New excitations in $^{142}$Ba and $^{144}$Ce : Evolution of $\gamma$ bands in the N=86 isotone
Résumé
New excited states in $^{142}$Ba and $^{144}$Ce are investigated by means of prompt $\gamma$-ray spectroscopy of the radiation following spontaneous fission of $^{252}$Cf. Measurements of angular correlations and the observed branchings allowed the assignment of spins and parities with confidence. The new measurements are reinforced by shell-model calculations where energy levels, electric transitions, and magnetic moments are consistent with experimental data. The presence of collectivity in the N=86 isotones is confirmed by clear signatures of soft triaxial γ bands in both nuclei.