Measuring small field profiles and output factors with a stemless plastic scintillator array.

Abstract

To fabricate a 1D stemless plastic scintillation detector (SPSD) array using organic photodiodes and to use the detector to measure small field profiles and output factors. An organic photodiode array was fabricated by spin coating a mixture of P3HT and PCBM organic semiconductors onto an ITO-coated glass substrate and depositing aluminum top contacts. Four bulk scintillators of various dimensions were placed on top of the photodiode array. A fifth scintillator was used that had been segmented by laser etching and the septa filled with black paint. Each detector array was first calibrated using a reference field of 95cm SSD, 5cm depth, and 10×10 cm2 field size for a 6 MV photon beam. After calibration, profiles were measured for three small field sizes: 0.5×0.5 cm2 , 1×1 cm2 , and 2×2 cm2 . Using the central pixel of the array, output factors were measured for field sizes of 0.5×0.5 cm2 to 25×25 cm2 . Small field profiles were compared to film measurements and output factors compared to ion chamber measurements. The segmented scintillator measured profiles that were in good agreement with film for all three field sizes. Output factors agreed to within 1.2% of ion chamber over the field size range of 1×1 cm2 to 25×25 cm2 . At 0.5×0.5 cm2 the segmented scintillator underestimated the output factor compared to film and a microDiamond detector. Bulk scintillators failed to produce a good agreement with film for measured profiles and deviations from ion chamber for output factors were apparent at field sizes below 5×5 cm2 . In comparison to a bulk scintillator of dimensions 5×5×0.5 cm3 the etched scintillator saw a reduction of 5.1, 7.1, and 10.5 times the signal for field sizes of 0.5×0.5 cm2 , 1×1 cm2 , and 2×2 cm2 , respectively. The reduction of signal comes from reduced cross-talk that was present in all of the bulk scintillator geometries to various degrees. A 1D SPSD array was demonstrated with various scintillator designs. The etched scintillator array demonstrated excellent small field profile measurements when compared to film and output factors (down to 1×1cm2 field size) when compared to micro ion chamber and diamond detector measurements.