On the limits of spin-exchange optical pumping of ³He

We have obtained improvement in the ³He polarization achievable by spin-exchange optical pumping (SEOP). These results were primarily obtained in large neutron spin filter cells using diode bar lasers spectrally narrowed with chirped volume holographic gratings. As compared to our past results with lasers narrowed with diffraction gratings, we have observed between 5% and 11% fractional increase in the ³He polarization P_He. We also report a comparable improvement in P_He for two small cells, for which we would not have expected an increase from improved laser performance. In particular, prior extensive studies had indicated that the alkali-metal polarization was within 3% of unity in one of these cells. These results have impact on understanding the maximum P_He achievable by SEOP, whether the origin of the improvement is from increased alkali-metal polarization or decreased temperature-dependent relaxation. We conclude that the most likely explanation for the improvement in P_He is increased alkali-metal polarization. We have observed P_He of between 0.80 and 0.85 in several large cells, which marks a new precedent for the polarization achievable by SEOP.