Narrow Results

The experimental data on the capture and evaporation residue cross-sections obtained in the ⁴⁸Ca+²⁰⁸Pb reaction were analyzed in the framework of the dynamical model based on the dinuclear system concept and advanced statistical method to clarify the reaction mechanism. The experimental excitation function of the capture reactions was decomposed into contributions of the fusion–fission, quasifission and fast-fission processes. Total evaporation residues and ones after neutron emission were only calculated and compared with the available experimental data.

The small cross-section of the synthesis of the superheavy elements (SHEs) is caused by the hindrance to complete fusion and small survival probability of the compound nucleus against fission. The hindrance to complete fusion is related with such peculiarities of the entrance channel as the mutual orientation angles of the axial symmetry of the colliding deformed nuclei, their mass (charge) asymmetry, the shell effects in their microscopic structure and orbital angular momentum. The appearance of the hindrance to complete fusion has been analyzed in the framework of the dinuclear system (DNS) model by the description of the experimental data obtained in the reactions of the SHE synthesis.

A nuclear molecule or a dinuclear system consists of two touching nuclei which carry out motion in the internuclear distance and exchange nucleons by transfer. The dinuclear system model can be applied to nuclear structure, fusion reactions leading to superheavy nuclei, multi-nucleon transfer and fission.

Two measurements of P_CN, the fusion probability, are described. In the first measurement, the value of P_CN was deduced for a typical cold fusion reaction, the ⁵⁰Ti + ²⁰⁸Pb reaction, by analysis of the fission fragment angular distributions. In the second measurement, P_CN was deduced, using the DNS model, from experimental measurements of the capture and EVR cross sections for the ¹²⁴Sn + ⁹⁶Zr reaction. Comparison of the deduced values of P_CN with various theoretical models of the synthesis of the heaviest nuclei are made.

Mass-energy distributions and capture cross sections have been measured in the reactions ⁴⁸Ca + ²⁰⁸Pb, ²³²Th, ²³⁸U, ²⁴⁴Pu, and ²⁴⁸Cm at energies around the Coulomb barrier. The properties of the fusion-fission and quasifission processes were studied in dependence on the interaction energies and ion-target combinations. In the reaction ⁴⁸Ca+²⁰⁸Pb the fusion-fission process is dominant. In the reactions of 48Ca with actinide targets the asymmetric quasifission becomes the dominant process. Nevertheless, the analysis of mass and energy distributions has shown that a significant part of symmetric fragments in the range ACN/2 ± 20 u may be attributed to fusion-fission process for the reactions ⁴⁸Ca+²³⁸U, ²⁴⁴Pu, and ²⁴⁸Cm. Using this analysis we estimated the fusion-fission crosssections σ_FF, the survival probabilities W_sur and the lower limits for fission barriers of ^254-256No, ^283-286Cn, ^289-292Fl, and ^293-296Lv compound nuclei.

The properties of the mass and energy distributions of fissionlike fragments formed in the reactions ³⁶S, ⁴⁸Ca, ⁴⁸Ti, ⁶⁴Ni + ²³⁸U at energies around the Coulomb barrier have been analyzed to define the systematic trend of compound nucleus fission and quasifission in hot fusion reactions with actinide targets. The measurements have been carried out at the U400 cyclotron of the FLNR, JINR using the double-arm time-of-flight spectrometer CORSET. The most probable fragment masses as well as total kinetic energies and their dispersions in dependence on the interaction energies have been investigated for asymmetric and symmetric fragments for the studied reactions. The fusion probabilities have been deduced from the analysis of mass and energy distributions. It was found that for the studied reactions fusion probability depends exponentially on mean fissility parameter of the system. For the reactions with actinide nuclei leading to the formation of superheavy elements the fusion probabilities are of several orders of magnitude higher than in the case of cold fusion reactions.

