Optimization of the in-beam OpenPET detector for carbon beam irradiation

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 ¹²C beam irradiation with practical intensities. The constructed OpenPET detector consists consisted of a 16 × 16 × 4 crystal array with four-layer DOl encoding capability and a position-sensitive photomultiplier tube (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 ¹²C beam were 290MeV/u and 1.0 × 10⁸ 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 1.0 × 10⁸ pps, although the position histogram was degrading slightly with increasing ¹²C beam intensity. The energy resolutions of 15.2% (1st layer), 14.6% (2nd layer), 13.6% (3rd layer) and 12.- % (4th layer) were obtained for single crystal elements. We concluded that the constructed detector was able to function as the OpenPET detector for in-beam imaging.