The neutron polarization of the NG-C beamline at the NIST Center for Neutron Research was measured as part of the aCORN neutron beta decay experiment. Neutron transmission through a polarized 3He spin filter cell was recorded while adiabatic fast passage (AFP) nuclear magnetic resonance (NMR) reversed the polarization direction of the 3He in an eight-step sequence to account for drifts. The dependence of the neutron transmission on the spin filter direction was used to calculate the neutron polarization. The time dependent transmission was fit to a model which included the neutron spectrum, and 3He polarization losses from spin relaxation and AFP-NMR. The polarization of the NG-C beamline was found to be ${mid}P_mathrm{n}{mid} leq 4times 10^{-4}$ with 90 % confidence.
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