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Measuring small field profiles and output factors with a stemless plastic scintillator array
Michael A Hupman1, Ian G Hill1, Alasdair Syme1,2,3
1Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada.
Purpose:
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.
Methods:
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 95 cm SSD, 5 cm 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.
Results:
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.
Conclusion:
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 × 1 cm2 field size) when compared to micro ion chamber and diamond detector measurements.
