Speaker
Description
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.
The observation of super-deformed structures (SD) at relatively low excitation energy in the mass A$\sim$40 around doubly-magic Z=N=20 $^{40}$Ca nucleus [1], is one of the most important discoveries of 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 compared to heavy nuclei, A$\sim$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 $\gamma$-ray tracking array was studied [2]. It was demonstrated for the first time that Coulomb excitation can probe SD structures. This work also provided the first direct experimental evidence for non-axially symmetric or triaxial SD shapes in the A$\sim$40 neighborhood. The experimental studies of the deformation in this region are ongoing, aiming at investigating the origin of the emerging SD structures coexisting with the normally deformed bands in the Ar-Ca-Ti isotopes.
Going towards the medium mass region, the nickel isotopes offer a unique laboratory to investigate shape evolution in the vicinity of another doubly-magic N=Z 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 also reported in $^{56}$Ni, explained as the result of $mp-mh$ excitations like in the 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 and the neighboring nuclei. To this end, the Coulomb excitation studies focused on the structure of $^{58,60,62}$Ni isotopes are currently undertaken. These, together with the recent findings from the $\gamma$-ray and electron spectroscopy measurements reporting the unexpectedly large $E0$ transition strengths for the 2$^+_2\to$2$^+_1$ transitions of $^{58,60,62}$Ni [4], shall bring crucial information enabling the further discussion on the magical properties of Ni isotopes.
In this talk, I will present recent results from measurements designed to approach the well-deformed structures in Ca and Ni isotopes using the Coulomb excitation method, performed at the INFN LNL and IJC Lab, and compare them with state-of-the-art theoretical calculations.
References:
[1] E. Ideguchi et al., Phys. Rev. Lett.87, 222501 (2001)
[2] K. Hadynska-Klek et al., Phys. Rev. Lett. 117, 062501 (2016) and K. Hadynska-Klek 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).