An LGAD-based Full Active Target for the PIONEER Experiment

PIONEER is a next-generation experiment to measure the charged-pion branching ratio to electrons vs. muons and the pion beta decay with an order of magnitude improvement in precision. This will probe lepton universality at an unprecedented level and test CKM unitarity at the quantum loop level. PIONEER was approved in 2022 to run at the PiE5 pion beamline at PSI. A high-granularity active target (ATAR) is being designed to provide detailed 4D tracking information, allowing the separation of the energy deposits of the pion decay products in both position and time. The chosen technology for the ATAR is Low Gain Avalanche Detectors (LGAD). These are thin silicon detectors with moderate internal signal amplification. To achieve a ~100\% active region, prototype technologies under development are being evaluated, such as AC-coupled LGADs (AC-LGADs). Since a range of deposited charge from Minimum Ionizing Particle (MIP, few 10s of keV) from positrons to several MeV from the stopping pions/muons is expected, the detection and separation of close-by hits in such a wide dynamic range will be a challenge. Furthermore, the compactness and the requirement of low inactive material of the ATAR present challenges for the readout system, forcing the amplification chip and digitization to be positioned away from the active region. This paper will introduce PIONEER and the ATAR, then show a selection of R\&D results on sensors and electronics.