Shape evolution in even-mass $^{98-104}$Zr isotopes via lifetime measurements using the $\gamma\gamma$-coincidence technique

TL;DR

This paper reports lifetime measurements of excited states in $^{98-104}$Zr isotopes using a precise $etaeta$-coincidence technique, providing insights into shape evolution and quantum phase transitions in these nuclei.

Contribution

It presents the first lifetime measurements using the Recoil Distance Doppler Shift method in $etaeta$-coincidence mode for $^{98-104}$Zr, offering new data to compare with theoretical models.

Findings

Observed abrupt changes in nuclear shape indicators around $^{100}$Zr

New lifetime values align with shape transition predictions

Experimental data support specific nuclear structure models

Abstract

The Zirconium (Z = 40) isotopic chain has attracted interest for more than four decades. The abrupt lowering of the energy of the first $2^+$ state and the increase in the transition strength B(E2; $2_1^\rightarrow 0_1^+$ going from $^{98}$Zr to $^{100}$Zr has been the first example of "quantum phase transition" in nuclear shapes, which has few equivalents in the nuclear chart. Although a multitude of experiments have been performed to measure nuclear properties related to nuclear shapes and collectivity in the region, none of the measured lifetimes were obtained using the Recoil Distance Doppler Shift method in the $\gamma\gamma$-coincidence mode where a gate on the direct feeding transition of the state of interest allows a strict control of systematical errors. This work reports the results of lifetime measurements for the first yrast excited states in $^{98-104}$Zr carried out to extract reduced transition probabilities. The new lifetime values in $\gamma\gamma$-coincidence and $\gamma$-single mode are compared with the results of former experiments. Recent predictions of the Interacting Boson Model with Configuration Mixing, the Symmetry Conserving Configuration Mixing model based on the Hartree-Fock-Bogoliubov approach and the Monte Carlo Shell Model are presented and compared with the experimental data.