Low-lying collective states in 120Cd populated by β decay of 120Ag: Breakdown of the anharmonic vibrator model at the three-phonon level

We have reinvestigated the β decay of 120Ag to levels in 120Cd. Significant deviations in the decays of low-lying states from that expected from a U(5) nucleus have been observed. The results from this paper which include the 02+ and 31+ states (which have now been correctly identified) and previous work on lighter Cd isotopes strongly suggest that the anharmonic vibrator description of the even-even Cd isotopes is inadequate. Analysis of the decay patterns and half-life information led to the determination that there are three β-decaying isomers in 120Ag, with Jπ assignments of (0^-, 1^-), 4(⁺), and 7(^-). In addition, we have identified four candidates of the quadrupole-octupole quintuplet. These states are Jπ=(5^-) at 2489.5 keV, (4^-) at 2208.4 keV, (2 ^-) or (3^-) at 2318.4 keV, and (1^-) or (2 ^-) at 2448.7 keV. All show strong transitions to the previously known 3^- octupole state at 1898.9 keV. The overall level scheme of 120Cd has been significantly expanded.