Evolution of shell structure in exotic nuclei

Abstract

The atomic nucleus is a quantum many-body system whose constituent nucleons (protons and neutrons) are subject to complex nucleon-nucleon interactions that include spin-and isospin-dependent components. For stable nuclei, already several decades ago, emerging seemingly regular patterns in some observables could be described successfully within a shell-model picture that results in particularly stable nuclei at certain magic fillings of the shells with protons and/or neutrons: N,Z = 8, 20, 28, 50, 82, 126. However, in short-lived, so-called exotic nuclei or rare isotopes, characterized by a large N/Z asymmetry and located far away from the valley of beta stability on the nuclear chart, these magic numbers, viewed through observables, were shown to change. These changes in the regime of exotic nuclei offer an unprecedented view at the roles of the various components of the nuclear force when theoretical descriptions are confronted with experimental data on exotic nuclei where certain effects are enhanced. This article reviews the driving forces behind shell evolution from a theoretical point of view and connects this to experimental signatures.

A nuclear periodic table

“…1; Garcia-Ramos and Heyde 2019), and we also include it in the same column. Even though the magic numbers may change in neutron-rich nuclei when the number of neutrons is much larger than the number of protons (Steppenbeck 2013;Otsuka 2020), in this paper we consider only those nuclei which are close to the beta-stability line and do not consider such effect. Though the heaviest element discovered so far is Oganessson (Z=118), the proton magic number after Z=82 is currently unknown.…”

Roles of Nuclear Forces on the Shell Evolutions Toward Driplines

“…(a) Neutron effective single-particle energies (ESPEs) for N=8 isotones obtained by using monopoles of SFO [9] and PSDMK2 [12]. (b) Energy levels of 12 Be obtained by shell-model calculations with SFO (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16) and PSDMK2 as well as experimental values [14].…”

“…We show in Fig. 1(b) the case for SFO (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16), where the p-shell part is taken to be TBE(6-16) interaction instead of POT(8-16) interaction [5]. Calculated B(E2) value is 8.1 fm 4 for the effective charges e p =1.2 and e n =0.3, while the experimental value is 6.4-7.6 fm 4 [14].…”

Realistic shell model and nuclei around 132Sn

“…In the light-and medium-mass regions, structural changes have been evidenced for nuclei with a large excess of neutrons, leading to the breakdown of the traditional magic numbers and the appearance of new ones. These findings have driven a great theoretical effort to understand the microscopic mechanism underlying the shell evolution, with special attention to the role of the different components of the nuclear force (see, for instance, [5]). It is of great interest to verify if peculiar properties, as those observed in lighter nuclei, may be also observed in the 132 Sn region.…”