Triaxial deformation of neutron-rich Zr nuclei explored by high-resolution in-beam γ-ray spectroscopy

Abstract

Lifetimes of excited states in the very exotic nuclides ¹⁰⁸Zr₆₈ and ¹¹⁰Zr₇₀ were measured for the first time applying high-resolution in-beam γ-ray spectroscopy at relativistic energies. The experiment was carried out at the Radioactive Isotope Beam Factory in RIKEN, using nucleon removal reactions of radioactive Nb ions to produce ¹⁰⁸Zr and ¹¹⁰Zr. Emitted γ rays from the excited states were measured by the High-resolution Cluster Array at RIBF (HiCARI) and lifetimes were extracted from the observed line shapes caused by relativistic Doppler-shift effects. In addition to the new lifetime information, the precision of the $E\left(2_1^{+}\right)$ and $R_{4/2}=E\left(4_1^{+}\right)/E\left(2_1^{+}\right)$ values of ¹¹⁰Zr could be significantly improved. The low-lying $2_2^{+}$ states in both nuclei are also newly observed, which are indicative of the triaxiality. The various physical quantities were compared with theoretical models to investigate the collectivity in Zr isotopes towards $N=70$. Together the experimental and theoretical results point at a reduction of the axial deformation while triaxial collectivity increases.