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

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 cm field size for a 6 MV photon beam. After calibration, profiles were measured for three small field sizes: 0.5 × 0.5 cm , 1 × 1 cm , and 2 × 2 cm . Using the central pixel of the array, output factors were measured for field sizes of 0.5 × 0.5 cm to 25 × 25 cm . 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 cm to 25 × 25 cm . At 0.5 × 0.5 cm 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 cm . In comparison to a bulk scintillator of dimensions 5 × 5 × 0.5 cm 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 cm , 1 × 1 cm , and 2 × 2 cm , 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 cm field size) when compared to micro ion chamber and diamond detector measurements.