J. Dudek

The 48Ti on 40Ca reactions have been studied at 300 and 600 MeV focusing on the fusion-evaporation (FE) and fusion-fission (FF) exit channels. Energy spectra and multiplicities of the emitted light charged particles have been compared to Monte Carlo simulations based on the statistical model.Indeed, in this mass region (A ~ 100) models predict that shape transitions can occur at high spin values and relatively scarce data exist in the literature about coincidence measurements between evaporation residues and light charged particles. Signals of shape transitions can be found in the variations of the lineshape of high energy gamma rays emitted from the de-excitation of GDR states gated on different region of angular momenta. For this purpose it is important to keep under control the FE and FF processes, to regulate the statistical model parameters and to control the onset of possible pre-equilibrium emissions from 300 to 600 MeV bombarding energy.

The nuclear mean-field theory and the group representation theory can be used to optimise the search for strong nuclear shell effects. The two theories allow to correlate the symmetry aspects with the presence of large gaps in the single-particle spectra, facilitate in this way the conditions of search for strong nucleonic- and nuclear-binding and thus for an increased nuclear stability. In this article we give a short overview of the related on-going research, focussing on the results of the TetraNuc Collaboration.

We discuss a point-group-theory based method of searching for new regions of nuclear stability. We illustrate the related strategy with realistic calculations employing the tetrahedral and the octahedral point groups. In particular, several nuclei in the Rare Earth region appear as excellent candidates to study the new mechanism.

A search for the self-consistent solutions for the chiral rotational bands in the N=75 isotones, 130Cs, 132La, 134Pr and 136Pm is performed within the Skyrme-Hartree-Fock cranking approach using SKM* and SLy4 parametrizations. The dependence of the solutions on the time-odd contributions in the energy functional is studied. From among the considered four isotones, self-consistent chiral solutions are obtained only in 132La. The microscopic calculations are compared with the 132La experimental data and with results of a classical model that contains all the mechanisms underlying the chirality of the collective rotational motion. Strong similarities between the HF and classical model results are found. The suggestion formulated earlier by the authors that the chiral rotation cannot exist below a certain critical frequency is further illustrated and discussed, together with the microscopic origin of a transition from the planar to chiral rotation in nuclei. We also formulate the separability rule by which the Tilted-Axis-Cranking solutions can be inferred from three independent Principal-Axis-Cranking solutions corresponding to three different axes of rotation.

Systematic searches for exotic shapes, hyperdeformation (HD) and Jacobi transitions, have been made in Hf, Nd, Ba, Xe, Sn and Cd nuclei during the last 4 years, guided by theoretical predictions. The most promising results showing patterns of rotational correlations (e.g. 2. order ridges of multiple rotational bands) are found for ¹²⁶Xe and ¹²⁶Ba when the very highest multiplicity cascades are selected by various techniques. In particular, the results on ¹²⁶Ba obtained in 3 different experiments, using both Gammasphere in Berkeley and Euroball IV in Strasbourg are discussed and compared to theoretical predictions and to simulations by a double‐potential statistical model for population and γ‐decay. A very surprising result was obtained in the last experiment at Euroball, in a full month running time, namely that the observed ridge structure depends very sensitively on the bombarding energy, which points to entrance channel effects. The analysis shows that a discrete HD yrast band intensity will be significantly less than 1 part in 10⁶ of the strongest bands populated in the reaction.

The high‐energy and discrete γ‐ray spectra, as well as the charged particle angular distribution have been measured in the reaction 105 MeV ¹⁸O+²⁸Si using the EUROBALL IV, HECTOR and EUCLIDES arrays in order to investigate the predicted Jacobi shape transition in light nuclei. A comparison of the GDR line shape data with the predictions of the thermal shape fluctuation model, based on the most recent rotating liquid drop LSD calculations, shows evidence for such Jacobi shape transition in hot, rapidly rotating ⁴⁶Ti. The found narrow low‐energy component in the GDR line shape is interpreted as the consequence both of the elongated shape and of the Coriolis effect.

We recall the main features of the Td (tetrahedral) symmetry in atomic nuclei and present realistic mean-field calculations supporting the existence such a symmetry all over the nuclear chart. A few potential candidate-nuclei are investigated and the possible experimental signatures of the tetrahedral symmetry are also briefly discussed.