Air-CLS Detector: A Modified Crosshair Light-Sharing PET Detector With Air Gaps in the U-Shape Light Path

We have developed a crosshair light-sharing (CLS) detector to obtain time-of-flight and depth-of-interaction (DOI) information; the detector consists of a 2-D crystal array with three layers of reflective material, and has a loop structure within a pair of crystal bars. In this work, we modified the detector structure by removing optical glue between the crystals forming the loop structure for the purpose of simplifying the assembly process. The modified CLS was made of fast lutetium-gadolinium oxyorthosilicate (LGSO) crystals with dimensions of $1.45\times 1.45\times 15$ mm3 that were optically coupled to the multipixel photon counter (MPPC) array. Most optical windows of the top and bottom layers of the new Air-CLS were so-called air gaps. Only the optical windows that contribute to maintaining the 3-D structure of the reflective material were optically bonded, and a grid of reflective material was formed within the MPPC protective cover. This approach also improved the coincidence resolving time (CRT). The Air-CLSs and previous room temperature vulcanized (RTV)-CLSs were read out by TOFPET2 application-specific integrated circuits, respectively. For Air-CLS (RTV-CLS), we obtained CRT of 188 ps (197 ps), energy resolution of 14.3% (13.1%), and DOI resolution of 3.6 mm (2.9 mm). The Air-CLS significantly simplifies the assembly process while achieving the CRT of less than 190 ps.