II. THE JPET-D4 PROTOTYPE
The jPET-D4 scanner (Fig. 1) has 5 rings of 24 detector blocks. This scanner is dedicated to brain scans, and the detector rings have a diameter of 390 mm. Each detector block consists of a 4-layered 16 x 16 array of GSO crystals (Hitachi Chemical Co., Japan) with different amounts of cerium (Ce) dopant (0.5 mol%Ce for 1st and 2nd layers, and 1.5 mol%Ce for 3rd and 4th layers) and a 256 channel flat-panel position-sensitive photomultiplier tube (H9500; Hamamatsu Photonics K.K., Japan). Each GSO crystal is sized 2.9 mm x 2.9 mm x 7.5mm. The basic idea for DOI discrimination is the coded reflector insertion [9] that controls the distribution of scintillation light so that each position can be identified by the Anger type calculation. Four-layer DOI positions are identified based on this idea after upper and lower two DOI positions are separated by the pulse shape discrimination [10]. At this stage, the coincidence time window has been optimized up to 10ns. The standard energy window is 400-
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600keV. Major specifications of the jPET-D4 are listed in Table 1.