Measurement of the neutron beam polarization of BL05/NOP beamline at J-PARC

► We have measured neutron beam polarization with an accuracy of 0.03% using a 3He neutron spin filter. ► This accuracy was achieved by flipping the 3He polarization with adiabatic fast-passage (AFP) NMR. ► The 3He spin flipping canceled most of systematic uncertainties. ► Such accuracy is necessary for the precise neutron beta decay measurements planned at BL05 of the J-PARC spallation neutron source.