Experiments on nuclear orientation at very low temperatures I, Establishment of a method of nuclear alinement and its application to cobalt-60

Experiments have been carried out which demonstrate the possibilities of the method of ‘magnetic hyperfine-structure alinement’ for the orientation of nuclei. Single crystals of composition (1% Co, 12% Cu, 87% Zn) Rb 2 (SO 4 ) 2 , 6H 2 O were cooled by adiabatic demagnetization to 0·01°K. The crystals contained a small quantity of radioactive 60 Co, which decays by β -emission followed by two γ -rays in cascade. The anisotropic distribution of the γ -rays from the oriented 60 Co nuclei was measured by an array of Geiger counters around the cryostat. The largest anisotropy observed was 33%, and the change in this anisotropy as the crystals warmed up to 1°K was fitted to theoretical curves where the only unknown parameter was the magnetic moment of 60 Co, for which a value of 3·5 ± 0·5 nuclear magnetons was deduced.