Nuclear alinement of cobalt and the decay of cobalt-55

Abstract

Cobalt-55 has been alined in a crystal of nickel, zinc fluosilicate which was magnetically cooled to 0.004°K, and the anisotropy of the 0.935 and 1.41 MeV radiations studied as a function of temperature. On the assumption that the spin of 55 Co is 7/2, the spins of the 0, 0.935 and 1.41 MeV levels in 55 Fe are 3/2, 5/2 and 7/2 respectively with presumably odd parity; the 0.935MeV γ -ray is a dipole-quadrupole mixture with δ = + 0.37 ± 0.04. The magnetic moment of 55 Co is 4.3 ± 0.3n.m. and the Fermi admixture in the β -transition to the 1.41 MeV level of 55 Fe is less than 10% . The crystals used consisted of an inactive core on to which a layer several millimetres thick and containing the cobalt activity ( 55 Co, 56 Co, 58 Co or 60 Co), was grown. Data concerning the effect of the nickel concentration in the active layer on the cooling properties of such crystals are presented. The rapidity and ease with which these crystals may be grown should make the technique applicable to isotopes with half-lives as short as 10h. New data on the 58 Co alinement indicates that a Fermi admixture of 1 ± 1 % is associated with the β -transition.