Narrow Results

A dynamical model is employed to determine some peculiar properties of the scission neutrons, i.e., those emitted at the rupture of the neck between the fragments during low energy nuclear fission. Thus, by a specially adapted procedure, we calculate the time-dependent decay rate. This, defines the character of the emission process: pulsed or exponentially. From the time dependence of the survival probability, we deduced the “half-life” of the scission neutron emission. The angular distribution of the scission neutrons with respect to the fission axis is calculated as well. This is obtained for sets of neutron wave functions defined by a given quantum number $\mathrm{\Omega }$ (projection of the total angular momentum on the symmetry axis). A strong dependence on $\mathrm{\Omega }$ was found, namely, wave functions with different $\mathrm{\Omega }$ have different most probable emission angles: from emission along the fission axis (1/2) to emission perpendicular to the fission axis (9/2). This result leads to a new interpretation of the measured angular distribution of the prompt fission neutrons. Finally, an extension of the dynamical model (i.e., with a time-dependent potential also after scission) is presented.

A data bank of negative and positive (80–1665MeV) pion-induced experimental fission cross-sections of heavy nuclei from ¹¹⁹Sn to ²³⁸U is compiled using present results and published data from the literature. Corresponding calculations of fission cross-sections, using the cascade exciton model (CEM) are also included in the compilation. Fission cross-sections compiled in the data bank are examined critically. Mass and energy dependences of fission cross-sections are analyzed. Fission cross-sections of ²³⁸U targets are the highest and scale down approximately, for other targets studied, with fissility, $\frac{{(Z\pm 1)}^2}{A}$. In the fissility expression, $Z$ and $A$ are atomic and mass numbers of the target while $\pm 1$ refer to positive and negative pion projectiles. The presented data bank is of interest for students and researchers involved in the investigation of energetic light particle-induced fission of heavy nuclei. Nuclear fission of heavy nuclei has been classified into three regimes. Phenomenological discussion of the fission process is also given.

In this paper, we employ the density-dependent relativistic mean-field theory to study how the triaxiality and pairing interaction affect the inner fission barriers of actinide nuclei. It was found that the triaxiality reduced the inner barriers and improved agreement with experimental values for many actinides. However, about 1–2 MeV discrepancy to the experimental values still remained for some of the considered nuclei. Such a discrepancy could be made further smaller by increasing the BCS pairing strength parameter. In this work, we demonstrated that adjusting the paring strength was effective in reproducing the experimental inner fission barriers as well as “pairing rotational energy” and binding energy in a consistent manner for nuclei where the effect of the triaxiality on the inner barriers was significant.

The mass and total kinetic energy distributions of the fission fragments in the fission of even-even isotopes of superheavy elements from Hs (Z=108) to Og (Z=118) are estimated using a pre-scission point model. We restrict to nuclei for which spontaneous fission has been experimentally observed. The potential energy surfaces are calculated with Strutinsky’s shell correction procedure. The parametrization of the nuclear shapes is based on Cassini ovals. For the just before scission configuration we fix α=0.98 [12], what corresponds to r_neck ≈ 2 fm, and take into account another four deformation parameters: α₁, α₃, α₄, α₆. The fragment-mass distributions are estimated supposing they are due to thermal fluctuations in the mass asymmetry degree of freedom just before scission. The influence of the excitation energy of the fissioning system on these distributions is studied. The distributions of the total kinetic energy (TKE) of the fragments are also calculated (in the point-charge approximation). In Hs, Ds and Cn isotopes a transition from symmetric to asymmetric fission is predicted with increasing neutron number N (at N≈168). Super-symmetric fission occurs at N≈160. When the excitation energy increases from 0 to 30 MeV, the peaks (one or two) of the mass distributions become only slightly wider. The first two moments of the TKE distributions are displayed as a function of the mass number A of the fissioning nucleus. A slow decrease of the average energy and a minimum of the width (at N≈162) is found.

Dissipation of superheavy nuclei is studied by means of TDDFT+ Langevin model. Much attention is paid to the energy dependence of the friction coefficient, identifying the thermodynamic property of nuclear medium for given nucleon numbers, neutronrichness, and energies. In this article, following the preceding work showing a systematics on Z = 92 to 100 nuclei, macroscopic friction coefficients for Z = 120 isotopes are derived from a microscopic framework. The comparison between Z = 92 and Z = 120 cases clarifies the similarity and the difference of fissions between heavy and superheavy nuclei. That is not only the completion of a systematic theoretical database, but also discovering the existence limit of chemical elements. In addition, the knowledge about the obtained fission probability and fission fragment yield are expected to be useful for superheavy synthesis taking place in accelerators.

