Narrow Results

Recent progress in the study of fission barriers of actinides and superheavy nuclei within covariant density functional theory is overviewed.

Two systematic sources of error in most current microscopic evaluations of fission-barrier heights are studied. They are concerned with an approximate treatment of the Coulomb exchange terms (known as the Slater approximation) in the self-consistent mean-fields and the projection on good parity states (e.g., of positive parity for the spontaneous fission of an even–even nucleus) of left–right reflection asymmetric intrinsic solutions (e.g., around the second barrier). Approximate or unprojected solutions are shown to lead each to an underestimation of the barrier heights by a few hundred keV.

In this paper, a simple semi-empirical formula is proposed for angular momentum-dependent fission barriers of actinides $(89\le Z\le 103)$. This formula produces angular momentum-dependent fission barriers with the simple inputs of atomic number (Z), mass number (A) and angular momentum ($\ell$). The fission barriers produced by the present formula are compared with that of experiments and other models. There is a good agreement between the present formula and experiments. The present formula could be used to evaluate the angular momentum-dependent fission barriers of actinides.

Quantitative predictions of fission product yields are relevant for the reliable operation of different modern nuclear applications. This concerns the realistic characterizations of the radio-toxicity of the fuel elements after the envisaged extended irradiation, as well as sub-critical assemblies, where the number of delayed neutrons from minor actinides is determined by the characteristic emission yields of the corresponding so-called pre-cursor isotopes. However, to be able to make more reliable quantitative predictions of fission characteristics requires the better understanding of the fission process itself. For this purpose a better knowledge about the distinct structure of the nuclear energy landscape around the fission barrier is indispensable. In particular, the question should be answered, whether the fission barrier is either double- or triple-humped or even multi-humped as been proposed within the multi-modal neck rupture model. Despite quite some effort based on different experimental techniques and theoretical approaches, this question remains still unanswered. There is still no consistent picture of the fission barrier available and hence, different sets of barrier parameters are in use, unable to describe the different observed phenomena in a coherent way. With the systematic investigation of shape isomer population, its decay modes as well as the branching ratio, precise information can be obtained to resolve the puzzling situation. The experimental approach will be discussed and results from first experiments presented.

Fission excitation functions have been measured for a chain of neighboring compound nuclei from ²⁰⁷Po to ²¹²Po. We present a new analysis which provides a determination of the fission barriers and ground state shell effects with nearly spectroscopic accuracy. The accuracy achieved in this analysis may lead to a future detailed exploration of the saddle mass surface and its spectroscopy.

Results are presented and discussed from an experiment searching for element 120. The reaction ⁵⁴Cr + ²⁴⁸Cm → ³⁰²120* was investigated at the velocity filter SHIP at GSI in Darmstadt. In a complementary study, α-decay chains of isotopes of known even elements were reviewed in order to prepare a comprehensive basis for comparison of experimental data among each other and with results of model calculations. Model dependent shell-correction energies and related heights of fission barriers were deduced from measured Q_α values and compared with results of macroscopic-microscopic models. The consequences for calculations of cross-sections are discussed.

After a successful production of isotopes of element 116 in the reaction ⁴⁸Ca + ²⁴⁸Cm, the reaction ⁵⁴Cr + ²⁴⁸Cm → ³⁰²120* was investigated at the velocity filter SHIP at GSI, Darmstadt, aiming to search for isotopes of element 120. One chain of events was observed, which is not created by chance with high probability. Parts of the chain agree with model predictions and measured data from a decay chain which could start at the isotope ²⁹⁹120. In a complementary study, model dependent shell-correction energies and related heights of fission barriers were deduced from measured Q_α values. The results are compared with predictions of macroscopic-microscopic models. The consequences for calculations of crosssections are discussed.

The current status of the application of covariant density functional theory to the description of fission barriers in actinides and superheavy nuclei is reviewed. The achievements and open problems are discussed.

We evaluate the performance of modern nuclear energy density functionals for predicting inner and outer fission barrier heights and energies of fission isomers of even-even actinides. For isomer energies and outer barrier heights, we find that the self-consistent theory at the HFB level is capable of providing quantitative agreement with empirical data.

Substantial information on nuclear shells which determine the stability of superheavy nuclei, can be achieved from the measurement of decay properties of isotopes of element 120. Using existing data measured up to elements 116 and 118 at FLNR in Dubna, estimates of shell-correction energies and fission barriers are deduced and compared with model calculations. The results indicate that the probability for re-separation of target and projectile like nuclei in the entrance channel of the reaction is less then assumed in some of the existing cross-section calculations. This finding together with technical reasons favors the reaction ⁵⁴Cr + ²⁴⁸Cm for synthesis of element 120. A first part of the experiment was already performed in 2011. The results will be presented in a forthcoming paper.