Calibration of LGADs for the PIONEER Rare-Pion Decay Experiment

PIONEER is a rare-pion decay experiment, currently in developmental stages, which will be mounted at the Paul Scherrer Institute (PSI) in Switzerland. The primary first stage goal of PIONEER will be to measure the branching ratio R_e/μ= (π⁺ →e⁺ )/(π⁺ →μ⁺ ) to a precision of 10^-4, providing the world’s most sensitive test of Lepton Flavor Universality. Achieving this measurement requires the development of a highly segmented active target (ATAR) that will provide 5D (x,y,z,t,E) information to track and identify the incoming pion and its decay channels. The ATAR will be composed of strips of Low Gain Avalanche Diode (LGAD) devices. To optimize performance to the necessary degree, these LGADs must be thoroughly characterized. A variety of test LGADs have been fabricated, varying in thickness and location of the amplification layer. We present recent progress on LGAD characterization, including calibration measurements using a betascope setup and test beam studies at the CENPA Tandem accelerator.