Quasifission In Reactions Research Articles

Entrance channel dependence of quasi fission in reactions leading to 206Po compound nucleus

The evaporation residue (ER) cross sections for the reaction F19+187Re→206Po⁎ are measured, in the excitation energy range of 86.1 to 118.4 MeV. The measured cross sections are compared with that of 30Si + 176Yb reaction populating the same compound nucleus. Theoretical calculations are performed using the coupled - channels calculations for the capture cross sections and statistical model calculations for the ER cross sections. The dependence of quasi fission on entrance channel parameters such as charge product (Z1Z2), mass asymmetry (α), target deformation (β2) and effective fissility (χeff) are studied.

Fission and Quasi-Fission in Reactions with Deformed Nuclei

The mass-energy distributions of fragments formed in reactions 16,18O + 232Th, 238U, and 22Ne + 232Th, 238U at energies near the Coulomb barrier are measured to study the role of multimodal fission in the reactions of light ions with strongly deformed actinide nuclei. It is found that at these energies, multimodal fission affects significantly the mass–energy and angular distributions of fragments that results in an increase of the width of mass distributions and large angular anisotropy as in the case of quasifission.

Simulation of fusion and quasi-fission in nuclear reactions leading to production of superheavy elements using the Constrained Molecular Dynamics model

Fusion dynamics and the onset of quasi-fission in reactions, leading to production of superheavy nuclei are investigated using the constrained molecular dynamics model. Constraints on the parameters of the nuclear equation of state are derived from experimental fusion probabilities. The obtained constraint on the modulus of incompressibility of nuclear matter K0=240−260 MeV is consistent with the results of previous study using the Boltzmann-Uehling-Uhlenbeck equation and also with constraints derived using the recent neutron star binary collision event GW170817. Unlike the modulus of incompressibility of symmetric nuclear matter, the stiffness of the density-dependence of symmetry energy influences the fusion probability only weakly.

Fusion and quasifission studies in reactions forming Rn via evaporation residue measurements

Background: Formation of the compound nucleus (CN) is highly suppressed by quasifission in heavy-ion collisions involving massive nuclei. Though considerable progress has been made in the understanding of fusion-fission and quasifission, the exact dependence of fusion probability on various entrance channel variables is not completely clear, which is very important for the synthesis of new heavy and superheavy elements.Purpose: To study the interplay between fusion and quasifission in reactions forming CN in the boundary region where the fusion probability starts to deviate from unity.Methods: Fusion evaporation residue cross sections were measured for the $^{28,30}\mathrm{Si}+^{180}\mathrm{Hf}$ reactions using the Hybrid Recoil Mass Analyser at IUAC, New Delhi. Experimental data were compared with data from other reactions forming the same CN or isotopes of the CN. Theoretical calculations were performed using the dinuclear system and statistical models.Results: Reduced evaporation residue cross sections were observed for the reactions studied compared with the asymmetric reaction forming the same CN, indicating fusion suppression in more symmetric systems. The observations are consistent with fission fragment measurements performed in the same or similar systems. Larger ER cross sections are observed with increase in mass in the isotopic chain of the CN.Conclusions: Fusion probability varies significantly with the entrance channels in reactions forming the same CN. While complete fusion occurs for the $^{16}\mathrm{O}+^{194}\mathrm{Pt}$ reaction, the fusion probability drops to approximately $60--70%$ for the $^{30}\mathrm{Si}+^{180}\mathrm{Hf}$ and less than $20%$ for the $^{50}\mathrm{Ti}+^{160}\mathrm{Gd}$ reactions, respectively, forming the same CN at similar excitation energies.

