Parameter Optimization of a Digital Photon Counter Coupled to a Four-Layered DOI Crystal Block With Light Sharing

We have developed four-layered depth-of-interaction (DOI) detectors based on light sharing. Reflectors, which are inserted in every two lines of crystal segments and shifted differently depending on each layer, project 3-D crystal positions to a 2-D position histogram without any overlapping after applying the Anger-type calculation. The DOI measurement itself has the potential to improve time resolution because the depth-dependent timing delay can be corrected. However, light sharing tends to increase variance of light paths inside the crystal block, thus resulting in worsened time resolution. Although we have reported advantages of our DOI detectors in terms of position and energy resolutions, we had not evaluated their potential for time resolution. In this paper, therefore, we measured timing performance with the help of a digital photon counter (DPC), which offers precise control of event triggering. There are several studies that have reported one-to-one coupling of the scintillator to the DPC pixel, but DPCs have not been studied well for light-sharing detectors. Therefore, in this work, we optimized measurement parameters of the DPCs for our four-layered DOI detector. The DOI detector consists of 256 LGSO crystals which are arranged in four layers of ${8} \times {8}$ arrays, coupled to the DPC array. Each crystal element is $2.9\times 2.9 \times 5~\hbox{mm}^{3}$ . Each die of the DPC array provides an individual timestamp. Crystal identification performance largely depended on the dark count rate of the DPC array, which can be reduced by means of cell inhibition. We measured several conditions of the inhibition rate of microcells and temperature. For increased inhibition rate, we observed degraded time resolution, although positioning performance and energy resolution were improved. Regarding the temperature dependency within 10 to $ - 7 \ ^\circ \hbox {C}$ , we found that time resolution was insensitive. At 10 $\ ^\circ \hbox {C}$ and 20% inhibition rate, average time resolution over all crystals was $267 \pm 32~\hbox{ps}$ (full width half maximum). Better positioning performance and energy resolution were obtained for colder temperatures.