  Accurately measuring the neutron beam polarization of a high flux, large area
neutron beam is necessary for many neutron physics experiments. The Fundamental
Neutron Physics Beamline (FnPB) at the Spallation Neutron Source (SNS) is a
pulsed neutron beam that was polarized with a supermirror polarizer for the
NPDGamma experiment. The polarized neutron beam had a flux of $\sim10^9$
neutrons per second per cm$^2$ and a cross sectional area of
10$\times$12~cm$^2$. The polarization of this neutron beam and the efficiency
of a RF neutron spin rotator installed downstream on this beam were measured by
neutron transmission through a polarized $^{3}$He neutron spin-filter. The
pulsed nature of the SNS enabled us to employ an absolute measurement technique
for both quantities which does not depend on accurate knowledge of the phase
space of the neutron beam or the $^{3}$He polarization in the spin filter and
is therefore of interest for any experiments on slow neutron beams from pulsed
neutron sources which require knowledge of the absolute value of the neutron
polarization. The polarization and spin-reversal efficiency measured in this
work were done for the NPDGamma experiment, which measures the parity violating
$\gamma$-ray angular distribution asymmetry with respect to the neutron spin
direction in the capture of polarized neutrons on protons. The experimental
technique, results, systematic effects, and applications to neutron capture
targets are discussed.
