Commissioning of SiPM detectors for the UCNA+ experiment

The UCNA+ experiment hosted at the ultracold neutron source at Los Alamos National Lab plans to measure the beta-asymmetry parameter A₀ to a precision of ≤0.2 %. This parameter can be used to determination λ, the ratio of the axial and vector weak coupling constants and when combined with measurements of the neutron lifetime is used to test the unitarity of the CKM quark mixing matrix and search for physics beyond the standard model. A key component of the upgrade from UCNA to UCNA+ are the SiPM-based electron detector packages at either end of the 2×2π solenoidal spectrometer. Each package is comprised of two 16-sided plastic scintillators which are readout by 128 SiPM to determine the energy and position of incident electrons. The detector packages will be installed and commissioned during the 2023 LANSCE accelerator cycle. We will present the results of offline testing at ETSU and initial commission and calibration at LANSCE.