Mechanical design and photodetector system of the first calorimeter prototypes based on oriented scintillating crystals

Scintillating crystals are a well-established technology for electromagnetic (EM) calorimetry.
When their crystalline lattice is properly oriented with respect to the incoming particle beam, however, coherent interactions with the periodic potential can strongly modify the EM shower development. For high-energy electrons and photons, the Lorentz-boosted electric field experienced along atomic strings or planes can approach the quantum electrodynamical strong-field regime, enhancing bremsstrahlung and the pair production probability and effectively shortening the radiation length. This leads to earlier and more compact EM showers compared to the amorphous case. The ORiEnted calOrimeter (OREO) project exploits these strong-field effects in ultra-fast PbWO4 (PWO-UF) crystals to realise an ultra-compact, longitudinally segmented electromagnetic calorimeter. In this paper, we present
the mechanical design and photodetection system of two OREO prototypes: a first configuration with an oriented upstream layer followed by a non-oriented layer, and a second configuration in which both layers can be operated in the oriented mode and aligned independently. The prototypes employ 3 × 3 matrices of PWO-UF crystals read out by SiPM arrays, with dedicated mechanics to achieve microradian-level control of the crystal orientation and to ensure light-tight operation.
Possible applications include forward calorimetry at future colliders, fixed-target and beam-dump experiments, and space-based 𝛾-ray detectors.