Neutron Polarimetry with Polarized 3He for the NPDGamma Experiment

Cold neutrons enable the study of the fundamental interactions of matter in low-energy, low-background experiments that complement the efforts of high-energy particle accelerators. Neutrons possess an intrinsic spin, and the polarization of a beam of neutrons defines the degree to which their spins are oriented in a given direction. The NPDGamma experiment uses a polarized beam of cold neutrons to make a high precision measurement, on the order of one part in 100 million, of the parity-violating asymmetry in the angular distribution of emitted gamma-rays from the capture of polarized neutrons on protons. This asymmetry is a result of the hadronic weak interaction (HWI) and is directly proportional to the long-range, weak interaction modeled by the exchange of a pion between two nucleons. The results of the NPDGamma experiment are dependent on the polarization of the neutron beam used in the capture reaction. The neutron polarization is measured using the large spin-dependent neutron capture cross section of polarized 3He to a precision of less than 2%, which does not significantly increase the total error of the measured gamma-ray asymmetry. Reported here is a description of the NPDGamma experiment, the work done to polarize 3He, and the results of the neutron beam polarimetry measurements.