Application of Polarized 3He Cells in Neutron Interferometry

We report a novel use of polarized $^3$He for studies of nucleon-nucleon (NN) interactions via neutron interferometry. Phenomenological NN models fail to accurately predict properties such as binding energies and scattering lengths of nuclear systems larger than two nucleons. Many different three nucleon interactions (3NI) exist that can be included with NN models to correct for some (but not all) of discrepancies between theory and experiment. We will present the results of a recent precision measurement done at the National Institute of Standards and Technology of the n-$^3$He incoherent scattering length, which plays an important role in the testing of 3NI models. In this experiment a small polarized $^3$He cell, polarized off-line by spin exchange optical pumping, was located within a neutron interferometer. The phase difference for the neutron polarization parallel and anti-parallel to the $^3$He polarization was measured. In an auxiliary measurement, polarized $^3$He cells were also employed to measure the neutron polarization and spin flipper efficiency to $<0.08$\,\% relative uncertainty. The incoherent scattering length was determined with a relative uncertainty of $<1$\,\%.