Optimization of mounting large crystal arrays to photomultiplier tubes

This work reports on the development of mounting techniques of multi-element crystal arrays to position sensitive photomultiplier tubes (PSPMTs) for the micro crystal element scanner (MiCES). MiCES will consist of four rings of 18 detector modules. The crystal array for each detector is a 22 × 22 grid of 0.8 × 0.8 × 10 mm mixed lutetium silicate (MLS) crystals set at 0.9 mm centers and framed for rigidity to compose a 19.94 mm square. The PSPMTs used are Hamamatsu R8520-00-C12. The MiCE crystal arrays are slightly larger than the dimensions of the anode strips of the PSPMT. Therefore the detector module assembly requires precision alignment of the crystal array with the PMT optics for optimum decoding. An alignment jig has been constructed to aid in the initial positioning of the crystal arrays. The crystal arrays are glued to the PSPMT using Histomount (National Diagnostics) glue. It has excellent transmittance at MLS peak emission wavelength. Its relatively long curing time allows crystal map measurements followed by slight repositioning of the crystal arrays before the glue fully sets. Tapping down the crystals was found to significantly improve the amount of light collected. The optimum location of the crystal array was found to be centered in X and offset by 0.6 mm toward the red pin 1 marker in Y for 17 out of the initial 18 detector modules constructed. One module required an alternate array offset of 0.25 mm in Y. A complete summary of the results and assembly technique are provided.