Rotational and vibrational excitations in 84Zr studied through in-beam and 84Nb β-decay spectroscopy

Low- and medium-spin states have been investigated in the even-even nucleus 84Zr by using in-beam γ-ray spectroscopy via the reactions 59Co(28Si,p2n) and 58Ni(32S,α2p) at beam energies of 99 and 135 MeV, respectively. Several new levels have been identified, including a new even-spin sequence of the γ-vibrational band. The previously known sequence has been found to be built on a 3+ state at 1575.3 keV representing the odd-spin sequence. In addition, the β+ /EC decay of mass-separated 84Nb sources has been studied by β- and γ-ray spectroscopy following the bombardment of a thin 58Ni target with 28Si ions at 94 MeV. Some low-lying daughter states up to spin and parity of 4+ have been identified in 84Zr, and a half-life of 9.5 ± 1.0 s has been measured for the 84Nb ground-state decay. The systematics of γ-vibrational states in even-even Zr nuclei and a comparison with model predictions indicate increasing γ softness and triaxiality as the spherical neutron shell closure at N = 50 is approached. Using an empirical estimate, a triaxiality of about 25° has been extracted for 84Zr. Furthermore, a negative-parity structure of 4- and 6- states which is similar to other N = 44 isotones has been identified in 84Zr as a two-quasineutron excitation.