Nuclear Structure of theN=82IsotonesCe140andLa139from the (α,α′) Reaction

Differential cross sections for elastic and inelastic scattering of 45-MeV $\alpha$ particles by the $N=82\mathrm{}$ isotones ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ce}$ and ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ have been measured. Both collective- and microscopic-model distorted-wave Born-approximation analyses have been performed to ascertain the collective nature of the levels excited; the wave functions used in the microscopic-model calculations were the eigenfunctions of the pseudo $L-S$ coupling scheme recently developed by Hecht and Adler. The results of the collective-model analysis of the ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ce}(\alpha ,{\alpha }^{\prime })$ data indicate that none of the levels excited are well described as mass vibrations. The microscopic-model calculations indicate that significant contributions from the core are needed in order to fit the magnitudes of the cross sections for transitions to the first $2^{+}$ (1.597 MeV) and $4^{+}$ (2.084 MeV) states of ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ce}$. The collective-model analysis of the ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}(\alpha ,{\alpha }^{\prime })$ data permits parity assignments to be made for all levels excited; unfortunately, it is not possible to understand the features of the ($\alpha ,{\alpha }^{\prime }$) spectrum in terms of a weak-coupling model. The microscopic-model calculations for the ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}(\alpha ,{\alpha }^{\prime })$ experiment indicate that transitions which require a "pseudospin flip" are strongly inhibited. The features of the experimental ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}(\alpha ,{\alpha }^{\prime })$ spectrum are consistent with this prediction. The $\Delta B=0\mathrm{}$ selection rule suggested by Hecht and Adler ($B$ is the total pseudospin) is approximately obeyed for both reactions.