Determining Absolute Polarization of Ultracold Neutrons in the UCNA Experiment

The UCNA experiment uses the decay of trapped ultracold neutrons (UCN) to measure the angular correlation A between the emitted electron's momentum and the direction of the neutron's spin. For a precision measurement of A, a similarly precise determination of the equilibrium neutron polarization \textbf{\textless P\textgreater }is required. By utilizing UCN, transport through a large (7T) B field provides \textasciitilde 100{\%} polarization, and a spin flipper allows state selection during loading phases. This spin flipper also measures the equilibrium polarization of the UCN population present in the spectrometer, after each hour-long beta-counting cycle. By including a neutron reflecting shutter the leading uncertainty in polarimetry measurements prior to 2011, resulting from the residual background population, was reduced to near zero. However, this modification also introduces new systematic corrections, requiring new run types to quantify. Among these corrections are effects from the spin flipper efficiency, spectral velocity conditioning, and depolarization feeding. We will review the analytic underpinning for these contributions, discuss additional measurements required to quantify these parameters, and present a Monte-Carlo analysis to determine the corrected depolarized fraction, and associated uncertainty.