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The four-dimensional Langevin model was applied to calculate a wide set of experimental observables for compound nuclei, formed in heavy-ion fusion-fission reactions. A modified one-body mechanism for nuclear dissipation with a reduction coefficient ks of the contribution from a “wall” formula was used for shapes parameters. Different possibilities of deformation-dependent dissipation coefficient for the K coordinate (γK) were investigated. Presented results demonstrate that the influence of the ks and γK parameters on the calculated quantities can be selectively probed. It was found that it is possible to describe experimental data with the deformation-dependent γK coefficient. One of the possibility is to use large values of γK ≃ 0.2 (MeV zs)−1/2 for compact shapes featuring no neck and small values of γK ≃ 0.0077 (MeV zs)−1/2 for elongated shapes.

It is well accepted that fission is a slow process dominated by nuclear viscosity. This is demonstrated by experimental observations, mainly based on measurements of fission fragment TKE, pre-scission light particles and GDR γ-ray multiplicities, energy spectra. The fission time-scale, as well as the nature of energy dissipation during the process and its dependence on the nuclear shape and temperature are the main aspects which have often been addressed. In spite of the extensive work, there are still many open questions on the dynamics of the fission process, mainly due to the lack of constraints on the models and to the different probes used. Intermediate fissily systems are particularly suited for this study as they present comparable cross sections in the fusion-fission and fusion-evaporation channels, allowing to measure observables in both channels, and therefore to further constrain the models. Furthermore, the path from equilibrium to saddle configuration is expected to dominate with respect to saddle-to-scission one, reducing the complexity of the physical process to be studied. We present here an in depth review of a study performed on the nucleus 132Ce, which shows the limits of the statistical model in accounting for an extended set of observables. We further present the analysis of the same data with a 3D Langevin dynamical approach which proves to be capable of reproducing the values of the whole set of observables. Our analysis reinforce the fact that extended data sets are essential to achieve reliable simulations to address the open questions on fission dynamics and strongly suggest the use of setups of high efficiency. [ABSTRACT FROM AUTHOR]

This paper briefly reviews the first results of a coordinated study aimed at the clarification of the interplay between spectator fragmentation and electromagnetic phenomena in ultrarelativistic heavy-ion collisions at CERN SPS energies. The available experimental and phenomenological results are put in the context of recent theoretical calculations. [ABSTRACT FROM AUTHOR]

The system of intermediate fissility 132Ce has been studied experimentally and theoretically to investigate the dissipation properties of nuclear matter. Cross sections of fusion-fission and evaporation-residue channels together with light charged particle multiplicities in both channels, their spectra, light charged particle-evaporation residue angular correlations, and mass-energy distribution of fission fragments have been measured. Theoretical analysis has been performed using a multidimensional stochastic approach coupled with a Hauser-Feshbach treatment of particle evaporation. The main conclusions are that the full one-body shape-dependent dissipation mechanism allows the reproduction of the full set of experimental data and that after a time τd=5×10-21 s from the equilibrium configuration of the compound nucleus, fission decay can occur in a time that can span several orders of magnitude. [ABSTRACT FROM AUTHOR]

A four-dimensional stochastic approach to dynamics of nuclear fission induced by heavy ions was applied to analyze the key observables from fusion-fission reactions resulted in the highly excited compound nuclei. We took into account not only three shape collective coordinates introduced on the basis of the c, h, α-parametrization but also orientation degree of freedom (K-state) -- spin about the symmetry axis. We systematically investigated the possible deformation dependence of the viscosity coefficient ks predicted by chaos theory and deformation dependence of the γK coefficient deduced by Lestone et al. In the framework of 4D dynamical model, we examined the correlation between dissipation strength and mass, energy and angular distributions of fission fragments. Our calculations demonstrate that the deformation dependent coefficient ks and γK value obtained by Lestone is suitable for simultaneous description of experimental massenergy and angular distributions of fission fragments in 4D model for heavy nuclei, in contrast with the 3D model. The influence of ks and γK parameters on the calculated results could be separated. [ABSTRACT FROM AUTHOR]

