Alignment of Three Odd-A Rare Earth Nuclei

The results of nuclear magnetic hfs alignment experiments on Ce139 are presented, and are discussed in relation to those previously obtained for Ce141 and Nd147. It is shown that the gamma-ray anisotropies, ε, in the case of Ce139 are consistent with a decay sequence 32+→52+→52+→72+, the gamma transition being M1 with ∼4% admixture of E2. Even such a small admixture of E2 has an important effect upon the anisotropy through the interference term. Other factors which might affect the anisotropy are discussed: electron spin-spin interactions are important, whereas reorientation effects due to long lifetimes in the intermediate state are not. Interference is also important in the case of the 92-kev gamma ray following the decay of Nd147, where ∼3% admixture of E2 radiation is needed to explain our results. It seems likely also that there is a similar effect in the 142-kev gamma ray following the decay of Ce141. The similarity of these mixtures (same magnitude and phase φ=0) may not be fortuitous, since in each case the gamma ray connects levels which are classified by the shell model as d52 and g72. In the cases of Ce141 and Nd147 the g72 state lies uppermost and ε is reduced by the E2 admixture, whereas ε is increased in Ce139, where the levels are inverted.