A performance study of ultrathin sensor planes for a highly compact and granular electromagnetic calorimeter of the LUXE experiment

The LUXE experiment will study laser electron scattering at the European XFEL at DESY to explore an uncharted domain in Quantum Electrodynamics. To measure the number and the energy spectrum of electrons and positrons, produced in the multi-photon Breit--Wheeler process over a wide range of multiplicity, two electromagnetic calorimeters are foreseen. In Monte Carlo simulations it was demonstrated that the necessary performance can be matched by highly compact and finely segmented calorimeters. A design is presented for a sampling calorimeter. Tungsten plates are used as absorber material, with a Molière radius of about 9.3mm. They are interspersed with active pad sensors to form a sandwich. To keep the Molière radius near the one of tungsten, the gap between tungsten plates must be kept small. Hence, thin sensor-planes are needed. Here, two technologies for ultra-thin sensor planes are investigated based on silicon (Si) and gallium arsenide (GaAs) pad sensors. In both, thin metal traces guide the signal on a pad to the sensor edge where the front-end ASICs are positioned. For the GaAs sensors, these traces are made of aluminium, embedded in the gaps between the pads, and for the Si case, Kapton fan-outs with copper traces are glued to the sensor. In addition, a dedicated front-end ASIC in 130-nm CMOS technology is developed for the readout. A full system test of two considered pad-structured sensors for the LUXE ECAL is reported. The sensors are positioned in a 5 GeV electron beam at DESY. A pixel telescope is used to measure the trajectory of each triggered electron. The signal on each pad is amplified and digitised. Data preprocessing is done using FPGAs. Results are presented on the signal-to-noise ratio, the homogeneity of the response of different pads, edge effects, cross talk and signals due to the readout traces on the GaAs substrate.