The Nab experiment will measure the ratio of the weak axial-vector and vector coupling constants λ = gA/gV with precision δλ/λ ∼ 3 × 10− 4 and search for a Fierz term bF at a level ΔbF < 10− 3. The Nab detection system uses thick, large area, segmented silicon detectors to very precisely determine the decay proton’s time of flight and the decay electron’s energy in coincidence and reconstruct the correlation between the antineutrino and electron momenta. Excellent understanding of systematic effects affecting timing and energy reconstruction using this detection system are required. To explore these effects, a series of ex situ studies have been undertaken, including a search for a Fierz term at a less sensitive level of ΔbF < 10− 2 in the beta decay of 45Ca using the UCNA spectrometer.

Solid deuterium (sD(2)) is used as an efficient converter to produce ultracold neutrons (UCN). It is known that the sD(2) must be sufficiently cold, of high purity and mostly in its ortho-state in order to guarantee long lifetimes of UCN in the solid from which they are extracted into vacuum. Also the UCN transparency of the bulk sD(2) material must be high because crystal inhomogeneities limit the mean free path for elastic scattering and reduce the extraction efficiency. Observations at the UCN sources at Paul Scherrer Institute and at Los Alamos National Laboratory consistently show a decrease of the UCN yield with time of operation after initial preparation or later treatment ("conditioning") of the sD(2). We show that, in addition to the quality of the bulk sD(2), the quality of its surface is essential. Our observations and simulations support the view that the surface is deteriorating due to a build-up of D-2 frost-layers under pulsed operation which leads to strong albedo reflections of UCN and subsequent loss. We report results of UCN yield measurements, temperature and pressure behavior of deuterium during source operation and conditioning, and UCN transport simulations. This, together with optical observations of sD(2) frost formation on initially transparent sD(2) in offline studies with pulsed heat input at the North Carolina State University UCN source, results in a consistent description of the UCN yield decrease.