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

The strongly spin dependent absorption of neutrons in nuclear spin polarized ³overlineHe opens the possibility to polarize beams of thermal and epithermal neutrons. An effective ³He neutron spin filter (NSF) requires high ³He nuclear polarization as well as a filter thickness corresponding to a gas amount of the order of 1 barl. We realized such a filter using direct optical pumping of metastable ³He ^∗ atoms in a ³He plasma at 1 mbar. Metastable exchange scattering transfers the angular momentum to the whole ensemble of ³He atoms. At present 3 × 10 ¹⁸³He- 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 ³He within 2 h. ³He 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 Å.