First observation of the intruder band in ¹⁰⁸Cd

The study of intruder and multiphonon configurations is of great interest in nuclear structure physics because it is related to single-particle and collective motion. The rarest of the stable cadmium isotopes was investigated with the powerful combination of two complementary experimental techniques: γγ spectroscopy following the β decay of ¹⁰⁸In, and population of excited states using the nonselective (α,n) fusion-evaporation reaction. The intruder band in ¹⁰⁸Cd up to its 4⁺ member is established on the measurement of branching ratios of in-band and intraband decays supported by upper limits for the lifetimes using Doppler shift attenuation method. The heavily suppressed absolute E2 transition strength out of the proposed intruder band indicates this band structure to be fairly pure. These findings are compared to neighboring Cd isotopes and related six quasiproton structures. An analysis of the multiphonon structures observed in ¹⁰⁸Cd and heavier Cd isotopes shows that the even mass cadmium chain forms a transitional path between the vibrational U(5) and the gamma-unstable O(6) limit with ¹⁰⁸Cd closer to the O(6) description.