Population of rotational bands in superheavy nuclei

Abstract

Using the statistical approach, we study the population of ground-state rota- tional bands of superheavy nuclei produced in the fusion-evaporation reactions 208 Pb( 48 Ca, 2n) 254 No, 206 Pb( 48 Ca, 2n) 252 No, and 204 Hg( 48 Ca, 2n) 250 Fm. We calculate relative intensi- ties of E2-transitions between the rotational states and entry spin distributions of the resid- ual nuclei, evaporation residue cross sections, and excitation functions for these reactions. Fermi-gas model is used for the calculation of level density, and damping of shell effects both with excitation energy and angular momentum is taking into account. The results are in a good agreement with the experiment data. theoretical approaches which predict the next doubly magic shell closure beyond 208 Pb. Reaching the region of superheavy nuclei in the vicinity of this closure, so called island of stability, is a challenge for the modern nuclear physics. The macroscopic-microscopic approaches predict the magic nucleus with Z=114 (1). The closed shell Z = 114 disappears in the framework of self-consistent mean-field models (2-6) which result the strong shell effects at Z=120-126. Although all these models provide close binding energies for nuclei with Z �114, their predictions for the spins of the ground states, quasiparticle spectra, and for the moments of inertia are rather different. Therefore, one can test the theoretical models by comparing their structure predictions with the spectroscopical data. Experimental study of rotational bands of superheavies is especially hopeful, as the limits of sta- bility of these nuclei in spin and excitation energy are governed by the fission barrier, which is mostly defined by shell component. The investigation of high-spin spectra also provides information about angular momentum dependence of the shell effects. Unfortunately, there are still a little experimental data for the heaviest evaporation residues, as their production cross sections are too small for the spec- troscopical analysis. However, using the in-beam studies, the structure of high-spin excited states up to � 20~ has been recently identified for the nuclei 254 No, 252 No, and 250 Fm, produced with the cross sections 0.2-3 µb (7-10). The nuclei studied are produced in fusion-evaporation reactions, which can be successfully de- scribed using the statistical approach. In Refs. (11-14) we have already calculated evaporation residue cross sections and excitation functions for superheavy isotopes using their properties, predicted by different theoretical models (15-18). The results are in a good agreement with the experimental data. In the present paper, we extend the statistical approach to describe the population of yrast rotational states in the residual superheavy nuclei and the production of evaporation residues in the complete fusion reactions 206,208 Pb( 48 Ca, 2n) 252,254 No and 204 Hg( 48 Ca, 2n) 250 Fm, taking into consideration the survival of compound nuclei against fission at di fferent angular momenta. The relative intensities of E2-transitions and entry spin distributions of the residual nuclei can be derived from the calculated population cross sections of the rotational states and compared with the experimental data (7-10).