The Electrode System in the Neutron Decay Region of the Nab Experiment

The Nab collaboration will determine two parameters in free neutron beta decay: (a) the electron-antineutrino correlation coefficient a to Δa/a<10^-3 and (b) the Fierz interference term b to Δb<3×10^-3. Part (a) will be done with a measurement of the two-dimensional electron energy and proton time-of-flight spectrum in the neutron beta decay. To achieve the desired experimental uncertainty we have certain requirements for the electric field in the neutron decay region. We will present our electrode system made from materials with low work function variations for the decay region, and the work function characterization result from Kelvin Probe.