Narrow Results

Cross-sections for the π^--induced fission of ²⁰⁹Bi and ¹¹⁹Sn have been measured using the most sensitive CR-39 solid-state nuclear track detector. In experiments, target–detector stacks were exposed to negative pions of energy 500, 672, 1068, and 1665 MeV at the Brookhaven National Laboratory, USA. An important aspect of the present paper is the comparison of pion-induced fission fragment spectra of above mentioned nuclei with the spontaneous fission fragment spectra of ²⁵²Cf. This comparison is made in terms of fission fragment track lengths in the CR-39 detectors. Measurement results are compared with calculations of Monte Carlo and statistical weight functions methods using the computer code CEM95. Agreement between measurements and calculations is fairly good for ²⁰⁹Bi target nuclei whereas it is indigent for the case of ¹¹⁹Sn. The possibilities of the trustworthy calculations, using the computer code CEM95, comparable with measurements of pion-induced fission in intermediate and heavy nuclei are explored by employing various systematics available in the code. Energy dependence of pion-induced fission in ¹¹⁹Sn and ²⁰⁹Bi is analyzed employing a newly defined parameter geometric-size-normalized fission cross-section . It is found that the collective nuclear excitations, which may lead to fission, become more probable for both ²⁰⁹Bi and ¹¹⁹Sn nuclei with increasing energy of negative pions from 500 to 1665 MeV.

Accurate information about the fission barrier is important for studying of the fission process. Fission barrier is needed for discovering the island of stability in superheavy region and searching of the superheavy elements. Furthermore, the astrophysical r-process is closely related to the fission barrier of the neutron-rich nuclei. In this study, by using artificial neural network (ANN) method, we have estimated the fission barrier heights of the Rf, Db, Ra and Ac nuclei covering 230 isotopes. For inner barrier calculation, we have used Rf and Db nuclei and the barrier heights have been determined between nearly 1 MeV and 7 MeV. The related mean square error value has been obtained as 0.108 MeV. For outer barrier calculation, we have used Ra and Ac nuclei and the heights have been determined between nearly 8 MeV and 28 MeV. The related mean square error has been obtained as 0.407. The results of this study indicate that ANN is capable for the estimations of inner and outer fission barrier heights.

In this paper, we apply the direct variational method to derive the nuclear deformation energy. The extended Thomas–Fermi approximation (ETFA) for the energy functional with Skyrme forces is used. We study the influence of the finite surface layer of the nuclear density profile function on the formation of the fission barrier and the scission configuration. Comparison of the variational approach with the traditional liquid drop model (LDM) is presented. We show the sensitivity of the numerical results to the surface diffuseness parameter.

Fission barrier of the heavy nucleus ${}^{250}$Cf is analyzed in a multi-dimensional deformation space. This space includes two quadrupole (${\epsilon }_2,\gamma$) and three hexadecapole deformation (${\epsilon }_{40},{\epsilon }_{42},{\epsilon }_{44}$) parameters. The analysis is performed within an unpaired macroscopic–microscopic approach. Special attention is given to the effects of the axial and non-axial hexadecapole deformation shapes. It is found that the inclusion of the nonaxial hexadecapole shapes does not change the fission barrier heights, so it should be sufficient to minimize the energy in only one degree of freedom in the hexadecapole space ${\epsilon }_4$. The role of hexadecapole deformation parameters is also discussed on the Lublin–Strasbourg drop (LSD) macroscopic and the Strutinsky shell energies.

The multidimensionally-constrained covariant density functional theories (MDC-CDFTs) have been developed to study the influence of octupole and triaxial deformations on the ground state and fission properties. In this paper, we present a brief review of the applications of MDC-CDFTs and discuss the results of a systematical study of even-$A$ uranium isotopes with the multidimensionally-constrained relativistic mean field (MDC-RMF) model which is one of the MDC-CDFTs with pairing correlations treated by using the Bardeen-Cooper-Schrieffer (BCS) approach. We examine in detail the two-dimensional potential energy surfaces $E({\beta }_{20},{\beta }_{30})$ of these U isotopes and discuss the ground state and fission properties as well as the third and fourth minima on the potential energy surfaces. The emphasis is put on the effects of octupole and triaxial deformations.

The study of superheavy nuclei (SHN) is on the frontier of modern nuclear physics. In recent years, we have carried out theoretical investigations of both the structure properties and the synthesis mechanism of SHN. In this contribution, we briefly review these progresses and focus on the study of potential energy surfaces and fission barriers of actinide nuclei by using the MDC-RMF model and that of the fusion mechanism by using the ImQMD model.

Many shape degrees of freedom play crucial roles in determining fission barriers. In multidimensionally constrained covariant density functional theories (MDC-CDFTs) which we have developed in recent years, both the axial and the spatial reflection symmetries are broken and all deformations described by β_λμ with even μ, including β₂₀, β₂₂, β₃₀, β₃₂, β₄₀, etc., are considered self-consistently. The MDC-CDFTs have been applied to the study of fission barriers and potential energy surfaces of actinide nuclei, third minima in potential energy surfaces of light actinides, shapes and potential energy surfaces of superheavy nuclei, the Y₃₂ correlations in N = 150 isotones and Zr isotopes, and the shape of hypernuclei. In this contribution we will present an introduction of the MDC-CDFTs and a short review of its applications on fission barriers.

The multidimensionally-constrained covariant density functional theories (MDC-CDFTs) have been developed to study the influence of octupole and triaxial deformations on the ground state and fission properties. In this paper, we present a brief review of the applications of MDC-CDFTs and discuss the results of a systematical study of even-A uranium isotopes with the multidimensionally-constrained relativistic mean field (MDC-RMF) model which is one of the MDC-CDFTs with pairing correlations treated by using the Bardeen-Cooper-Schrieffer (BCS) approach. We examine in detail the two-dimensional potential energy surfaces E(β₂₀; (β₃₀) of these U isotopes and discuss the ground state and fission properties as well as the third and fourth minima on the potential energy surfaces. The emphasis is put on the effects of octupole and triaxial deformations.