Radiation damage of the multi-pixel photon counter to be used for in-beam PET in carbon therapy

We are developing the OpenPET for in-beam imaging in carbon ion therapy. We have succeeded in showing a proof-of-concept by developing small OpenPET prototypes that we used in the HIMAC. One of the major issues to realize the OpenPET is radiation damage to detectors because a number of fragmented particles are incident on the detectors located downstream from the target. While we are currently using photo-multiplier tubes in the OpenPET, some groups reported feasibility of use of a semiconductor photo-detector in hadron therapy. In this paper, therefore, we tested radiation hardness of the multi-pixel photon counter (MPPC) to show feasibility of its use in the OpenPET for carbon ion therapy. We used a single pixel MPPC (S10931-050) to evaluate radiation hardness. The experiment was performed in the PH2 course of the HIMAC. The energy of the ¹²C beam was 290MeV/u and beam intensity was 1.6 × 10⁹ particles per second (pps) which was ten times higher than the typical clinical beam intensity. The carbon ions entered and stopped in a water phantom (10 × 10 × 20 mm³). The MPPC was 30 cm from the back end of the water phantom at an angle of 30 degrees. We measured the energy spectrum for ²²Na point source and the dark count spectrum before and after 10, 20, 30, 40, 50, 70 min irradiations. We found pulse height and energy resolution of the energy spectra were degraded as the irradiation time was increased.