Speaker
Description
Mid-shell Cd nuclei were traditionally considered to be the best examples of vibrational nuclei. Recent studies that combined detailed $\gamma$-ray spectroscopy with sophisticated beyond-mean-field calculations had suggested [1,2] that the low-lying 0$^+$ states in $^{110,112}$Cd possessed prolate, triaxial, and oblate shapes with rotational-like bands built upon them. If confirmed, this would have major implications on structural interpretations of nuclei in the Z = 50 region, and perhaps beyond. Soon afterwards a similar picture was suggested for $^{106}$Cd [3,4].
The low-energy safe Coulomb-excitation technique represents an ideal tool to study nuclear deformation. It enables a direct determination of electromagnetic transition matrix elements between low-lying excited states including spectroscopic quadrupole moments and signs. Those can be further analysed in terms of quadrupole invariants [5,6] yielding model-independent information on shape parameters of individual states. This requires, however, extensive sets of high-precision experimental data.
A multi-faceted experimental program to ascertain the deformation of low-energy states in $^{110}$Cd has been initiated. We seek to firmly establish the shape of the first three lowest-lying 0$^+$ states through the use of the rotation-invariant sum rules for $E$2 transitions. Coulomb-excitation measurements were performed using various reaction partners: $^{14}$N and $^{32}$S beams with EAGLE at HIL UW (Warsaw, Poland) [7], $^{60}$Ni beam with AGATA at LNL (Legnaro, Italy) [8] and $^{110}$Cd beam on a $^{208}$Pb target with GRETINA at ANL (Argonne , USA). These measurements have been complemented by an experiment performed at TRIUMF-ISAC with the GRIFFIN spectrometer examining the decays of Ag/In that will provide high-precision data on $\gamma$-ray branching ratios and transition mixing ratios.
Recent results on quadrupole deformation parameters obtained for the ground and excited 0$^+$ states will be presented [7]. These experimental findings will be discussed in the context of new calculations based on the general quadrupole collective Bohr Hamiltonian model involving two variants of interactions: SLy4 and UNEDF0. Information on the $E$2 properties contained in the shape invariants $\langle Q^2\rangle$ and $\langle Q^3{\rm cos}(3\delta)\rangle$} will be provided, showing which collective parameters are determined by the data, as well as the goodness of collective model descriptions in terms of the emergence of shape coexistence.
References
[1] P. Garrett et al., Phys. Rev. Lett. 123 (2019) 142502.
[2] P. Garrett et al., Phys. Rev. C 101 (2020) 044302.
[3] M. Siciliano et al., Phys. Rev. C 104 (2021) 034320.
[4] D. Kalaydjieva , PhD thesis, Universite Paris-Saclay, 2023.
[5] K. Kumar et al., Phys. Rev. Lett. 28 (1972) 249.
[6] D. Cline, Annu. Rev. Nucl. Part. Sci. 36 (1986) 683.
[7] K. Wrzosek-Lipska et al., Phys. Lett. B 875 (2026) 140315.
[8] I. Z. Piętka et al. , Acta Phys. Pol. B Proc. Suppl. 18 (2025) 2-A26.