Mapping quadrupole collectivity in the Cd isotopes: The breakdown of harmonic vibrational motion

The stable Cd isotopes have long been used as paradigms for spherical vibrational motion. Extensive investigations with in‐beam γ spectroscopy have resulted in very‐well‐established level schemes, including many lifetimes or lifetime limits. A programme has been initiated to complement these studies with very‐high‐statistics β decay using the 8π spectrometer at the TRIUMF radioactive beam facility. The decays of 112In and 112Ag have been studied with an emphasis on the observation of, or the placement of stringent limits on, low‐energy branches between potential multi‐phonon levels. A lack of suitable 0+ or 2+ three‐phonon candidates has been revealed. Further, the sum of the B(E2) strength from spin 0+ and 2+ states up to 3 MeV in excitation energy to the assigned two‐phonon levels falls far short of the harmonic‐vibrational expectations. This lack of strength points to the failing of collective models based on vibrational phonon structures.