Structure of superheavy nuclei

A short review of the results on the structure of superheavy nuclei is presented.


Introduction
Superheavy nuclei form a new region of the nuclide chart whose structure is only started to be investigated. It is interesting if their structure is similar to that of nuclei which have been already studied or unexpected features will be discovered. The specific feature of atomic nuclei is a self consistent mean field whose characteristics determine a shape of atomic nucleus, its excitation spectra and decay modes. The following theoretical approaches are used to determine the properties of self consistent mean field: the microscopic-macroscopic method and the self consistent mean field models (relativistic or non-relativistic) based on nuclear energy density functional. Both approaches are phenomenological, however, in the second one the nuclear mean field is consistently determined with a good description of the global nuclear properties.

Shell effects
We use in our calculations the two-center shell model potential (TCSM) which is useful for the description of nuclear structure and reactions [1]. The parameters of the nuclear average mean field potential were set to describe the spins and parities of known rare earth, actinides and superheavy nuclei [2]. Weak dependence on (N-Z) was incorporated in the momentum dependent part of the single particle Hamiltonian. The ground state spins of nuclei with N=147-161 are presented in Tables I and II. One can see a good description up to Sg isotopes.
The information on the proton magic number after Z=82 is contained also in the spectra of the proton two-quasiparticle states. While for nuclei with Z ≤ 118 the calculated energies of the first proton two-quasiparticle states are smaller than 1.2 MeV, in 296;298 120 the calculated energies of the first proton two-quasiparticle states are above 1.9 MeV (Figure 1) [3]. This indicates a large gap in the proton single-particle spectrum.
The measurements of the α -decay spectra of superheavy nuclei can give us an information on the isomers. Around 250 Fm one can find both protons and neutrons pairs of levels close to the Fermi level that can be coupled to low-lying states with high K, such as (9/2 − [734] × 7/2 + [624]) n 8 − for neutrons, or (9/2 + [624] × 7/2 − [514]) p 8 − and (7/2 − [514] × 7/2 + [633]) p 8 − for proton pairs. The lowest calculated states in the N=155 isotonic chain have ∆ K ≥ 3. Therefore, the longliving isomeric state is expected in N=155 isotones ( Figure 2) [4]. If the energy of this isomer is small, its lifetime is long enough and the α -decay could occur from this isomer. The calculated ground state spin changes in 257 No when the value of β 4 decreases. Therefore, a small change of deformation could cause the inversion of the levels 1/2 + and 7/2 + which are close in energy in dense spectrum.
The lowest calculated states in 263 Sg and 265 Hs have ∆ K ≥ 4. So, the first excited states in 263 Sg and 265 Hs can be related to the observed states at 130 keV and above 300 keV, respectively. The lowest 3/2 + state can be isomeric in 257 Fm, 259 No, and 261 Rf (Figure 3) [4] .

α -decay
In 285 Fl the single-particle state 9/2 [604] could be the isomeric one from which α -decay can occur with T α ≈ 3 ms. The α -decay of 277

Phase transition
Phase transition phenomenon, especially, shape phase transition is a very interesting subject of investigations which became widespread in nuclei removed out of the valley of stability. It is interesting to look for the possible manifestation of this phenomenon in superheavy nuclei. There are several known indicators of the phase transitions. One of them is a level density parameter. In Figure 4 the calculated proton a Z and neutron a N level density parameters are shown for superheavy nuclei belonging to the α -decay chains containing 296,298,300 120 isotopes, calculated using two-center shell model [6].
As seen, a Z has a minimum at Z=120 and a N has a minimum at N=184. This indicates Z=120 and N=184 as a possible proton and neutron magic numbers, respectively. The proton level density parameter has a maximum at Z=112 indicating Z=112 as a transition point from deformed to spherical nuclei. Figure 4. Calculated neutron (a N ) and proton (a Z ) level density parameters as a function of neutron number N (upper part) and proton number Z (lower part). The nuclei from alpha-decay chains containing 296 120, 298 120, 300 120 are marked by closed circles, open circles, and stars, respectively.