Octupole correlations in the 90 Zr and 91 Zr isotopes

The 𝐵⁡(𝐸⁢3) transition strengths in 90Zr and 91Zr isotopes were determined based on the half-life measurements of the first 3− and (11/2)− states, i.e., 𝑇1/2=7.8⁢(16) ps and 500(16) ps, respectively. The plunger method and fast-timing techniques were employed. The 𝐵⁡(𝐸⁢3) values of 16(3) and 20.5(7) W.u. for 90Zr and 91Zr were derived, pointing to a sizable octupole collectivity in both cases. The 90Zr value complements the systematics of the octupole strength in even-even Zr isotopic chain, revealing a smooth trend between 𝐴=90 and 96, which is in agreement with the pattern of octupole strength predicted by quasiparticle random phase approximation calculations. In 91Zr, the hybrid configuration mixing model describes the (11/2)− low-energy strength in terms of a coupling of the unpaired neutron with the 3− octupole excitation of the 90Zr core, predicting a slight decrease of octupole strength which is in contrast with the experimental result. This suggests a more complex structure for the (11/2)− state in 91Zr than predicted by the weak-coupling limit.