Quasifission Process Research Articles

Trajectory analysis for fusion path in superheavy-mass region

We propose an effective method for the precise investigation of the fusion–fission mechanism in the superheavy-mass region, using the fluctuation–dissipation model. The trajectory calculation with friction is performed in the nuclear deformation space using the Langevin equation. In the reaction 48Ca + 244Pu, the trajectories are classified into the fusion–fission process, the quasi-fission process and the deep quasi-fission process. By analyzing the time evolution of each trajectory, the mechanism of each process is clearly revealed, i.e., it is explained why a trajectory takes a characteristic path in this model. We discuss, in particular, the condition under which the fusion path is followed, which is crucial in the discussion of the possibility of synthesizing superheavy elements.

The influence of the entrance channel dynamics on the evaporation residue formation

The dynamical effects of the entrance channel on the competition between quasifission and fusion processes, and on the evaporation residue formation are investigated. We have analyzed the results and compared our calculations with the experimental data obtained in the 16 O + 204 Pb and 96 Zr + 124 Sn reactions having very different mass asymmetries and leading to the 220 Th * compound nucleus. We have found that different partial capture cross-sections $\sigma_\ell^{\rm cap}(E)$ for these reactions lead to different fusion-quasifission competitions and, consequently, to different partial fusion cross-sections $\sigma_\ell^{\rm fus}(E)$ of the compound nucleus formed in the two reactions. The dynamical conditions also affect the fission-evaporation competition of the excited intermediate nuclei along the CN de-excitation cascade and, consequently, the evaporation residue formation.

Dynamical calculation for fusion–fission probability in superheavy mass region, where mass symmetric fission events originate

We discuss dynamically the fusion–fission mechanism in the superheavy mass region. By analyzing the mass distribution of fission fragments, we distinguish between the fusion–fission process and the quasi-fission process. We investigate these two processes using the fluctuation–dissipation model. The three-dimensional Langevin equation is employed. We classify the dynamical process by analyzing the trajectory in the nuclear deformation space. In the superheavy mass region, we found that 90–99% of mass symmetric fission events come from the quasi-fission process, in which the system hardly reaches the spherical region. The fusion cross section is also estimated precisely.

Analysis of fusion–fission dynamics in superheavy mass region by pre-scission neutron multiplicities

We discuss thc fusion-fission process in superheavy mass region. By analyzing the mass distribution of fission fragments, we can distinguish between fusion-fission process and quasi-fission process. We investigate these two processes using the fluctuation-dissipation model. In the dynamical calculation, we employ the three-dimensional Langevin equation. We find 90 ∼ 99% of mass symmetric fission events come from the quasi-fission process in the superheavy mass region. In order to classify the fusion-fission process more precisely. we analyze the pre-scission neutron multiplicity. © 2004 Elsevier B.V. All rights reserved.

Shell effects in fission and quasi-fission of heavy and superheavy nuclei

Results of the experiments aimed at the study of fission and quasi-fission processes in the reactions 12 C+ 204 Pb, 48 Ca+ 144,154 Sm, 168 Er, 208 Pb, 244 Pu, 248 Cm; 58 Fe+ 208 Pb, 244 Pu, 248 Cm, and 64 Ni+ 186 W, 242 Pu are presented in the work. The choice of the above-mentioned reactions was inspired by recent experiments on the production of the isotopes 283 112, 289 114 and 283 116 at Dubna [1],[2] using the same reactions. The 58 Fe and 64 Ni projectiles were chosen since the corresponding projectile-target combinations lead to the synthesis of even heavier elements. The experiments were carried out at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR, Russia), the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL, Italy) and the Accelerator of the Laboratory of University of Jyvaskyla (JYFL, Finland) using the time-of-flight spectrometer of fission fragments CORSET[3] and the neutron multi-detector DEMON[4],[5]. The role of shell effects and the influence of the entrance channel on the mechanism of the compound nucleus fusion-fission and the competitive process of quasi-fission are discussed.

Shell effects in fission and quasi-fission of heavy and superheavy nuclei

Results of the experiments aimed at the study of fission and quasi-fission processes in the reactions 12C+ 204Pb, 48Ca+ 144,154Sm, 168Er, 208Pb, 244Pu, 248Cm; 58Fe+ 208Pb, 244Pu, 248Cm, and 64Ni+ 186W, 242Pu are presented in the work. The choice of the above-mentioned reactions was inspired by recent experiments on the production of the isotopes 283112, 289114 and 283116 at Dubna [1],[2] using the same reactions. The 58Fe and 64Ni projectiles were chosen since the corresponding projectile-target combinations lead to the synthesis of even heavier elements. The experiments were carried out at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR, Russia), the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL, Italy) and the Accelerator of the Laboratory of University of Jyvaskyla (JYFL, Finland) using the time-of-flight spectrometer of fission fragments CORSET[3] and the neutron multi-detector DEMON[4],[5]. The role of shell effects and the influence of the entrance channel on the mechanism of the compound nucleus fusion-fission and the competitive process of quasi-fission are discussed.

