Nuclear Spin States in Molecular Crystals

Neutron beam attenuation by a sample immersed in liquid helium was measured. Temperatures below 4 deg K were achieved by evacuating the liquid helium and reducing the boiling point. The neutron beam was derived from a reentrant hole in an aqueous nnoderator immersed in liquid helium. A velocity spectrunn of the emergent beam was best fitted by a Maxwell distribution having a temperature of 40 deg K. Time-dependent cross sections of acetylene and spectral distributions from aqueous moderators at 4 deg K were measured and the resuits are graphically represented. Nuclear spin states found in water and acetylene are expected in many molecular crystals, the rotational energy levels being shifted to higher temperatures by the crystal potential. (L.N. N.)