Detection System for Neutron $beta$ Decay Correlations in the UCNB and Nab experiments

We describe a detection system designed for precise measurements of angular correlations in neutron $beta$ decay. The system is based on thick, large area, highly segmented silicon detectors developed in collaboration with Micron Semiconductor, Ltd. The prototype system meets specifications for $beta$ electron detection with energy thresholds below 10 keV, energy resolution of $sim$3 keV FWHM, and rise time of $sim$50 ns with 19 of the 127 detector pixels instrumented. Using ultracold neutrons at the Los Alamos Neutron Science Center, we have demonstrated the coincident detection of $beta$ particles and recoil protons from neutron $beta$ decay. The fully instrumented detection system will be implemented in the UCNB and Nab experiments, to determine the neutron $beta$ decay parameters $B$, $a$, and $b$.