Scintillation Detector Design Study for Prompt Gamma Photon Detection in Proton Therapy Monitoring

Proton therapy is currently one of the most advanced radiotherapy technologies. However, due to proton range uncertainty during proton dose delivery, proton range monitoring is of vital importance for proton therapy. In recent years, there has been increased interest of utilizing prompt gamma imaging (PGI) for online proton range monitoring due to its high yield, real-time emission property and immunity to biological wash-out effect. However, the $2 \sim 8$ MeV high energy of prompt gamma makes detector design of PGI system challenging and the saturation effect of SiPM, which is a kind of commonly used photodetector in scintillation detector design, should be carefully considered.In this work, we propose a scintillation detector design based on BGO crystal and SiPM (SensL FJ30035) for the PGI system we are now developing. A numerical simulation considering the optical photon generation and single microcell response of SiPM pixel was performed to evaluate the saturation of SiPM under the proposed detector design and compared with that based on LYSO crystal. The results demonstrate that BGO-based detector design features similar detector efficiency but significantly reduced saturation effect of SiPM compared with LYSO-based detector design for high energy prompt gamma detection. A detector module consisting of a $12 \times 12$ BGO block (single crystal size: $3.5 \times 3.5 \times 30$ mm3) and an $8 \times 8$ SiPM array was further developed and an experiment platform was set up to evaluate the detector performance in terms of 2D flood map and energy resolution under $^{\mathbf {22}}$Na source irradiation. In the 2D flood map, each crystal element can be clearly identified, indicating good positioning capability of the proposed detector design. The energy resolution of the proposed detector design is $16.4 \pm 1.4$ % for 1275 keV gamma photons. Based on these initial results, we conclude the proposed detector has high detection efficiency, negligible SiPM saturation, good positioning capability and reasonable energy performance and is feasible for prompt gamma detection.