Development of a four-layer DOI detector composed of Zr-doped GSO scintillators and a high sensitive multi-anode PMT

We are developing an open-type PET scanner "Open PET" dedicated to in-beam imaging for carbon ion therapy. In this paper, we developed a 4-layered depth of interaction (DOI) detector suitable for the OpenPET. In a small OpenPET prototype we have previously developed, LGSO scintillator was used in expectation of its large scintillation photon yield and high stopping power. However, in in-beam PET use, we found that self luminescence of Lu-176 radioisotope degraded signal-to-noise ratio because activity generated through auto-activation was very limited. Therefore, in this study, we selected Zr-doped GSO (GSOZ) scintillation crystals as a Lu-free scintillator. On the other hand, we should pay attention to decreased light yield of GSOZ compared with LGSO. To compensate this, we employed a MA-PMT having a super bialkali (SBA) photo cathode, which had about 1.3 times higher quantum efficiency than a conventional bialkali (BA) photo cathode. The effect of using the SBA-PMT was evaluated in terms of energy resolution and crystal identification performance. The results showed that the energy resolution of the detector with the SBA-PMT was improved by about 10% as compared to the detector with the BA. The crystal identification performance, which was degraded at the periphery for the BA-PMT such as a few crystal responses overlapped each other, was improved by the use of the SBA-PMT.