Polarization Measurement of an Unpolarized Neutron Beam for Nab

The purpose of the Nab experiment is to accurately characterize unpolarized correlation coefficients in the neutron beta-decay. Any unwanted polarization of the neutrons would have a systematic impact on "a", which determines the angular correlation between the electron and antineutrino, for the neutron beta decay. The Nab experiment is trying to measure "a" to a relative accuracy on the order 10^-3 so which also poses an upper limit on the neutron beam polarization of 2*10^-5. By performing Adiabatic Fast Passage (AFP) to reverse the polarization of He-3 cell placed on the beamline and comparing the transmission rates of neutrons in different polarization states of the helium cell. We will present an overview of the method and results of polarimetry measurements.