Metastability-exchange optical pumping of 3He for neutron polarizers

Research is underway at NIST and IU to develop neutron polarizers that are based on polarized 3He. Such polarizers rely on the strong spin dependence of the cross section for neutron capture by polarized 3He. Two methods can produce the high density of polarized 3He gas (1019-1020 cm-3) required for an effective neutron polarizer: spin-exchange optical pumping, which is performed directly at high pressure (1-10 bar), and metastability-exchange optical pumping, in which the gas is polarized at low pressure (1 mbar) and then compressed. While we are pursuing both methods, progress in the metastable method will be discussed. The features of the metastable method are the high rate at which the gas can be polarized and the inherent separation of the optical pumping and target cells. In a landmark achievement, researchers at the Univ. of Mainz have developed a piston compressor that can fill a 130 cm3 cell to a pressure of 7 bar of 45% polarized 3 He gas in 2 hours. We plan to develop a compressor and test it at the NIST Cold Neutron Research Facility. We have constructed a metastable-pumping apparatus at NIST and have obtained 76% polarization with a pumping rate of 1.2 x 1018 atoms/sec in a 0.4 mbar, 270 cm3 cell