Coulomb Excitation of Rare-Earth Nuclei with Alpha Particles

Abstract

Using our doubly charged He beam at 6 Mev we have excited most low-lying (rotational) states in the nuclei between $Z=60$ and $Z=73$. Assignments were made with isotopically enriched targets where available. On the whole the predictions of the "strong coupling" approximation of the unified model are strikingly borne out by our observations. All ${2}^{+}$ first-excited states of even-even nuclei lying below \ensuremath{\sim}600 kev were observed, some of which were previously unknown. Two excited states were found in most odd-$A$ nuclei as revealed by direct and/or cascade radiation to the ground state as well as coincidence measurements. The positions of the first two rotational levels agree with the simple interval rule in all established cases within the experimental accuracy. The majority of these states are not found in beta-ray spectroscopy. The reduced electric-quadrupole transition probabilities for all these transitions were measured and found to be some 50 to 100 times greater than those expected for an independent particle. Good agreement was found with some values of these quantities available from direct lifetime observations, giving us confidence in the correctness of both the theoretical expression for the total excitation cross section, and the experimental approach for its determination. A systematic decrease in these transition probabilities is evident in even-even nuclei upon approaching the closed-neutron shell at $N=82$; a corresponding increase in the energy of the ${2}^{+}$ levels reflects the common origin of both phenomena, namely the decrease of the intrinsic deformation of the nuclei. However, the absolute magnitude derived for this deformation depends upon which manifestation is used.Some evidence for a difference in splitting constants $\frac{{\ensuremath{\hbar}}^{2}}{2\mathcal{I}}$ between even-even, even-odd, and odd-even nuclei is found. Within the rather large uncertainties the transition probabilities do not seem to show a corresponding variation.