Evolution of signatures of quasifission in reactions forming curium

Background: Quasifission, a fission-like reaction outcome in which no compound nucleus forms, is an important competitor to fusion in reactions leading to superheavy elements. The precise mechanisms driving the competition between quasifission and fusion are not well understood.Purpose: To understand the influence reaction parameters have on quasifission probabilities, an investigation into the evolution of quasifission signatures as a function of entrance channel parameters is required.Methods: Using the Australian National University's (ANU) CUBE detector for two-body fission studies, measurements were made for a wide range of reactions forming isotopes of curium. Important quasifission signatures---namely, mass-ratio spectra, mass-angle distributions, and angular anisotropies---were extracted.Results: Evidence of quasifission was observed in all reactions, even for those using the lightest projectile (${}^{12}\text{C}+{}^{232}\text{Th}$). But the observables showing evidence of quasifission were not the same for all reactions. In all cases, mass distributions provided some evidence of the possible presence of quasifission but were not sufficient in most cases to clearly identify reactions for which quasifission was important. For reactions using light projectiles (${}^{12}$C, ${}^{28,30}$Si, ${}^{32}$S), experimental angular anisotropies provided the clearest signature of quasifission. For reactions using heavier projectiles (${}^{48}$Ti, ${}^{64}$Ni), the presence of mass-angle correlations in the mass-angle distributions provided strong evidence of quasifission and also provided information about quasifission timescales.Conclusions: The observable offering the clearest signature of quasifission differs depending on the reaction timescale.

Symmetric and asymmetric quasifission in reactions with heavy ions

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 of binary fragments as well as their cross sections have been measured for a wide range of composite systems with Z=82–122 formed in the reactions with 22Ne, 26Mg, 48Ca, 58Fe and 86Kr ions at energies around the Coulomb barrier. The experiments were carried out using a double-arm time-of-flight spectrometer of binary reaction products CORSET. The results of the experimental investigation of the influence of the entrance channel properties on the competition between fusion-fission and quasifission for the "warm" fusion reactions is discussed.

Study for fusion-fission and quasifission in reactions using 238U target nucleus

Fragment mass distributions for fission after full momentum transfer (FMT) were measured for the 30Si,31P,34,36S,40Ar + 238U reactions at bombarding energies around the Coulomb barrier. The experiment was carried out at the JAEA tandem accelerator facility. We observed strong variation of the mass distribution on beam energy and projectile nucleus. In the reaction of 36S +238U we observed a transition from symmetry to asymmetry mass distributions when the beam energies were decreased from the above-barrier to sub-barrier values. The mass asymmetry was AL /AH = 74/200, which corresponds to the fission valley leading to the nuclei close to the doubly closed-shell nuclei 78Ni /208Pb. The fission channel is populated by quasifission, which is the disintegration without forming the compound nucleus. The incident-energy dependence is strongly correlated with the prolate deformation of 238U and the orientation at the initial impact. The results indicate that the reaction starting at the polar collisions on 238U has larger quasifission probability. The mass distributions are nicely reproduced by a model calculation using Langevin equation. The 40Ar + 238U reaction also has the similar mass asymmetry (AL/AH ≈ 78/200) in quasifission. In the reactions of 31P + 238U and 30Si + 238U, mass asymmetries are AL/AH 81/188 and 90/178, respectively. The results suggest that the system produced by the reaction using lighter projectile approaches closer to the shape of the compound nucleus even when quasifission occurs. In the reactions of 30Si +238U and 34S+ 238U, we also measured the evaporation residue (ER) cross sections to obtain information on the fusion probability. From the ER cross sections for the 34S+238U reaction it was suggested that the symmetric fission is also dominated by quasifission.

Competition of fusion and quasifission in reactions leading to production of superheavy elements

The mechanism of fusion hindrance, an effect observed in the reactions of cold, warm, and hot fusion leading to production of superheavy elements, is investigated. A systematics of transfermium production cross sections is used to determine fusion probabilities. The mechanism of fusion hindrance is described as a competition of fusion and quasifission. Available evaporation residue cross sections in the superheavy region are reproduced satisfactorily. Analysis of the measured capture cross sections is performed and a sudden disappearance of the capture cross sections is observed at low fusion probabilities. A dependence of the fusion hindrance on the asymmetry of the projectile-target system is investigated using the available data. The most promising pathways for further experiments are suggested.

