An Array of Spherically Dimpled Scintillating Cells for an Integrated Readout Layer

The CALICE collaboration is developing calorimetry for particle flow algorithm (PFA) based detectors. To measure particle shower development, a finely segmented calorimeter optimized for PFA must consist of millions of channels. A scintillator-based calorimeter with cells of size 30 x 30 x 3 mm³ readout with silicon photo-multipliers (SiPMs) in situ shows great promise. The construction and assembly challenges of a highly segmented scintillator-SiPM calorimeter could be greatly reduced with full integration of the readout electronics into active calorimeter layers. An integrated readout layer (IRL) comprised of a multi-channel electronics board instrumented with SiPMs paired to a solid uniform array of scintillator cells would eliminate much of the labor intensive production associated with the fiber readout cells. We discuss the development and performance of injection molded polystyrene arrays of 2x2 dimpled scintillator cells with a thickness of 3 mm and cell area of about 900 mm². The spherically dimpled cells ensure uniformity of response for a SiPM positioned at the center of the cell. The performance of an array of molded cells is compared to an array of cells uniformly machined from a solid sheet of polyvinyltolulene cast scintillator with 3 mm thickness. To improve the performance of the molded array the cell dimple depth was optimized. The light output, uniformity, and cross talk of the molded and machined cells irradiated with a Sr-90 source and measured with a SiPM in current mode are reported. Plans for initial particle beam tests of the complete IRL are also discussed.