TH-AB-201-11: A Systematic Characterization of the Low Energy Response of Plastic Scintillation Detectors

Abstract

Purpose: To characterize the low energy behavior of scintillating material used in plastic scintillation detectors. Methods: Four plastic scintillation detectors (PSDs) were developed using three different scintillating material (BCF-10, BCF-12, BCF-60) and one scintillator-free fiber. The scintillators were 1 mm in diameter and 10 mm-long, coupled to a 15 meter long clear PMMA optical fiber. A photomultiplier tube was connected to this fiber to collect the scintillator’s light signal. All fibers were covered by a light-tight polyethylene jacket. The PSDs were exposed to AAPM accredited low energy beam qualities ranging from 20 kVp to 250 kVp. Their response described the dose in polystyrene and was compared to the dose in air measured by a primary calibration chamber located at the beam exit. The mean energy and the spectral method were used to extract the effective mass energy-absorption coefficient using NIST database for each beam quality reference spectrum. Attenuation in air and in the fiber jacket were also considered. The scintillators expected response was compared to the experimental measurements and an energy-dependent correction factor was identified to account for low energy quenching occurring within the scintillators. Results: The clear optical fiber response was below 0.5% of the scintillator’s light, which indicated that a negligible amount of fluorescence contamination was produced. The scintillators response increased by a factor 3.65 ± 0.07 between a 20 kVp and a 250 kVp beam. The correction factor varied with beam energy and remained above 84% for beam qualities of 30 kVp and above, but drops for very low energy (20 kVp) to about 77%. Conclusion: This work describes the PSD energy dependence to low energy beam qualities and identifies a correction factor related to low energy quenching. A detailed comprehension of intrinsic scintillator response is essential when designing accurate PSD dosimeters for radiology purposes. This work was supported in part by a NIH/NCI SBIR grant R44CA153824 and by grant # 435510 from the NSERC. JB is supported by a FQRNT doctoral research scholarchip and acknowledges partial support by the NSERC CREATE Medical Physics Research Training Network grant # 432290.