Competition between complete fusion and quasi-fission in dinuclear system

A model based on the dinuclear system concept is suggested for the calculation of the competition between complete fusion and quasi-fission in reactions with heavy nuclei. The fusion rate through the inner fusion barrier in mass asymmetry is found by using the Kramers-type expression. The calculated cross-sections for the heaviest nuclei are in a good agreement with the experimental data. The experimentally observed rapid fall-off of the cross-sections of the cold fusion with increasing charge numberZ of the compound nucleus is explained.

Process of complete fusion of nuclei within the framework of dinuclear system concept

A new approach, the dinuclear system concept (DNS-concept), was elaborated for the description of the process of complete fusion of nuclei. The DNS-concept has revealed the fusion barrier of a new type and the competition between complete fusion and quasi-fission in reactions with massive nuclei. The DNS-concept has been used for the analysis of reactions of superheavy element (SHE) synthesis.

Model of competition between fusion and quasifission in reactions with heavy nuclei

The fusion process is studied by a model as the time evolution of a dinuclear system due to the diffusion in the mass asymmetry degree of freedom. The diffusion process in the relative distance between the centers of the interacting nuclei is responsible for the quasifission. The important point in the evolution of the dinuclear system to the compound nucleus is the appearance of a fusion barrier along the mass asymmetry degree of freedom. The model has the advantage that it treats the competition between the processes of complete fusion and quasifission in the asymmetric dinuclear system. A multidimensional Fokker-Planck equation and a Kramers-type expression are used to calculate the fusion rate. Due to the competition between the processes of complete fusion and quasifission, the fusion probability strongly decreases with decreasing mass asymmetry in the entrance channel of the reaction which is in agreement with experimental data.

Model of competition between complete fusion and quasi-fission in reactions with massive nuclei

A model is proposed for the calculation of the competition between complete fusion and quasi-fission in a massive symmetric dinuclear system (DNS). The model is based on the DNS-approach to the analysis of complete nuclear fusion. The model has been to calculate the cross-sections of evaporation residues in the 100Mo+ 100Mo and 110Pd+ 110Pd reactions.

Competition between complete fusion and quasi-fission in reactions between massive nuclei. The fusion barrier

A model of the competition between complete fusion and quasi-fission channels in fusion of two massive nuclei is proposed. The calculation of the evaporation residue cross sections in the reactions 100Mo + 100Mo and 110Pd + 0794 0989 110Pd within this model gives a good agreement with the experimental data. The fusion barrier of a new type in the complete fusion of two massive nuclei is revealed.

Erratum: Complete fusion and quasifission in reactions between heavy ions

Erratum: Complete fusion and quasifission in reactions between heavy ions

Complete fusion and quasifission in reactions between heavy ions.

It is shown that fission anisotropies from reactions with projectiles lighter than A=20 are well described by the standard theory, in which it is assumed that the distribution of K values (projection of total spin I onto the symmetry axis) is determined at the fission saddle point. Angular anisotropies of symmetric fragments from heavy ion reactions with projectiles heavier than A=24 are, however, substantially larger than expected on the basis of this theory, an observation which recently led to the suggestion that the K values adjust adiabatically during the descent from saddle to scission such that the observed final K distributions reflect a thermal equilibrium at the scission point. The predictions of such scission point models are compared with the available experimental data and it is shown that they fail to reproduce the observed trends as a function of bombarding energy for most systems. It is concluded that the observed deviation from the saddle point theory can be explained as a dynamical inhibition of complete fusion with heavy projectiles. An analysis is carried out in which the fraction of the symmetric fragment cross section originating from the fission decay of completely fused compound nuclei can be estimated. The systematics of the inhibition to complete heavy ion fusion is discussed in the framework of the extra push model.

Compound nucleus fission and quasi-fission in reactions of 238U with 16O and 27Al

Fission fragment yields have been measured as a function of fragment mass, angle and energy for the 238U + 16O, 27Al fusion reactions at 6 MeV/u. The observed anisotropies for the two systems indicate that a large fraction of the 238U + 27Al cross section is associated with the quasi-fission (or fast fission) reaction, whereas the 238U + 16O data are consistent with compound nucleus formation, followed by fission.