 cm² field size. The response was increased by 7% and 9% at 1 cm and 2 cm depths, respectively, with a 15×15 cm² field size. For the 2 mm fiber, the detected intensity was increased by 9% and 19% when the fiber was at 1 cm and 2 cm depths, respectively, irradiated by a 10×10 cm² field size due the effect of magnetic field. The response was increased by 4% and 5% at depths of 1 cm and 2 cm, respectively, with a 15×15 cm² field size.
Conclusion: The amount of Cherenkov light collected by transport fibers, manifested as a “stem effect” in fiber optic dosimeters, increases in the presence of magnetic field. This increase depends on many factors including the fiber’s core size, field size, and depth of measurement.
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