A Design of Scintillator Tiles Read Out by Surface-Mounted SiPMs for a Future Hadron Calorimeter

Precision calorimetry using highly granular sampling calorimeters is being developed based on the particle flow concept within the CALICE collaboration. One design option of a hadron calorimeter is based on silicon photomultipliers (SiPMs) to detect photons generated in plastic scintillator tiles. Driven by the need of automated mass assembly of around ten million channels stringently required by the high granularity, we developed a design of scintillator tiles directly coupled with surface-mounted SiPMs. A cavity is created in the center of the bottom surface of each tile to provide enough room for the whole SiPM package and to improve collection of the light produced by incident particles penetrating the tile at different positions. The cavity design has been optimized using a GEANT4-based full simulation model to achieve a high response to a Minimum Ionizing Particles (MIP) and also good spatial uniformity. The single-MIP response for scintillator tiles with an optimized cavity design has been measured using cosmic rays, which shows that a SiPM with a sensitive area of only $\mathbf{1\times1~mm^2}$ (Hamamatsu MPPC S12571-025P) reaches a mean response of more than 23 photon equivalents with a dynamic range of many tens of MIPs. A recent uniformity measurement for the same tile design is performed by scanning the tile area using focused electrons from a $\mathbf{^{90}Sr}$ source, which shows that around 97% (80%) of the tile area is within 90% (95%) response uniformity. This optimized design is well beyond the requirements for a precision hadron calorimeter.