Polarized $^3$He Neutron Spin Filter for Parity-Odd Asymmetry Measurement on 0.88-eV p-Wave Resonance of $^{81}$Br

The Neutron OPtics Time Reversal Experiment (NOPTREX) collaboration plans to conduct a sensitive search for time reversal invariance violation in polarized neutron transmission through polarized nuclei by taking advantage of the very large amplification of symmetry-violating effects in p-wave resonances of certain heavy nuclei. As a step toward this experiment we are remeasuring parity violation in selected nuclei to greater precision at LANSCE. One such candidate is $^{81}$Br with a longitudinal asymmetry of $A = 0.024 \pm 0.004$ at the 0.88-eV resonance. We aim to measure this asymmetry to 5\% accuracy. This requires an intense source of polarized neutrons at eV energies. We plan to use a polarized $^3$He neutron spin filter based on the very large spin dependent neutron absorption cross-section of neutrons on $^3$He. In the $^3$He system under construction at Indiana University, $^3$He gas is polarized by spin-exchange optical pumping (SEOP). Key components include a $\mu$-metal shielded solenoid and $^3$He gas cell both generously provided by ORNL. This talk will describe the proposed $^{81}$Br experiment, motivation for choice of $^3$He SEOP for NOPTREX, and projected performance of the $^3$He spin filter.