Temperature Characterization of the Nab Experiment's Highly-Pixelated, Thick, Large-Area Silicon Detectors

The Nab experiment, currently 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 (relative precision of ~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-pixelated, 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. To meet Nab’s high precision goals, gain shifts in the detectors must be understood to the 1 x 10^-4 level. I performed a series of temperature dependent 207-Bi source studies in the Nab spectrometer and measured the detector thermal profile to constrain what level of temperature control and temperature dependent corrections will be necessary to meet the Nab precision requirements.