The demise of vibrational excitations in the Cd isotopes

11h - GANIL Seminar room  (105)

A coffee will be served 15 mn before

The nuclear structures of the even‐even Cd isotopes near stability, especially  110‐116Cd, were long thought to be prime examples of low‐lying vibrational motion. Their level schemes display a nearly harmonic spacing of one, two, and three‐phonon levels. Due to their importance as paradigms of vibrational motion, their structures were previously investigated by a variety of reactions, such as a series of (α,2n) reactions and (n,n’gamma) reactions. These reactions established the location of levels and their main decay branches, and in many cases their lifetimes as well. The outcome of these systematic studies revealed some surprising discrepancies from the vibrational picture that were consistent across the isotopic chain suggesting a breakdown in vibrational motion. However, many key transitions necessary to assess the degree of collectivity possessed by low‐spin states at high excitation energy remained unobserved.

In order to complement the data used to test the collectivity present in the Cd isotopes, we have initiated a programme of extensive beta‐decay experiments using the 8π spectrometer at the TRIUMF radioactive‐beam facility. The goal of these experiments is to achieve a sufficient sensitivity to weak, low‐energy branches  amongst the multi‐phonon levels so that the collective branches would either be observed, or very stringent upper limits set. The measurement of the decay of 112In/112Ag to 112Cd has allowed a nearly complete mapping of the E2 strength in low‐spin levels up to ~3 MeV, and has 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 does not appear due to fragmentation (i.e., mixing) amongst the levels below ~3  MeV. In addition, single-nucleon transfer experiments performed with the Q3D spectrometer at Munich has observed the 5- member of the quadrupole-octupole coupled quintuplet as one of the strongest populated states in the spectrum, suggesting a re-interpretation is necessary for these excitation also.

These new data on the Cd isotopes raises the issue that if our long‐standing paradigms of vibrations can no longer be considered as vibrational nuclei, are there any spherical vibrational nuclei?