The path to the synthesis of super heavy elements is strongly hindered by the competition between fusion and quasi-fission in heavy ions reactions at near barrier energies. We report on a recent experimental investigation performed at the ALTO facility of IPN Orsay, France on the heavy-ion collision around the Coulomb barrier of ³²S+¹⁹⁷Au. The fission-fragment mass and total kinetic energy were measured by the double arm mass spectrometer for binary fragments CORSET, using the double time of flight approach. The asymmetric structure on top of the predominant symmetric one may suggests the contribution from either quasi-fission or pre-equilibrium fission or might be driven by shell effects in the compound nucleus fission. To get further insight on such puzzling cases situated at the crossroads of the various opened channels, we extended the measurement to coincident prompt γ -rays, of low and high energy. For this purpose, the prompt γ-rays following fission were detected by the coupled high efficiency ORGAM array consisting of 10 Compton suppressed Germanium detectors (BGO shielded), located at backward angles and the first cluster of the future PARIS calorimeter consisting of 9 close packed phoswich detectors and another single phoswich, together covering a wide angular and energy range.

The properties of the mass and energy distributions of fissionlike fragments formed in the reactions ⁴⁸Ca,⁵⁸Fe + ²⁰⁸Pb, ³⁶S,⁴⁸Ca,⁴⁸Ti,⁶⁴Ni + ²³⁸U, ⁴⁸Ca + ²³²Th,²⁴⁴Pu,²⁴⁸Cm at energies around the Coulomb barrier have been analyzed to define the systematic trend of compound nucleus fission and quasifission in cold and hot fusion reactions. The measurements have been carried out at the U400 cyclotron of the FLNR, JINR using the double-arm time-of-flight spectrometer CORSET. The fusion probabilities have been deduced from the analysis of mass and energy distributions. It was found that for the studied reactions fusion probability depends exponentially on mean fissility parameter of the system. For the reactions with actinide nuclei leading to the formation of superheavy elements the fusion probabilities are of several orders of magnitude higher than in the case of cold fusion reactions.

We show that the microscopic TDHF approach provides an important tool to shed some light on the nuclear dynamics leading to the formation of superheavy elements. In particular, we discuss studying quasifission dynamics and calculating ingredients for compound nucleus formation probability calculations.

Superheavy elements are primarily formed through heavy ion fusion reactions. Formation of a fully equilibrated compound nucleus is a critical step in this reaction mechanism but can be hindered by orders of magnitude by quasifission, a process in which the dinuclear system breaks apart prior to full equilibration. To provide a complete description of heavy-ion fusion it is important to characterize the quasifission process. The interplay between the fusion-fission and quasifission reaction channels was explored by measuring fission mass distributions in eight different combinations of Cr+W reactions, with varying neutron-richness, at the Australian National University. The reactions were measured in two energy regimes: one at 13% above the Bass fusion barrier and one at 52.0 MeV of excitation energy in the compound nucleus, ${\text{E}}_{\text{CN}}^{*}$. For the systems measured at E_c.m./ V_Bass = 1.13 the dependence on the neutron-richness is clear. However, for the reactions at ${\text{E}}_{\text{CN}}^{*}$ = 52.0 MeV, the dependence is less clear and additional factors are shown to play a vital role, especially the influence of deformation on the effective fusion barrier. The present work demonstrates that quasifission is an important process in competition with heavy-ion fusion in reactions with intermediate mass projectiles, particularly with more neutron-rich systems.

The main aim of the present study is to evaluate the fusion probabilities and investigate competing quasifission process in the reactions with heavy ions leading to the formation of superheavy composite systems. The mass-energy distributions, as well as capture cross-sections of fission-like fragments for the reactions of ²²Ne, ²⁶Mg, ³⁶S, ⁴⁸Ca, ⁵⁸Fe and ⁶⁴Ni ions with actinides leading to the formation of superheavy compound systems with Z=102-120 at energies near the Coulomb barrier have been measured. The relative contribution of quasifission to the capture cross section becomes dominant for superheavy composite systems. Fusion-fission cross sections were estimated from the analysis of mass and total kinetic energy distributions.