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Evaluation of a novel inorganic scintillator for applications in low dose rate (LDR) brachytherapy using both TE-cooled and room temperature silicon photomultipliers (SiPMs)

This work considers the use of an optical fiber sensor, employing a Gd2O2S:Tb inorganic scintillator, for applications in LDR brachytherapy for prostate cancer. Gd2O2S:Tb is characterized by a scintillation decay time of ~500 μs, implying that each primary gamma interaction produces a series of single photons, requiring the use of adequate detectors, such as Silicon Photomultipliers (SiPMs). These devices suffer from a significant Dark Count Rate (DCR), undermining system sensitivity. This work reports the result of a feasibility study where identical SiPMs, but different packages, are compared. Specifically, a room temperature SiPM in a ceramic package and a TE-cooled SiPM in a TO8 package. In the former, the optical fiber is in direct contact with the sensor surface, while in the latter there is a separation of ~3 mm. The signal, measured as Photon Count Rate (PCR), in excess of the DCR, was measured in a water phantom at distances of 5 mm and 30 mm from an I125 source. For the TE-cooled SiPM, the DCR dropped by ~96% as expected, and the PCR dropped by ~80%, compared to the room-temperature SiPM, due to reduced light acceptance. However, incorporating an optical coupling system into the TE-cooled SiPM, to improve acceptance, resulted in sensitivity increases of 332% and 296% at distances of 5 mm and 30 mm respectively, compared to the room-temperature SiPM. It is hoped that these improvements in sensitivity, will allow for accurate monitoring of the dose-rate from LDR sources, within the clinically relevant treatment volume for prostate cancer.