Probing quadrupole and octupole collectivity in ¹⁰⁶Cd via unsafe Coulomb excitation

Stable cadmium isotopes were thought to have spherical ground states due to their phonon-like level structure, however, this was put into question in recent studies that suggested shape coexistence in ^110,112Cd [1]. Further evidence supporting the presence of ground-state deformation in mid-shell Cd nuclei came from a Coulomb-excitation study of ¹⁰⁶Cd [2] in line with Beyond-Mean-Field (BMF) calculations [3]. This intriguing structural puzzle was addressed in more detail using "unsafe" Coulomb-excitation data, obtained as a byproduct of lifetime measurements [4] performed at GANIL using the AGATA and VAMOS++ spectrometers.

Excited states of ¹⁰⁶Cd were populated via inelastic scattering on a ⁹²Mo target at beam energies exceeding the "safe" Coulomb excitation energy [5] by 8-40%, depending on the scattering angle. The balance between the Coulomb and nuclear interactions in the population of individual excited states and the effects introduced by the Coulomb-nuclear interference on the experimental excitation cross sections were explored as a function of the scattering angle between the collision partners. Coupled-channel codes FRESCO [6] and GOSIA [7] were employed to demonstrate that unsafe Coulomb-excitation data can be used to extract spectroscopic information, such as quadrupole and octupole transition strengths.

Selected results will be presented, including the first measurement of B(E3) values involving several negative-parity states, and discussed in terms of a possible quadrupole-octupole coupling scenario. The extracted B(E2) values will be compared with new BMF calculations using the symmetry-conserving configuration mixing method, which includes intrinsically rotating states [8].

[1] P.E. Garrett et al., Phys. Rev. Lett. 123, 142502 (2020).

[2] T.J. Gray et al., Phys. Lett. B 834, 137446 (2021).

[3] M. Siciliano et al., Phys. Rev. C 104 (2021) 034320.

[4] M. Siciliano et al., Phys. Lett. B 806, 135474 (2020).

[5] D. Cline, Annu. Rev. Nucl. Part. Sci. 36, (1986) 683.

[6] I.J. Thompson, Comput. Phys. Rep. 7, 167 (1988).

[7] T. Czosnyka et al., Bull. Am. Phys. Soc. 28, (1983) 745.

[8] D. Kalaydjieva et al., to be submitted to EPJ A (2025).