Structure of even-even Zr isotopes with 52$\leq$N$\leq$58 neutrons

Abstract

Excited levels in $^{92}$Zr have been studied in cold-neutron capture reaction using the EXILL Ge array at ILL Grenoble. Excited levels in $^{94}$Zr and $^{98}$Zr nuclei have been studied using high-statistics $γ$-coincidence data from neutron-induced fission of $^{235}$U measured with EXILL. The goal of was to search for new levels in $^{92,94,98}$Zr and to improve spin-parity assignments to excited levels. Total of 54 new levels, 180 new $γ$ transitions and 70 new or improved spin-parity assignments have been determined in the three isotopes in the present work. A precise neutron binding energy of 8634.81(2) keV has been determined in $^{92}$Zr. In $^{94}$Zr a Gamow-Teller transition is proposed. The (9$^-$), 3894.1-keV level in $^{98}$Zr is likely a few ns isomer. A new technique of estimating half-lives in a picosecond range, developed in this work, provided 61 half-lives and 60 B($π,λ$) rates for transitions in $^{96,98}$Zr and $^{94,96}$Sr. New-type systematics, backed by Large Scale Shell Model calculations have been used to classify 2$^+$ excitations in Zn-Zr even-even nuclei of the 50$\le$N$\le$60 range. This information and new systematics of 0$^+$ excitations in Sr and Zr isotopes explain the evolution of collectivity in zirconium isotopes, showing the role of various single particle excitations in the phase transition and coexistence of the region. The role of the $ν$9/2$^+$[404] extruder orbital as a catalyst in creating 0$^+$ excitations and the deformation change in the region has been discussed.