A four-dimensional dynamical model was developed and applied to study fission characteristics in a wide range of a fissility parameter. Three collective shape coordinates and the K coordinate were considered dynamically from the ground-state deformation to the scission into fission fragments. A modified one-body mechanism for nuclear dissipation with a reduction coefficient ks of the contribution from a "wall" formula has been used in the study. The inclusion of the K coordinate in the dynamical consideration and use of the "chaos-weighted wall formula" with a deformation-dependent scaling factor ks(q1) lead to fairly good reproduction of the variances of the fission-fragment mass distribution and the prescission neutron multiplicity for a number of fissioning compound nuclei in a wide fissility range. The four-dimensional dynamical calculations describe better experimental prescission neutron multiplicity and variances of fission-fragment mass distribution for heaviest nuclei with respect to a three-dimensional dynamical model, where the K coordinate is assumed to be equal to zero. The estimate of a dissipation coefficient for the orientation degree of freedom, γK ≃ 0.077 (MeV zs)-1/2, is good for heavy nuclei and a larger value of γK ≃ 0.2 (MeVzs)-1/2 is needed for nuclei with mass ACN ≃ 200. [ABSTRACT FROM AUTHOR]

The time evolution of nuclei at high excitation energy and angular momentum is studied by means of three-dimensional Langevin calculations performed for two different parameterizations of the macroscopic potential: the Finite Range Liquid Drop Model (FRLDM) and the Lublin-Strasbourg Drop (LSD) prescription. Depending on the mass of the system, the topology of the potential energy surface (PES), in the deformation space, is observed to be different within these two approaches. When incorporated in the dynamical calculation, the FRLDM and LSD models are observed to give similar results in the heavy mass region, whereas the predictions can be strongly dependent on the PES for fission of medium-mass nuclei. The shape-dependent congruence energy is only slightly modulated on the top of the bulk LD part although it changes the PES and the barrier height. [ABSTRACT FROM AUTHOR]

A four-dimensional dynamical model based on Langevin equations was developed and applied to calculate a wide set of experimental observables for the reactions 16O + 208Pb → 224Th and 16O + 232Th → 248Cf over a wide range of excitation energy. The fusion-fission and evaporation residue cross sections, fission fragment mass-energy distribution parameters, prescission neutron multiplicities, and anisotropy of angular distribution of fission fragments could be reasonably reproduced using a modified one-body mechanism for nuclear friction with a reduction coefficient of the contribution from a wall formula ks ≃ 0.25 and a dissipation coefficient for the orientation degree of freedom (K coordinate) γK ≃ 0.077 (MeVzs)-1/2. Inclusion of the K coordinate into calculation of potential energy changes the stiffness of the nucleus with respect to mass asymmetry coordinate for the values of K ≠ 0 and results in a shift of the Businaro-Gallone point towards larger Z2/A values. The experimental data on the fission fragment mass-energy distribution parameters together with mean prescission neutron multiplicity for heavy fissioning nuclei are reproduced through the four-dimensional Langevin calculations more accurately than through three-dimensional calculations. [ABSTRACT FROM AUTHOR]

A novel experimental approach for studying dissipative effects in nuclear fission has been developed at GSI, Darmstadt. Fragmentation of radioactive heavy-ion beams is employed to prepare fissile nuclei in well-defined initial conditions and the fission-fragment nuclear charge distribution is used for investigating pre-saddle dynamics in detail. The undeniable manifestation of transient effects at high temperature is demonstrated and the influence of the initial deformation is clearly evidenced for the first time. [ABSTRACT FROM AUTHOR]

The average multiplicity of gamma rays emitted by fragments originating from the fission of 226Th nuclei formed via a complete fusion of 18O and 208Pb nuclei at laboratory energies of 18O projectile ions in the range E lab = 78–198.5 MeV is measured and analyzed. The total spins of fission fragments are found and used in an empirical analysis of the energy dependence of the anisotropy of these fragments under the assumption that their angular distributions are formed in the vicinity of the scission point. The average temperature of compound nuclei at the scission point and their average angular momenta in the entrance channel are found for this analysis. Also, the moments of inertia are calculated for this purpose for the chain of fissile thorium nuclei at the scission point. All of these parameters are determined at the scission point by means of three-dimensional dynamical calculations based on Langevin equations. A strong alignment of fragment spins is assumed in analyzing the anisotropy in question. In that case, the energy dependence of the anisotropy of fission fragments is faithfully reproduced at energies in excess of the Coulomb barrier ( E c.m. − E B ≥ 30 MeV). It is assumed that, as the excitation energy and the angular momentum of a fissile nucleus are increased, the region where the angular distributions of fragments are formed is gradually shifted from the region of nuclear deformations in the vicinity of the saddle point to the region of nuclear deformations in the vicinity of the scission point, the total angular momentum of the nucleus undergoing fission being split into the orbital component, which is responsible for the anisotropy of fragments, and the spin component. This conclusion can be qualitatively explained on the basis of linear-response theory. [ABSTRACT FROM AUTHOR]

