Pulsed NMR in the nuclear spin ordered phases of solid ³He in a silver sinter

To obtain the exact spin structure of the nuclear magnetically ordered phases of solid ³He, in the BCC lattice called U2D2 and high field phase, both occurring below about 1 mK, we started a project of neutron scattering from the solid at the Hahn-Meitner Institut, Berlin. This experiment faces three main difficulties: to cool the solid to temperatures below 1 mK (or even much lower in the case of the HCP lattice), to keep it there under neutron flux, and to grow a single crystal within the sintered material needed for this purpose. As a first step we have performed pulsed NMR measurements in the ordered phases of solid ³He in a silver sinter of 700 Å particle size down to temperatures of 600 μK at various molar volumes. The samples remained in the ordered state for as long as 110 h.