Using Electron Backscattering for Timing Synchronization in the Nab Experiment

The Nab experiment is a precision study of unpolarized neutron decay. Cold neutrons decay in flight inside a custom asymmetric spectrometer. The goal of Nab is to measure a, the electron-neutrino coefficient, and b, the Fierz interference term that vanishes in the standard model (SM). A measured value of a at the Nab goal precision of Δa/a ≈ 10-3 will provide an independent competitive evaluation of the Vud element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. The Nab precision goal for the measurement of b is Δb ≈ 0.003. In this way, Nab will contribute to a critical test of CKM unitarity, and, combining a and b, sensitive new tests of physics beyond the SM.

Nab's spectrometer measures and detects protons and electrons in coincidence with two pixelated silicon detectors placed asymmetrically apart from the decay region. Following a 5m long drift region (to determine their time of flight), decay protons are accelerated towards the upper detector by an accelerating potential of -30kV. For Nab to reach its precision goals, the timing bias between the two detectors must be understood to less than 300 ps. Recent systematic tests have indicated a need to continuously synchronize the detectors system to achieve this goal. Using electrons that backscatter between detectors for synchronization is enticing due to the inherent existence of this process during the normal running mode and may be crucial to properly understanding data that has already been taken.