[Show abstract][Hide abstract] ABSTRACT: The Clear-PEM scanner was developed by the Portuguese PET Consortium under the framework of the Crystal Clear Collaboration for detection of early stage breast cancer. The scanner relies on a modular design in which detection modules are readout by low-noise ASICs that extract the pulse shape information for energy and timing measurements. The developed module has a dual readout, in which avalanche photodiodes arrays are used to extract the light from 20 mm long 2×2 mm<sup>2</sup> cross section LYSO:Ce crystals, with a typical depth of interaction (DOI) resolution of 2.2 mm FWHM. The packing fraction of the module is 64%. In order to further improve the detection sensitivity of the scanner, by increasing the packing fraction while maintaining the already achieved DOI resolution, experimental work on the upgrade of the detector module was carried out. The work consisted on the exploration of arrays with 30 mm long crystals as well as the use of crystals with 2.2×2.2 mm<sup>2</sup> cross section wrapped in a specular reflector, yielding a larger packing fraction, of about 78%. Energy, DOI resolution and the relative light collection were measured for polished and unpolished crystal arrays. For the polished crystal array, the variation of the photopeak position within the same matrix, was 6% r.m.s. For the 30 mm unpolished crystal array the energy resolution was 14.7% with a DOI resolution of 2.7 mm FWHM. An approach to improve the light collection efficiency is reported in which individual crystals have a mixture of different number of polished and un-polished longitudinal faces. Using crystals with two longitudinal sides polished, leaving the remaining as-cut, allows extracting the DOI coordinate with a resolution of 2.8 mm FWHM, an energy resolution of 16% and the improvement in light collection of about 30%, in respect to the unpolished matrices. Based on these measurements a new set of detector modules were produced and characterized using the Clea-
r-PEM frontend and data acquisition electronics. Preliminary results for the total light collection, light collection asymmetry and energy resolution are reported.