A stochastic approach that treats fission dynamics on the basis of three-dimensional Langevin equations is used to calculate the mass-energy distributions of fragments originating from the fission of compound nuclei whose fissility parameter lies in the range Z²/A = 34-42. In these calculations, use was made of the liquid-drop model allowing for finite-range nuclear forces and the diffuseness of the nuclear surface in calculating the potential energy and a modified one-body mechanism of viscosity in describing dissipation. The emission of light prescission particles is taken into account on the basis of the statistical model. The calculations performed within three-dimensional Langevin dynamics reproduce well all parameters of the experimental mass-energy distributions of fission fragments and all parameters of the prefission-neutron multiplicity for various parameters of the compound nucleus. The inclusion of the third collective coordinate in the Langevin equations leads to a considerable increase (by up to 40-50%) in the variances of mass-energy distributions in relation to what was previously obtained from two-dimensional Langevin calculations. For the parameters of the mass-energy distributions of fission fragments and the parameters of the prefission-neutron multiplicity to be reproduced simultaneously, the reduction coefficient ks must be diminished at least by a factor of 2 (0.2 ≤ k[sub s] ≤ 0.5) in relation to that in the case of total onebody viscosity (k[sub s] = 1). [ABSTRACT FROM AUTHOR]

Dynamical calculations based on the four-dimensional stochastic Langevin approach are extended to the description of fragment isotopie distributions in fission at moderate-to-high excitation energy. Benefiting from previous theoretical work on charge-asymmetry relaxation times, and existing experimental charge-dispersion curves from low-energy fission, charge fluctuations are introduced at the scission point in an effective way. Comparison with recent suited measurements shows a good description over the full fragment production. The development proposed in this work can easily be implemented in other codes used in the field. [ABSTRACT FROM AUTHOR]

The potential energy surface plays a decisive role in nuclear fission. Together with inertia and viscosity, it influences the trajectory of the system, and the properties of the fission fragments result from the puzzling interplay between static and dynamical effects. A careful study on the influence of the parametrization of the potential energy landscape in heavy-ion-induced fission is performed. Dynamical calculations are done within the stochastic Langevin approach in a three-dimensional deformation space. Various prescriptions of the potential energy surface are considered, probing two different Liquid Drop models and the deformation dependence of the Wigner/congruence energy. A wide set of observables, including cross sections, particle multiplicities, and integral, as well as isotopic and isobaric, distributions of fission and evaporation products, is analyzed. Nuclei close to the Businaro-Gallone point are confirmed to be well suited for investigating the Liquid Drop parametrization, while the influence of the deformation-dependent Wigner/congruence energy is difficult to demonstrate unambiguously in fission at high excitation energies. [ABSTRACT FROM AUTHOR]

An innovative procedure to determine the mass-angle distribution of fragments from quasifission is proposed. It is based on the subtraction of the contribution from compound-nucleus fission from the experimental correlation spectrum which is in most cases a mixture of fusion-fission and quasifission. The former is calculated with a four-dimensional dynamical model of fission within the stochastic Langevin theory. The proposed approach is benchmarked using measurements for different entrance channels, ranging from below to above the onset of quasifission, all leading to the same 202Po composite system. The sensitivity of the procedure is evaluated. Its potential for isolating the fragment-mass and emission-angle information pertaining to the sole quasifission process is discussed. [ABSTRACT FROM AUTHOR]