Fusion and Quasifission within the Dinuclear System Model

The dinuclear system model considers a configuration of two touching nuclei which exchange nucleons. The microscopical justification of the model is presented. The fusion and quasifission processes are described in the reactions of synthesis of heavy and superheavy nuclei. The dependence of evaporation residue cross sections on isotopic composition of colliding nuclei is analyzed. The results agree with the available experimental data.

Formation of heavy and superheavy elements by reactions with massive nuclei

The effects of the entrance channel and shell structure on the experimental evaporation residues have been studied by analyzing the 32S + 182W, 48Ti + 166Er and 60Ni + 154Sm reactions leading to 214Th*; the 40Ar + 181Ta reaction leading to 221Pa*; the 48Ca + 243Am, 248Cm, 249Cf reactions leading to the 291115, 296116 and 297118 superheavy compound nuclei, respectively. The fusion mechanism and the formation of evaporation residues of heavy and superheavy nuclei have been studied. In calculations of the excitation functions for capture, fusion and evaporation residues we used such characteristics as mass asymmetry of nuclei in the entrance channel, binding energies and shape of colliding nuclei, potential energy surface, driving potential, partial-fusion cross-sections and survival probability of the compound nucleus, $\Gamma_{\rm n} / \Gamma_{\rm f}$ ratio at each step along the de-excitation cascade of the compound nucleus. The calculations have allowed us to make useful conclusions about the mechanism of the fusion-fission process, which is in competition with the quasifission process, and the production of the evaporation residues.

Entrance Channel Effect on the Formation of Heavy and Superheavy Nuclei

We study the effect of the entrance channel and the shell structure of reacting massive nuclei on the fusion mechanism and the formation of evaporation residues of heavy and superheavy nuclei. In the framework of the combined dinuclear system concept and advanced statistical model, we analyze the 40 Ar + 176 Hf, 86 Kr + 130 Xe and 124 Sn + 92 Zr reactions leading to 216 Th * ; the 32 S + 182 W, 48 Ti + 166 Er, and 60 Ni + 154 Sm reactions leading to 214 Th * ; the 40 Ar + 181 Ta reaction leading to 221 Pa * ; the 48 Ca + 248 Cm reaction leading to the 296 116 compound nucleus. In our calculations of the excitation functions for capture, fusion and evaporation residues we use the relevant variables such as mass-asymmetry of nuclei in the entrance channel, relative distance between nuclear centers, shell effect and shape of colliding nuclei and such characteristics of the reaction mechanism as potential energy surface, driving potential, the dependence of capture, fusion cross sections and survival probability of compound nucleus on the orbital angular momentum. As a result we obtain a beam energy range for the capture of the nuclei before the system fuses and the Γ n / Γ f ratio at each step along the de-excitation cascade of the compound nucleus. Calculations allow us to reach useful conclusions about the mechanism of the fusion–fission process, that is in competition with the quasifission process, and the production of the evaporation residues.

Synthesis of heavy and superheavy elements by reactions of massive nuclei

of complete fusion with the quasifission process is strongly related to the intrinsic fusion barrier B ∗ and the quasifission barrier Bqf, as well as the size of the well in the nucleus–nucleus potential. In our calculations of the excitation functions for capture, fusion, and evaporation residues, we use the relevant variables such as mass asymmetry of nuclei in the entrance channel, potential energy surface, driving potential, spin distribution, and surviving probability of compound nucleus that are responsible for the mechanism of the fusion–fission process. As a result, we obtain a beam energy window for the capture of the nuclei before the system fuses and the Γn/Γf ratio at each step along the deexcitation cascade of the compound nucleus. Calculations performed in the framework of the model taking into account the nuclear shell effect and shape of colliding nuclei allowus to reach useful conclusions about the mechanism of the fusion –fission process and the production of the evaporation residues. We analyze the 40 Ar + 176 Hf, 86 Kr + 130 Xe, and 124 Sn + 92 Zr reactions leading to 216 Th ∗ ;t he 32 S + 182 Wa nd 60 Ni + 154 Sm reactions leading to 214 Th ∗ ;t he 48 Ca + 248 Cm reaction leading to the 296 116 compound nucleus; and the 48 Ca + 249 Cf reaction leading to the 297 118 compound nucleus. c � 2003 MAIK “Nauka/Interperiodica”.

