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Full Spectrum Cerenkov Correction for Plastic Scintillation Dosimetry

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t monajemi

T.T. Monajemi Dept of Radiation Oncology, Dalhousie University, Halifax, NS, Canada Dept of Medical Physics, Nova Scotia Health Authority


SU-K-205-8 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 205

Purpose: To develop a novel Cerenkov correction method for use with plastic scintillation detectors (PSD) for dosimetry measurements in photon and electron beams.

Methods: A PSD fiber element (Kuraray, 8mm length, 1mm diameter) was coupled to 50 feet of Eska optical fiber. The optical fiber was connected via SMA connector to a spectrophotometer (Examplar Plus, BWTek). Relative dose factor (RDF) and PDD data were acquired for 6 and 18 MV photon beams and PDD data were acquired for electrons (6 – 16 MeV). PSD measurements were compared to ionization chamber data. A scintillation-only spectrum was acquired by exposing the PSD to 100 kVp photons on an orthovoltage x-ray unit. Following acquisition of all scintillator-based data, Cerenkov spectra were acquired by removing the PSD element and irradiating the optical fiber only, under various conditions: length of optical fiber in beam, beam energy, beam type (photons vs electrons), beam direction. All Cerenkov spectra were normalized to unit area and the average spectrum was calculated. Calculation of the true scintillation signal from the PSD measurements was performed by modeling the total spectrum (scintillation plus Cerenkov) as a linear combination of the scintillation-only spectrum and the Cerenkov-only spectrum. The detector system was calibrated by determining the area under the scintillation-only component of the total spectrum per unit dose using a 6MV photon beam under reference conditions.

Results: The Cerenkov-only spectrum was found to have only a very minor dependence on any of the variables listed above. The use of the average Cerenkov spectrum did not introduce any significant uncertainty to the dosimetric results. The agreement between PSD and ion chamber curves was within 1%.

Conclusion: We found the subtraction of the scaled apriori-determined Cerenkov spectrum (from optical fibre) from the total scintillation spectrum (PSD+optical fibre) a reliable calibration method applicable to PSD dosimetry.

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