量研学術機関リポジトリ「QST-Repository」は、国立研究開発法人 量子科学技術研究開発機構に所属する職員等が生み出した学術成果（学会誌発表論文、学会発表、研究開発報告書、特許等）を集積しインターネット上で広く公開するサービスです。 Welcome to QST-Repository where we accumulates and discloses the academic research results(Journal Publications, Conference presentation, Research and Development Report, Patent, etc.) of the members of National Institutes for Quantum and Radiological Science and Technology.
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We are developing the OpenPET which is well suited to in-beam PET in carbon ion therapy thanks to the open space. The in-beam PET can image the distribution of positron emitters during beam irradiation for in-situ dose monitoring. Therefore careful designing and testing of OpenPET detectors are important because secondary particles generated in a target degrade detector performance in the OpenPET geometry and the quantity of positron emitters produced in in-beam PET is very small. In this paper, we optimized the final design of the in-beam OpenPET detector. Some detector components were changed from our previous work for use under practical situations. We carried out in-beam tests of this new detector for carbon beam irradiation with practical intensities. The constructed OpenPET detector consists consisted of a 16x16x4 crystal array with four-layer DOI encoding capability and a PS-PMT. The scintillator material was changed from LGSO that included radio isotopes to GSOZ. In order to compensate for the lower light yield of GSOZ, we applied the PS-PMT (R10552-100-M64) with the super bialkali photocathode which realizes 30% higher quantum efficiency than the conventional bialkali type. A large current divider circuit was applied since the PMT current exceeded the dynamic range of a normal divider circuit under the targeted practical situations. Irradiation tests were done in HIMAC. The energy and intensities of the C-12 beam were 290MeV/u and 10^8 -10^9 particles per second (pps), respectively. The detector was positioned 30cm from the backside of the water phantom at an angle of 30deg. We obtained sufficient crystal identification performance around 10^8 pps, although the position histogram was degrading slightly with increasing C-12 beam intensity. The average energy resolution of 14.9% was obtained for 511keV gamma rays. We concluded that the constructed detector was able to function as the OpenPET detector for in-beam imaging.
Optimization of the in-Beam OpenPET Detector for Carbon Beam Irradiation
