Nuclei near closed shells N = 20 and N = 28
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ei near Closed Shells N = 20 and N = 28 Yu. E. Penionzhkevich and S. M. Luk’yanov PACS numbers: 27.30.+t; 21.10.–k; 21.10.Dr; 21.10.Ft; 21.10.Gv DOI: 10.1134/S1063779606020031
INTRODUCTION In the previous reviews [1–3], we described in detail the properties of neutron-rich nuclei (from helium up to oxygen isotopes), which are far from the stability line. We consider this nuclear region as the most interesting because, namely here, the neutron-rich bound nuclei are recently obtained, and their properties are investigated. Meanwhile, experimental investigations of heavier nuclei in the region of closed neutron shells N = 20 and N = 28 show that their properties (binding energy and deformation) strongly vary as they move away from the stability line. In addition, new effects manifest themselves, because of the change (decrease or increase) in the stability of these nuclei near the boundaries of nucleon stability. The found effects result in the essentiality to revise the theoretical concepts about the properties of such nuclei, also including the occurrence of new magic numbers characteristic for the shell effects. As early as 1975, Thibault et al. [4] found the coexistence of two types of deformation in the magic nucleus 31Na. Within the framework of the shell model, the deformation of the ground state in this nucleus was explained by the strong correlation between the 2p–2n excitation energies for the sd and pf shells. This fact manifests itself by increasing the binding energy of Na isotopes near the shell N = 20. Subsequent investigations showed that similar effects are also observed for neutron-rich O, F, Ne, Na, and Mg isotopes. For the majority of light nuclei, the so-called coexistence of two shapes (spherical and deformed) manifests itself in the ground state near the shells N = 20 and N = 28. This resulted in revising the existing shell models for the prediction of the light nuclei stability line and in the occurrence of new shell numbers N = 16 and 26. This problem is important for the future development of our concepts about nuclear-matter properties for large isospin (exotic nuclei), therefore we tried to systematize the currently available information on the properties of neutron-rich nuclei near the boundaries of nucleon stability, close to the shell numbers N = 20 and N = 28.
PHENOMENOLOGY OF CLOSED SHELLS AND MATTER DISTRIBUTION The study of nuclear density, using the scattering of electrons and x rays (emitted from muonic atoms), on stable nuclei showed that nucleons are distributed in a nuclear volume with a density that is constant in the center, decreasing as it approaches the nuclear boundary (Fig. 1). In light stable nuclei (N = Z), nucleons are distributed in a nuclear volume with the radius Rc = 1.08A1/3 fm (A corresponds to the number of nucleons, ρp(r) ≈ ρn(r) are the proton and neutron densities, respectively, and the central nucleon density is ρ0 = 0.17 nucleon/fm3. To compensate the repulsive interaction in heavy stable nuclei, the number of neutrons exceeds that of protons, leading to th
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