Using Low-Energy Protons to Characterize Semiconductor Detectors for Neutron Decay Experiments

The Neutron-a-b (Nab) experiment, stationed at Oak Ridge National Laboratory’s Spallation Neutron Source, is performing a high-precision characterization of the weak interaction via free neutron beta decay. Nab aims for a relative precision of 1⋅10^-3 on the angular correlation between beta and neutrino momenta (little-a) and 3⋅10^-3 absolute precision on the Fierz interference term (little-b). These measurements will weigh in on the discrepancies in modern determinations of the ratio of axial-vector to vector coupling (lambda) and play a critical role in probing first-row unitarity of the CKM matrix. This presentation covers the characterization campaign performed to understand our large-area, thick, segmented silicon detectors. I will discuss the operational principles, experimental expectations, and detail the capabilities of the detection system, as well as the testing facility used to undertake this campaign. Several studies were performed across the phase space to fully calibrate and characterize these silicon detectors – including the silicon temperature, diode bias voltage, and incident energy. These results play an instrumental role in allowing the Nab experiment to achieve its uncertainty goals.