Collective Nuclear Structure in the Even-Even Samarium Isotopes

The level structure and transition matrix elements between collective states in the even-even isotopes of samarium ($A=148-154\mathrm{}$) have been investigated using Coulomb excitation induced by ${\mathrm{O}}^{16}$ ions. Both total and differential cross sections were measured for the de-excitation gamma radiation and for gamma radiation in coincidence with the inelastically scattered oxygen ions. Internal-conversion data also supplemented some phases of these studies. The exising information on these nuclei has been extended to include higher spin states and previously unmeasured transition moments for both even- and odd-parity states. The predictions of the available phenomenological collective models have been compared with this information and it has been found that although the latter models can account for some of the general features of the observed excitation spectra and transition matrix elements, they are deficient in explaining the details. In ${\mathrm{Sm}}^{152}$, where specific attention was devoted to the levels associated with the beta and gamma vibrations, the results indicate that for nuclei near the edges of the deformed region the axially symmetric model, with a first-order perturbation treatment of the rotational-vibrational interaction, is not adequate to explain the structure of low-lying states, and that this mechanism can account for only 9% of the observed coefficient of the second-order term in the expansion of the excitation energy in an angular-momentum power series. The more exact account of centrifugal stretching, developed by Davydov and Chaban, presently produces the best over-all fit, but a number of discrepancies with the data still remain and are discussed.