Detector Cooling, Temperature Dependent Studies, and Waveform Analysis for the Nab Experiment

The Nab experiment, currently (2025) taking data on the Fundamental Neutron Physics Beamline at the ORNL Spallation Neutron Source, uses an unpolarized neutron beam to precisely measure two of the free neutron beta decay correlation parameters to probe physics beyond the Standard Model. The electron-neutrino correlation coefficient, a, will give us access to investigate CKM unitarity, and the Fierz interference term, b, will enable us to put bounds on the existence of scalar and tensor currents in the weak interaction. We aim to measure a with unprecedented precision (∆a/a ≈ 1 x 10⁻³) to enable a first row CKM unitarity check on the same level of precision as those made with super-allowed fermi decays. The Nab experiment uses two highly-segmented, thick, large-area silicon detectors at either end of a 7 m tall magnetic spectrometer to measure the electron energy and the proton time of flight, which can be used to construct nearly the full phase space of neutron decay, and make determinations of a and b. This work encompasses the finalization of the detector cooling system design, implementation, and characterization. The detector cooling system is now surpassing design goals. It is capable of cooling the upper detector to ~100 K and the lower detector to ~115 K with better than +/- 0.5 K stability. The system is also capable of stabilizing detector temperatures over a dynamic range from the minima up to room temperature, so it is well-suited to support temperature dependent studies. This feature was utilized to study the temperature dependent gain shift in three of the Nab detectors. Waveform analysis of 207-Bi source signals was carried out to show that the gain shift on each of the studied detectors was below the suggested requirement of 1 x 10⁻⁴ for +/- 0.5 K. These waveform analysis techniques were then applied to investigate the proton detection efficiency. A method for estimating the proton detection efficiency's impact on ∆a/a was explored, the ∆a/a for one pixel was reported, and the results will be useful to future analyses.