Magnetometry of the Nab Spectrometer

The Nab experiment uses a novel asymmetric superconducting magnetic spectrometer and two large-area segmented Si detectors to extract the neutron beta decay electron-neutrino correlation coefficient \textit{a} and the Fierz interference term \textit{b} from the proton momentum and electron energy spectrum. Nab was designed to achieve a $10^{-3}$ relative uncertainty in \textit{a}, and this requires a detailed characterization and analysis of the magnetic field in the spectrometer. Using a Hall probe calibrated to better than $10^{-3}$ relative precision and the laser trackers that can measure distances within tens of microns, we mapped a field that ranged from tens of gauss to 4.2 Tesla over 7 meters. This talk will cover the measurement technique, results of the field characterization scheme, and the field expansion methods used.