Design and Performance Evaluation of a BGO + SiPM Detector for High-Energy Prompt Gamma Imaging in Proton Therapy Monitoring

Proton therapy is attracting increasing attention nowadays owing to its accurate dose delivery to the target tumor. However, this advanced treatment modality is subject to proton beam range uncertainty, which could lead to overdose in healthy tissues. In the past decades, prompt gamma imaging (PGI) has gained increasing interest and been proved promising for realtime proton beam monitoring. The detection of 2-8 MeV high energy prompt gamma, however, poses a serious challenge for PGI system design, especially, for the detector design. In this article, we propose a prompt gamma detector design for a PGI system under development in our lab. The detector was composed of a 12 x 12 BGO block coupled to an 8 x 8 SiPM array with a crystal pixel size of 3.5 mm x 3.5 mm x 30 mm. A numerical simulation model was developed to evaluate the SiPM saturation effect caused by the finite number of microcells in each SiPM pixel while a huge number of optical photons might impinge the SiPM pixel under high energy gamma ray irradiation. Experiments under high energy gamma isotopes (
²²
Na - 511 keV, 1.275 MeV,
⁸⁸
Y - 898 keV, 1.836 MeV, and
²³²
Th - 2.6 MeV) irradiation were conducted to evaluate positioning performance, energy response linearity, and energy resolution (ER) of the detector. Root mean squared (RMS) metric was defined to assess the detector's positioning performance quantitatively. The numerical simulation results demonstrated that the SiPM saturation effect of the proposed detector design is negligible up to 10-MeV gamma photon detection. In the experimental studies, the obtained energy response linearity was 0.9997±0.0005, indicating almost no SiPM saturation effect for up to 2.6-MeV gamma photon detection with the proposed detector design. ER of 31.96% (511 keV), 26.67% (898 keV), 20.19% (1.275 MeV), 17.38% (1.836 MeV), and 15.88% (2.6 MeV) were achieved. With increased gamma photon energy, the crystal elements in the flood maps became clearer with reduced RMS value of the flood maps, indicating improved positioning performance. Based on the simulation and experimental results, we conclude that the proposed detector design is feasible for the PGI system with good energy and positioning performance as well as excellent anti-saturation capability for high energy gamma ray detection.