On the nuclear structure of the Cd isotopes: do they possess low-lying vibrational states?

Abstract:  The nuclear structures of the even-even Cd isotopes near stability, especially 110-116Cd, were long thought to be prime examples of spherical nuclei possessing low-lying vibrational states. Their level schemes display nearly harmonic spacings of one, two, and three-phonon levels. Due to their importance as paradigms of vibrational motion, their structures have been investigated by a variety of reactions. These reactions were essential for establishing the location of levels and their main decay branches, together with level lifetimes, but for many levels they lacked the sensitivity to probe the weak low-energy decay branches that are necessary to assess the degree of collectivity that the states possess. In order to complement the data used to test the collectivity present in the Cd isotopes, high-statistics [beta]-decay experiments using the 8[pi] spectrometer at the TRIUMF radioactive-beam facility have been performed. The goal of these experiments was to achieve a sufficient sensitivity to weak, lowenergy branches amongst the multi-phonon levels so that the collective branches would either be observed, or very stringent upper limits set. Thus far, we have examined the decay of 110In to 110Cd, and 112In/112Ag to 112Cd. These experiments have revealed that the individual low-spin multi-phonon states do not decay in the expected manner. Further, and much more surprising, the missing E2 strength is not due to fragmentation (i.e., mixing) amongst the levels. While the breakdown of vibrational motion at the 3-phonon level is perhaps not surprising, combining data from complementary measurements, especially Coulomb excitation and E0 strengths, has enabled us to rule out the existence of the 0+ member of the 2-phonon triplet. This lack of the E2 strength has forced a re-evaluation of the structure, suggesting that the Cd isotopes do not possess low-lying multiphonon vibrations about a spherical shape as envisioned in the Bohr picture. The results on the Cd isotopes raises the issue that if our long-standing paradigms of spherical vibrations can no longer be considered as such, are there any spherical vibrational nuclei?