An attempt to unfold the respective influence of the fusion and fission stages on typical fission observables, and namely the neutron prescission multiplicity, is proposed. A four-dimensional dynamical stochastic Langevin model is used to calculate the decay by fission of excited compound nuclei produced in a wide set of heavy-ion collisions. The comparison of the results from such a calculation and experimental data is discussed, guided by predictions of the dynamical deterministic HICOL code for the compound-nucleus formation time. While the dependence of the latter on the entrance-channel properties can straigthforwardly explain some observations, a complex interplay between the various parameters of the reaction is found to occur in other cases. A multidimensional analysis of the respective role of these parameters, including entrance-channel asymmetry, bombarding energy, compound-nucleus fissility, angular momentum, and excitation energy, is proposed. It is shown that, depending on the size of the system, apparent inconsistencies may be deduced when projecting onto specific ordering parameters. The work suggests the possibility of delicate compensation effects in governing the measured fission observables, thereby highlighting the necessity of a multidimensional discussion. [ABSTRACT FROM AUTHOR]

Dynamical calculations based on a four-dimensional stochastic Langevin approach are used to study fission in the Businaro-Gallone region. The use of such a model to map the Businaro-Gallone transition is proposed for the first time, exploiting recent theoretical developments and computing resources. The significance of the approach is supported by a systematic comparison with measurements for a set of systems. Experimental observations on fission-fragment charge distributions, which remained unexplained within a unified framework so far, are described by the dynamical model in a consistent manner. A detailed analysis reveals the interplay of dynamical effects, going beyond statistical-models interpretation and suggesting to revisit the commonly admitted idea about the prevalence of static effects in the Businaro-Gallone region. [ABSTRACT FROM AUTHOR]

In l29Xe + natSn central collisions from 8 to 25 MeV/nucleon, the three-fragment exit channel occurs with a significant cross section. We show that these fragments arise from two successive binary splittings of a heavy composite system. The sequence of fragment production is determined. Strong Coulomb proximity effects are observed in the three-fragment final state. A comparison with Coulomb trajectory calculations shows that the time scale between the consecutive breakups decreases with increasing bombarding energy, becoming quasisimultaneous above excitation energy E* = 4.0 ± 0.5 MeV/nucleon. This transition from sequential to simultaneous breakup was interpreted as the signature of the onset of multifragmentation for the three-fragment exit channel in this system. [ABSTRACT FROM AUTHOR]

In peripheral heavy-ion collisions at ultrarelativistic energies, usually only parts of the colliding nuclei effectively interact with each other. In the overlapping zone, a fireball or quark-gluon plasma is produced. The excitation energy of the heavy remnant can range from a few tens to several hundreds of MeV, depending on the impact parameter. The decay of these excited spectators is investigated in this work for the first time within a dynamical approach based on the multidimensional stochastic Langevin equation. The potential of this exploratory work to understand the connection between electromagnetic fields generated by the heavy spectators and measured pion distributions is discussed. [ABSTRACT FROM AUTHOR]

The nuclear properties of 48Cr composite α-like nuclei produced at 60 MeV of excitation energy via the 24Mg+24Mg reaction were investigated. This excitation energy corresponds to a resonance with a narrow width (170 keV) observed in the elastic and inelastic channels, which was interpreted as a highly deformed state. To gain insight on the deformation of this state exclusive measurements of light charged particles were carried out with 8πLP apparatus at Laboratori Nazionali di Legnaro and compared to statistical model predictions. The measured of α-particle energy spectra, α-evaporation residues, α-α, and α-α-α correlations indicate the limitation of the rotating liquid drop model in describing the nuclear shape of the compound nucleus along the decay cascade. To reproduce the full set of experimental data very elongated nuclear shapes had to be considered, with an axis ratio 3:1 at the resonance angular momentum. This large deformation is consistent with previous findings for α-like nuclei and with the predictions of the cranked cluster model. [ABSTRACT FROM AUTHOR]

We report on a novel experimental approach for studying the dissipative spreading of collective motion in a metastable nuclear system, using, for the first time, highly fissile nuclei with spherical shape. This was achieved by fragmentation of 45 radioactive heavy-ion beams at GSI, Darmstadt. The use of inverse kinematics and a dedicated experimental setup allowed for the identification in atomic number of both fission fragments. From the width of their nuclear-charge distributions, a transient time of (3.3+/-0.7)x10(-21) s is deduced for initially spherical nuclei.