Deformation of ¹¹⁰Cd in its ground and excited 0⁺ states

Since the 1950’s, the nuclear science community has largely accepted the work of A. Bohr and B. Mottelson, for which they received the Nobel Prize in 1975, that the lowest excitations that occur in the nucleus arise from collective behaviour. One can explain the principle of collective nuclear excitations in analogy to the motions of fans in a football stadium; each of them may stand up and run around – like individual protons and neutrons that can be promoted from one orbital to the next – but they can also participate in a “wave” that propagates around the stadium, which is a collective phenomenon. In the atomic nucleus, the collective excitations may be rotations or vibrations that involve multiple particles in the nucleus, or even the majority of them.

For over 40 years, cadmium isotopes have played a key role in the study of the collective behaviour in atomic nuclei. Initially, it was believed that their structures could be described within a collective vibrational model, in which the surface of the nucleus oscillates (vibrates) around a spherical equilibrium shape. Subsequent experimental results inspired new calculations using advanced theoretical models of atomic nuclear structure, which predict that the individual excited states in ¹¹⁰Cd and ¹¹²Cd nuclei adopt various ellipsoidal shapes (for example, elongated like a rugby ball or zucchini, or flattened like a Frisbee or a pumpkin). In parallel, alternative theoretical approaches were developed, that are able to reconcile the existing experimental data on these nuclei with their quasi-spherical shapes.

Recent experiments conducted at the Heavy Ion Laboratory UW made it possible to discriminate between these two fundamentally different interpretations of the structure of cadmium nuclei by determining the shapes of two key states in the ¹¹⁰Cd nucleus. Both the ground state and the excited 0⁺ state deviate from a spherical shape, with the former, in particular, resembling a kiwifruit (or an ellipsoid with all axes having different lengths). Furthermore, the measured quadrupole moments (i.e., charge distribution parameters) of the lowest two spin-2 states are significantly different from zero, ruling out the hypothesis that the structure of these nuclei could result from their vibrations around a spherical shape. These findings were recently published in the prestigious journal Physics Letters B.

The new results on ¹¹⁰Cd  are part of a broad international research programme aimed at determining the shapes of the low-excited 0⁺ states in even-even cadmium nuclei, using a variety of complementary experimental techniques and the most advanced detection systems. Within this programme, Iwona Piętka is preparing a PhD thesis based on experimental data collected with the state-of-the-art gamma-ray spectrometer AGATA. The work is carried out at the Heavy Ion Laboratory UW, within the Doctoral School of Exact and Natural Sciences at the University of Warsaw, under supervision of dr Katarzyna Wrzosek-Lipska i dr hab. Leszek Próchniak.

K. Wrzosek-Lipska, I.Z. Piętka, L. Próchniak, P.E. Garrett, M. Zielińska, T. Abraham, J.M. Allmond, F.L. Bello Garrote, H. Bidaman, V. Bildstein, S. Buck, C. Burbadge, M. Chiari, R. Coleman, G. Colombi, G. Colucci, A. Diaz Varela, D.T. Doherty, S. Dutt, B. Greaves, K. Hadyńska-Klęk, J. Heery, M. Hlebowicz, D. Hymers, J. Iwanicki, G. Jaworski, B. Jigmeddorj, D. Kalaydjieva, M. Kisieliński, M. Komorowska, N. Kopeć, M. Kowalczyk, J. Kowalska, K. Krutul-Bitowska, R. Kumar, A. Mai Quynh, N. Marchini, T. Marchlewski, K.R. Mashtakov, M. Matejska-Minda, C. Michelagnoli, A. Nannini, P.J. Napiorkowski, B. Olaizola, F. Oleszczuk, M. Palacz, E. Pasquali, E.E. Peters, M. Rocchini, E. Sahin, J. Samorajczyk-Pyśk, M. Saxena, A. Stolarz, J. Srebrny, A. Tucholski, A. Trzcińska, M. Venhart, J.L. Wood, S.W. Yates, T. Lidar

Deformation of the 0⁺_1,2 states in ¹¹⁰Cd from low-energy Coulomb excitation,
Physics Letters B, Volume 875, 2026, 140315, ISSN 0370-2693,
https://doi.org/10.1016/j.physletb.2026.140315.