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The concept of magic numbers constitutes a cornerstone of nuclear physics research. Over the past few decades, it has been well established that closed-shell nuclei exhibit spherical shapes in their ground states. However, as nucleons are added to or removed from complete shells, they tend to deform the microscopic mass distribution. Studies of deformed shapes are of key importance for the advancement of nuclear structure physics, especially the origin of the observed enhanced deformation in the vicinity of doubly-magic mass nuclei. Furthermore, the observation of shape coexistence, in which several distinct nuclear shapes are observed to be present within a very small energy range in the nucleus, represents one of the most striking phenomena to be reported in atomic nuclei.
The observation of super-deformated structures (SD) at relatively low excitation energy in the mass A~40 around doubly-magic Z=N=20 40-Ca nucleus [1], is one of the most important discoveries of the modern nuclear physics. Remarkably, the observed SD structures in these lighter nuclei are even less well understood than in heavier regions. Due to the vast reduction of nucleons involved and therefore complexity in comparison to heavy nuclei, A~40 constitutes an ideal ground for investigating exotic structures within various theoretical frameworks. Recently, the electromagnetic structure of the nucleus 42-Ca with the advanced AGATA γ-ray tracking array was studied [2]. It was demonstrated, for the first time, that it is possible to probe SD structures with Coulomb excitation. This work also provided the first direct experimental evidence for non-axially symmetric or triaxial SD shapes in the A~40 neighbourhood. The experimental studies of the deformation in this region are ongoing, aiming at investigating the origin of the emerging of SD structures coexisting with the normally deformed bands in the Ar-Ca-Ti isotopes.
Going more north-east from 40-Ca, the nickel isotopes offer a unique laboratory to investigate shape evolution in the vicinity of another doubly-magic nucleus 56-Ni (Z=N=28), which should exhibit similar structural properties to those observed in the Z=N=20 region. Indeed, observation of the SD structures was reported in 56Ni, being explained as the result of mp-mh excitations like in case of 40-Ca [3]. However, recently the questions on the validity of Z/N=28 as a good magic number have been brought up triggering the discussion on the deformation in the nickel region, including the signatures of shape coexistence. Microscopic and collective properties in the vicinity of 56-Ni shall be evaluated with the dedicated measurements of the deformation, also in the neighboring nuclei. To this end, the Coulomb excitation studies focused on the structure of 58,60,62-Ni isotopes are currently undertaken at several laboratories, including INFN LNL, IJC Lab Orsay and HIL Warsaw. These, together with the recent findings from the gamma-ray and electron spectroscopy measurements where the unexpectedly large E0 transition strengths for the 2+2 → 2+1 transitions of 58,60,62-Ni were reported [4], shall bring crucial information enabling the further discussion on the magical properties of Ni isotopes.
In this talk I will present the recent results from the Coulomb excitation measurements focused on Ca and Ni isotopes and the comparison with the state-of-the-art theoretical calculations.
[1] E. Ideguchi et al., Phys. Rev. Lett. 87, 222501 (2001)
[2] K. Hadyńska-Klęk et al., Phys. Rev. Lett. 117, 062501 (2016) and K. Hadyńska-Klęk et al., Phys. Rev. C97, 024326 (2018)
[3] D. Rudolph et al., Phys. Rev. Lett. 82, 3763 (1999)
[4] L.J. Evitts et al., Phys. Lett. B 779, 396 (2018)