After more than half a century of research addressing the synthesis and nuclear structure of superheavy nuclei (SHN) a boost for its progress is expected from the advent of new instrumentation. An order of magnitude in beam intensity increase is envisaged to be provided by new powerful accelerators like the new DC280 cyclotron at the SHE factory of FLNR/JINR or the superconducting LINAC at SPIRAL2 of GANIL. In addition new ion-optical installations like the separator-spectrometer set-up S3 with two complementary detection systems, SIRIUS and LEB, will provide a substantial sensitivity increase for decay spectroscopy after separation (DSAS), as well as for alternative and complementary methods like high precision mass measurements and laser spectroscopy. DSAS has proven in the past to be a powerful tool to study the low lying nuclear structure of heavy and superheavy nuclei. Single particle levels and other structure features like K isomerism, being important in the fermium-nobelium region as well as for the route towards spherical shell stabilized SHN, have been investigated almost up to the limit posed by the sensitivity of present-day instrumentation. Precision mass measurements and laser spectroscopy will offer the possibility to study alternative features like binding energies, charge radii and quadrupole moments. At the magnetic spectrometer VAMOS of GANIL with the recently improved mass resolution and the development of Z identification, deep-inelastic reactions like multi-nucleon transfer can be used to reach more neutron-rich nuclei in the region of light actinides, possibly being extended towards higher Z.

Scission and prompt neutrons provide indirect clues of the physical processes at play near the scission point when the two fission fragments are finally and forever separated. A scission model based on the approximation of a sudden rupture of the neck between the two nascent fragments is presented. It is used to compute the average number of scission neutrons per fission event in the symmetric fission of ²³⁶U. Once the fragments are fully accelerated, they will release their intrinsic excitation energy by emitting so-called prompt neutrons and gamma-rays. Monte Carlo simulations of this evaporation process are presented in the case of the first-chance fission of ²³⁵U. Future developments expected in the arenas of experiment, theory and evaluation are discussed.

Structure properties of many even-even actinides have been calculated using the Gogny D1S force and the Hartree-Fock-Bogoliubov approach as well as the configuration mixing method. Results on rotational states, shape isomers and fission barriers are here discussed.

In order to advance our understanding of the fission process we need to measure changes in fission product yields as a function of excitation energy of the fissioning system, as well as study the correlations between kinetic energy, mass and charge of the products. In addition, fission product yields are used for diagnostics in nuclear technology and there is therefore interest in reducing the uncertainties in their yield.

The SPIDER instrument is based on the 2v-2E technique for measuring the mass of fission products, and has been used to study spontaneous fission of ²⁵²Cf, as well as thermal neutron-induced fission of ²³³U, ²³⁵U, and ²³⁹Pu. The current instrument has two spectrometer “arms”, which provides sufficient detection efficiency for studying fission at thermal neutron energies. In order to study the change in fission yields at fast energies higher detection efficiency is needed, and a new version of SPIDER is therefore under construction. The new instrument will have a total of 16 individual arms, bringing the total solid angular coverage to about 1%.

We present a consistent framework for treating the energy and angular-momentum dependence of the nuclear shape evolution in the fission process. It combines microscopically calculated level densities with the Metropolis walk method and it contains no new parameters. The treatment can elucidate the influence on the shape dynamics of warm nuclei of pairing and shell effects as a function of the excitation energy.

In this work we study a stabilization of internal cluster structure of the nuclear system under the fluctuations of the asymmetry value used as a constraint in the shell correction calculations. Persistent cluster magic structures play a main role in origination of various fission modes. We get quantitative results for clustering in theoretical description of actinide multimodal fission by means of analyzing the high dimensional potential energy surface’s topological complexity. It is shown that traditional adiabatic description is insufficient in the regions of the deformation characterized by the strong internal shell structures rearrangement and therefore it is necessary to take nonequilibrium effects of reclusterization into account.

It was recently found that remarkably accurate fission-fragment mass distributions can be obtained by treating the nuclear shape evolution as a Metropolis walk on previously calculated five-dimensional potential-energy surfaces; this novel method is briefly reviewed here.

Fission fragments play an important role in nuclear reactors evolution and safety. However, fragments yields are poorly known : data are essentially limited to mass yields from thermal neutron-induced fissions on a very few nuclei. SOFIA (Study On FIssion with Aladin) is an innovative experimental program on nuclear fission carried out at the GSI facility, which aims at providing isotopic yields on a broad range of fissioning systems. Relativistic secondary beams of actinides and pre-actinides are selected by the Fragment Separator (FRS) and their fission is triggered by electromagnetic interaction. The resulting excitation energy is comparable to the result of an interaction with a low-energy neutron, thus leading to useful data for reactor simulations. For the first time ever, both fission fragments are completely identified in charge and mass in a new recoil spectrometer, allowing for precise yields measurements. The yield of prompt neutrons can then be deduced, and the fission mechanism can be ascribed, providing new constraints for fission models. During the first experiment, all the technical challenges were matched : we have thus set new experimental standards in the measurements of relativistic heavy ions (time of flight, position, energy loss).This communication presents a first series of results obtained on the fission of 238U; many other fissioning systems have also been measured and are being analyzed presently. A second SOFIA experiment is planned in September 2014, and will be focused on the measurement of the fission of ²³⁶U, the analog of ²³⁵U+n.