  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 ${\mid}P_\mathrm{n}{\mid} \leq 4\times 10^{-4}$ with 90 %
confidence.
