Influence of secondary particles from heavy ion irradiation to in-beam OpenPET detectors

We are investigating the influences of heavy ion irradiation on the detectors for in-beam OpenPET by using the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National institute of Radiological Sciences (NIRS). "OpenPET" which consists of two detector rings separated axially is suitable for in-beam PET in heavy ion therapy due to the open space between the separated detector rings. All of the annihilation gamma rays from irradiated volumes enter obliquely into the OpenPET detectors. Therefore, detectors which can measure depth of interaction (DOI) are required to reduce parallax error. Previously, we have succeeded in crystal identification using the DOI detector consisting of four-layer crystal blocks and a 256-channel position-sensitive photomultiplier tube by optimization of the reflector arrangement. We plan to adopt DOI detectors using this method as the OpenPET detectors; however, secondary particles entering into this type detector have a possibility of degradation of detector performances. In the experiment described here, the energy and intensity of the ¹²C beam were 290 MeV/u and 10⁸ -10⁹ particle per second (pps). Carbon ions entered into a water phantom and the influences of irradiation, especially fragment particles, on DOI detectors were evaluated. In this presentation, we report the results of this experiment.