Evaluation of detectors for the UCNA+ experiment

Electron detection in the original UCNA experiment used a combination of multi-wire gas proportional counters (MWPC) and plastic scintillators for energy, time, and position reconstruction. Scattering effects on the inactive material of the MWPC were a significant portion of the overall systematic uncertainty of the experiment, limiting the ultimate sensitivity to ΔA/A ≈ 0.6%. To reach a ≤0.2% measurement of the beta-asymmetry parameter, A₀, the UCNA+ will incorporate a significant upgrade to the detector package: the MWPC's, light guides and traditional photo-multiplier tubes will be replaced with 128 SiPMs directly coupled to the scintillator. Initial testing has demonstrated that this configuration will improve the light collection by a factor of 5 and provide ≈2.5 mm position reconstruction. We will report the design, testing, and installation of these detectors.