The effect of the entrance channel on the fission of 216Ra

We have studied mass-energy distributions (MED) of fission fragments using two projectile-target combinations, 12C + 204Pb and 48Ca + 168Er, leading to the same compound nucleus 216Ra at the excitation energy E*∼40 MeV. It has been found that the contribution of the asymmetric mode in the case of the former reaction is 1.5%, and it is 30% in the case of the latter one. We connect such a sharp increase in the yield of asymmetric products in the 48Ca + 168Er reaction with the quasifission process, the MED of which have a clearly expressed shell structure. The characteristics of the fission fragment MED are of such a kind that they can be interpreted by analogy with the low-energy fission of heavy nuclei as a manifestation of an independent mode of nuclear decay which competes with the classical fusion-fission process.

Unexpected entrance-channel effect in the fission of216Ra*

We have studied the mass and energy distributions of fission fragments from the two reactions ${}^{12}\mathrm{C}{+}^{204}\mathrm{Pb}$ and ${}^{48}\mathrm{Ca}{+}^{168}\mathrm{Er}$ that lead to the same compound nucleus ${}^{216}{\mathrm{Ra}}^{*}.$ Despite the fact that the excitation energy was around 40 MeV in both cases, the contribution from asymmetric fission in the first reaction is only around 1.5% but is about 30% in the second. This marked increase in the yield of asymmetric products is connected to the quasifission process, in which important shell effects become evident. The mass-energy distributions are interpreted in terms of an independent decay mode competing with the normal fusion-fission process and possibly leading to a significant suppression of the fusion cross section itself.

Synthesis of Superheavy Elements in Dinuclear System Model

The dinuclear system model is used to describe the fusion of heavy ions to superheavy nuclei and the quasifission process in the same reactions. The results agree with the present available experimental data.

Fusion-Fission Process of Superheavy Elements with Fluctuation-Dissipation Model

The fusion-fission process for the synthesis of superheavy elements is discussed based on fluctuation-dissipation dynamics. Recent data from Dubna on a fusion-fission cross section derived from fission experiments in the reactions 4 8 Ca+ 2 0 8 Pb, 4 8 Ca+ 2 3 8 U, 4 8 Ca+ 2 4 4 Pu and 4 8 Ca+ 2 4 8 Cm are analyzed using a three-dimensional Langevin calculation. We found that the quasi-fission process contributes to the yield of mass symmetric fission fragments. In superheavy mass region, the dynamical deformation of fragments is very important. The one- or two-dimensional calculation is not enough to describe the dynamics of fusion-fission process.

Synthesis of superheavy nuclei: Nucleon collectivization as a mechanism for compound nucleus formation

A consistent systematic analysis of the synthesis of very heavy nuclei is performed within a ``standard'' theoretical approach without any adjustable parameters and additional simplification. Good agreement with experimental data was obtained in all the cases up to synthesis of the 102 element. It was confirmed that a process of the compound nucleus formation, starting from the instant when two heavy nuclei touch and proceeding in strong competition with the fission and quasifission processes, plays an important role in the asymmetric synthesis of superheavy elements with ${Z}_{\mathrm{CN}}>~104$ as well as in the symmetric fusion at ${Z}_{\mathrm{CN}}>~90.$ A new mechanism of the fusion-fission process for a heavy nuclear system is proposed, which takes place in the ${(A}_{1}, {A}_{2})$ space, where ${A}_{1}$ and ${A}_{2}$ are two nuclei, surrounded by a certain number of common nucleons $\ensuremath{\Delta}A.$ These nuclei gradually lose (or acquire) their individualities with increasing (or decreasing) the number of collectivized nucleons $\ensuremath{\Delta}A.$ The driving potential in the ${(A}_{1}, {A}_{2})$ space is derived, which allows the calculation of both the probability of the compound nucleus formation and the mass distribution of fission fragments in heavy ion fusion reactions.

Quasifission process in a transport model for a dinuclear system

A new method is suggested for calculating the charge and mass distributions of quasifission products. The quasifission is treated within a transport model describing the evolution of a dinuclear system in charge (mass) asymmetry and the decay of this system along the internuclear distance. The calculated yields of these products are in agreement with recent experimental data for the hot fusion reactions leading to superheavy nuclei. The quasifission distributions in cold Pb-based fusion reactions are predicted.

Capture and fusion dynamics in heavy-ion collisions

The dependence of the difference between the processes of capture and complete fusion is studied as a function of the entrance channel in the framework of the combined dynamical and statistical model. The excitation functions for the 126 Ba compound nucleus formed in the 19 F + 107 Ag and 28 Si + 98 Mo reactions were calculated to compare the E min and E max values of the beam energy window for the fusion cross section. The importance of the competition between quasi-fission and complete fusion in interpreting the experimental data obtained for the 40 Ar + nat Ag reaction is discussed. The increase in the difference between the experimental evaporation residue production and the extracted fusion cross section at increasing 40 Ar beam energy is explained as an increasing effect of the quasi-fission process.

