DOI detection capability of 3D crystal array standing over two PMTs

Absence of scintillation crystal in the region between adjacent PMTs, PMT blind region, will cause loss of data sampling and less sensitivity in a PET or another system. We propose design of a depth of interaction (DOI) detector to fill up the PMT blind region with crystals. In the design, a 3-dimensional (3D) array of single kind crystals stands over two PMTs. The crystal array is optically coupled to the PMTs directly and materials between crystal elements are chosen among optical film, coupling material and just remained as air gap to control scintillation light path in the crystal array instead of coupling a light guide on the bottom of the array as is generally done. Not only reflector but translucent film is also utilized as optical film in the DOI detector design. In this paper, first, we seek for an available DOI crystal array design in simple setting with a PMT, and then we demonstrate its DOI detection capability in the setting with two PMTs. The DOI detector is developed with the intention of the use in jPET-RD system; the PET system we have developed for small animals. In the system, there is about 6 mm interval in axial direction between outer anodes of adjacent two PMTs. The cross section of the crystal elements is 1.46 mm × 1.46 mm in jPET-RD detector so that 2-layers of four crystals are involved in the PMT blind region. We introduce the design of the 2-layer DOI crystal array for the region and the results of its performance evaluation.