Room: Track 3
Purpose: Fiber Bragg gratings (FBGs)-based dosimeters are known in the nuclear industry to monitor extremely large dose of radiation. In this work, we set out to increase FBG dosimeter sensitivity for radiotherapy measurements at clinical dose levels.
Methods: FBGs are written through the coating of a standard 125 microns diameter polyimide-coated silica fiber with the phase-mask technique and femtosecond pulses. Shift in wavelength is recorded with an optical interrogator. The detector is embedded in a plastic piece which we hypothesized enhances FBG temperature sensitivity enough to allow radio-induced temperature increase to be detected. Such temperature sensitivity depends mostly on the plastic thermal expansion coefficient, while the radio-induced temperature increase depends on plastic heat capacity. Hence, different types of plastics, having different thermal properties, are used to establish the effects on detector response to a 6 MV beam (CLINACiX). We also characterize the detector response for different plastic thicknesses.
Results: We compared the total measured wavelength shift to the total calculated wavelength shift due to radio-induced temperature increase in plastic. A slope of 0.94 is obtained for this comparison which implies good accordance between measured and calculated values. This suggests that our dosimeter works as a calorimeter. Radiation induces a linear shift of the FBG reflected wavelength for all plastic type. The higher response (0.09 pm/Gy) is obtained using polypropylene whereas the lowest response (0.06 pm/Gy) is obtained using polyetheretherketone (PEEK). Moreover, we obtained a 0,4 pm signal increase by increasing the dosimeter plastic surrounding thickness from 0.2 to 0.55 cm.
Conclusion: We successfully increased FBG dosimeters sensitivity and it should be noted that further optimization is yet to come with plastics having a calculated shift reaching up to 0.5 pm/Gy. A statistical error of 0.03 pm is measured on data points which leads to a detection limit of 0.06 Gy.