Heating and fluctuations in elastoplastic systems

Starting from the equations of motion of a simple system possessing the properties of elastic and plastic bodies, we construct its Lagrangian and Hamiltonian functions and also the Rayleigh dissipation function. This allows us to find the rate of heating of the system and to analyze the fluctuations of basic observables. Introducing into the Hamilton-Rayleigh equation of motion a random force producing on average the same effects as a dissipation function, we arrive first at the Langevin equations describing the fluctuations and then at a kinetic equation for the distribution function defined in the space of the collective variables. In this way a rather general scheme is established for solving dynamical problems in different and more complex elastoplastic systems, in nuclear physics and maybe even in physics of molecules and atomic clusters. In a preliminary study, the model is applied to estimate the probability of the quasi-fission process coming from the thermal fluctuations of the nuclear shape.

Giant dipole resonance decay from fusion-fission and quasifission of hot thorium nuclei.

Giant dipole resonance (GDR) {gamma} rays were measured in kinematic coincidence with fission fragments in the reactions {sup 16}O+{sup 208}Pb at 140 MeV, and {sup 32}S+{sup nat}W at 185, 215, and 230 MeV bombarding energy, leading to {sup {similar to}216,224}Th at a temperature of {ital T}{approx}1.8 to 2.1 MeV. The experiment determined the {gamma}-ray spectrum and the {gamma}-ray-fission fragment angular correlation as a function of fragment mass, kinetic energy, and total kinetic energy release. The coincidence {gamma}-ray spectra are fitted successfully and consistently in terms of the statistical decay of the hot compound system and of the fission fragments, when a large nuclear dissipation ({gamma}=10) and, for the {sup 32}S+{sup nat}W reaction, the GDR {gamma} emission during the quasifission process is included. The {gamma}-ray-fission fragment angular correlation indicates a deformed compound system in {sup 224}Th of either strongly prolate ({beta}=0.3) or noncollective oblate ({beta}={minus}0.1) shape. This is consistent with, but does not prove, a transition to a liquid drop shape having occurred at {ital T}{approx}1.8 MeV. The quasifission process is successfully included using regular extra-push and extra-extra-push energies and a quasifission lifetime {tau}{sub QF}=(20--40){times}10{sup {minus}21} sec. This is about ten times shorter than the compound nucleus fission lifetime in {supmore » 224}Th at this temperature.« less

Time scales for fusion-fission and quasifission from giant dipole resonance decay.

Giant dipole resonance (GDR) \ensuremath{\gamma} rays in coincidence with fission fragments were measured in the reactions $^{16}\mathrm{O}$${+}^{208}$Pb at 120 and 140 MeV, $^{24}\mathrm{Mg}$${+}^{196}$Pt at 150 MeV, and $^{32}\mathrm{S}$${+}^{\mathrm{nat}}$W at 185 MeV. \ensuremath{\gamma}-ray--fission angular correlation measurements for $^{16}\mathrm{O}$${+}^{208}$Pb (120 and 140 MeV) and $^{32}\mathrm{S}$${+}^{\mathrm{nat}}$W confirm the strong contribution of prescission \ensuremath{\gamma} rays to the total \ensuremath{\gamma}-ray spectrum. The data were analyzed with a modified statistical model code which includes the effects of nuclear dissipation in the fission process, \ensuremath{\gamma} decay at equilibrium and between saddle and scission, and the decay of the mononucleus formed in quasifission reactions. The extracted value for the nuclear friction coefficient of \ensuremath{\gamma}=10\ifmmode\pm\else\textpm\fi{}3 at temperatures between 1 and 2 MeV is much larger than the upper limit for the ground state and indicates strongly overdamped large-scale mass motion. The extracted total fission time scale ${\mathrm{\ensuremath{\tau}}}_{\mathrm{fiss}}$(\ensuremath{\gamma}=10)\ensuremath{\simeq}2.9\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ s at 64 MeV excitation energy agrees with that extracted from recent neutron multiplicity experiments. A strong dependence of the GDR \ensuremath{\gamma}-ray yield on the target-projectile mass asymmetry is interpreted in terms of the quasifission process. \ensuremath{\gamma}-ray spectra and correlations could be fitted with the assumption of a GDR contribution from the mononucleus but required an arbitrary reduction of the total fission fragment excitation energy by 30 MeV. The quasifission lifetime was estimated to ${\mathrm{\ensuremath{\tau}}}_{\mathrm{qf}}$=(2--9)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ s.