Crystal shape effect on nuclear orientation thermometry at ultra-low temperatures

Abstract

Absolute temperature measurement (T < 100 mK region) from anisotropy of gamma radiations emitted from oriented60Co nuclei in a single crystal of hcp cobalt is found to depend on the shape of the crystal. This dependence is attributed to some closure magnetic domains not oriented along thec-axis of the disc shaped crystal studied. A long rectangular strip of cobalt crystal is found to give correct angular distribution ofγ-radiations and, therefore, suitable for thermometry.