Orateur
Description
The structure of atomic nuclei can be described using the single-particle picture, especially for nuclei with magic numbers such as 8, 20, 28, 50, 82, and 126. In these nuclei, there are significant energy gaps between occupied and valence orbitals. However, in reality, the atomic nucleus is a correlated system where nucleons occupy valence orbits, leading to a diffuse Fermi surface. The N=40 shell gap in 68Ni, which is part of the HO-like magic numbers 8 and 20 was studied in terms of this diffuseness which is important for characterizing magicity. The occupancy of fpg neutron orbitals in Stable Ni isotopes (from 58Ni to 64Ni) was investigated and partial filling of g9/2 orbitals starts below N=40 [1]. The shell evolution which is the disappearance of conventional magic numbers, and the appearance of new ones have been observed in the nuclei far from stability. One example of this phenomenon is 78Ni which has conventional magic numbers of Z=28 and N=50.
Recently, measurements of 78Ni have shown that the N=50 magic number state is preserved, but there is evidence of a nearby prolate shape [2]. The energy levels provide information about magicity, but the amplitude of the N=50 shell gap and its evolution by adding neutrons from 68Ni are still unknown. To determine the evolution of the N=50 gap between the d5/2 and g9/2 neutron orbitals, we need to start by determining it in 68Ni at N=40. This can be estimated from the excitation energy of the 5/2+ states in 69Ni, obtained by adding a single neutron to the d5/2 orbital of 68Ni through the (d, p) reaction. Spectroscopic factors can be used to determine the centroid of the 5/2+ states. The hole contribution can be derived from the neutron removal reaction 68Ni(p, d)67Ni. The N=50 gap can then be derived from the difference between the centroids of the d5/2 and g9/2 orbitals. By applying neutron-neutron matrix elements derived in this region, we can infer the size of the N=50 gap in 78Ni and compare it with calculations for 78Ni [3].
In this study, in addition to determining the N=50 shell gap, it is also interesting to study the spin-orbit splitting of the 2p, 1f (hole), and 1g (particle) orbitals in 68Ni. By performing the neutron removal reaction from 68Ni, we can obtain spectroscopic information about the 2p hole states in 68Ni. Also, we can deduce information about the location of the g7/2 particle strength in 69Ni and f7/2 hole strength in 67Ni from the neutron adding and removal reactions, respectively. The neutron spin-orbit splitting between g7/2 and g9/2 and between f5/2 and f7/2 is expected to be around 9 MeV and 7 MeV, respectively, but the strength is likely to be fragmented, especially for the g7/2 orbital.
To summarize, In this study, we present the preliminary results of neutron adding and neutron removal reactions from the 68Ni nucleus to study the neutron Fermi surface at N =40, the g9/2 − d5/2 spacing at N =40, the 2p SO splitting, as well as the 1g and 1f SO splittings.
References:
[1] J.P. Schiffer et al., Phys. Rev. Lett. 108, 022501 (2012).
[2]R. Taniuchi et al., Nature 569, 53–58 (2019).
[3]O. Sorlin and M.G. Porquet, Phys. Scr. T152, 014003 (2013).
