Realization of a broad band neutron spin filter with compressed, polarized He-3 gas

The strongly spin dependent absorption of neutrons in nuclear spin polarized (3) (H) over right arrow e opens the possibility to polarize beams of thermal and epithermal neutrons. An effective He-3 neutron spin filter (NSF) requires high He-3 nuclear polarization as well as a filter thickness corresponding to a gas amount of the order of 1 bar1. We realized such a filter using direct optical pumping of metastable He-3* atoms in a He-3 plasma at 1 mbar. Metastable exchange scattering transfers the angular momentum to the whole ensemble of He-3 atoms. At present 3 x 10(18) He-3-atoms/s are polarized up to 64%. Subsequent polarization preserving compression by a two stage compressor system enables to prepare NSF cells of about 300 cm(3) volume with 3 bar of polarized He-3 within 2 h. He-3 polarizations up to 53% were measured in a cell with a filter length of about 15 cm. By this cell a thermal neutron beam from the Mainz TRIGA reactor was polarized. A wavelength selective polarization analysis by means of Bragg scattering revealed a neutron polarization of 84% at a total transmission of 12% for a neutron wavelength of 1 Angstrom.