Speaker
Description
In recent years, shape coexistence in atomic nuclei has been recognized as ubiquitous across the nuclear chart [1]. The structure of nuclei exhibiting shape coexistence reflects an intriguing interplay between microscopic and macroscopic nuclear effects, the understanding of which is one of the greatest challenges of modern nuclear theory. This interplay manifests in the $A\approx100$ region with neutron number N=60, with the Sr and Zr isotopes exhibiting the most rapid onset of ground-state deformation observed across the nuclear landscape. In this context, $^{96}Sr$ represents a transitional nucleus located at the precipice of the region of deformed ground-state shapes. From a structural point of view, the observed sudden onset of deformation can be explained by an inversion of coexisting regular and “intruder” configurations (multiparticle-multihole excitations) where the former possesses a spherical shape, and the latter is deformed. This interpretation is supported both by theoretical calculations [2,3,4] and experimental data for Sr [5,6]. Notably, the $\gamma$-ray decay transition probabilities of the $2^+_1$ state in $^{96}Sr$ and the $2^+_2$ state in $^{98}Sr$, and the spectroscopic quadrupole moments of these states measured in a Coulomb-excitation study [5,6], were found to be consistent with the $0^+_1(^{96}Sr)\leftrightarrow 0^+_2(^{98}Sr)$ inversion scenario. On the contrary, there is still lack of experimental evidence for the correspondence of the low-lying $0^{+}$ states in $^{96}Sr$ to the ground-state configuration $0^+_1$ of $^{98}Sr$. A neutron-transfer reaction was performed at TRIUMF [7] and spectroscopic factors were measured for the excited $0^{+}$ states. Combining these results with the extremely large $\rho^2(E0;0^+_3\rightarrow 0^+_2)$ value of 160(40) [8] led to a suggestion of triple shape coexistence occurring in $^{96}Sr$, with a weakly deformed ground state and two excited $0^{+}$configurations: a deformed and a spherical one, which undergo strong mixing.
Our new experimental study of $^{96}Sr$ aims to shed light on the inversion scenario via the measurement of transition probabilities in a Coulomb excitation experiment performed at TRIUMF. The $^{96}Sr$ radioactive ion beam was produced and delivered by the ISAC-II facility. The $\gamma$ rays from Coulomb-excited states in $^{96}Sr$ were detected by the TIGRESS spectrometer [9], while the scattered beam ions were detected by the S3 detector of the BAMBINO array [10]. An increase of two orders of magnitude in the statistics of the $2_{1}^{+}\rightarrow0^{+}_{1}$ transition was achieved compared to the previous Coulomb-excitation experiment performed at ISOLDE [5,6]. The population of several states in $^{96}Sr$ was observed, including the $4_{1}^{+}$, $0_{2}^{+}$ and $3_{1}^{-}$ ones. Finally, our Coulomb-excitation experiment provided as well high-quality data on $^{96}Y$, allowing a distinction to be made between single-particle and core-coupled states.
Preliminary results of the ongoing data analysis will be presented.
[1] K. Heyde and J.L. Wood, Rev. Mod. Phys. 83, 1467 (2011).
[2] T. Togashi et al., Phys. Rev. Lett. 117, 172502 (2016).
[3] N. Gavrielov et al., Phys. Rev. C. 105, 014305 (2022).
[4] J.E. García-Ramos et al., Phys. Rev. C. 102, 054333 (2020).
[5] E. Clément et al., Phys. Rev. Lett. 116, 022701 (2016).
[6] E. Clément et al., Phys. Rev. C 94, 054326 (2016).
[7] S. Cruz et al., Phys. Lett. B 786, 62 (2018).
[8] T. Kibédi et al., Prog. Part. and Nucl. Phys. 123, 103930 (2022).
[9] G. Hackman and C. E. Svensson, Hyperfine Int. 225 241 (2014).
[10] C. Y. Wu et al., Technical report LNLL/Rochester 2007 TRIUMF activity (2007).