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A stochastic approach to describing the dynamics of the nuclear-fission process is applied to study the effect of the number of dimensions on the fission rate within the dynamical model used. The time dependence of the fission rate is calculated on the basis of a multidimensional Langevin equation without taking into account particle evaporation. The one-, two-, and three-dimensional cases are considered for the example of the “ c, h, α” parametrization of nuclear-surface shapes. The calculations are performed for a large number of compound nuclei whose parameter Z2/ A falls within the range 20 ≤ Z2/ A ≤ 40. The stationary level of the fission rate is found to increase considerably upon going over from the one- to the three-dimensional case. This increase is especially pronounced for light fissile nuclei in the vicinity of the Businaro-Gallone point. Also, the stationary fission-rate level obtained from our dynamical simulation is compared with its counterpart calculated by the Kramers formula generalized to the multidimensional case. [ABSTRACT FROM AUTHOR]

The impact of non-Markovian effects on the fission rate and time is studied on the basis of a generalized one-dimensional Langevin equation. Relevant calculations were performed under the assumption of an exponentially correlated random force appearing in Langevin equations and a constant correlation time. An increase in the fission rate and a decrease in the mean fission time with increasing random-force correlation time were revealed. [ABSTRACT FROM AUTHOR]

A multidimensional stochastic approach to fission dynamics on the basis of three-dimensional Langevin equations is applied systematically to calculating the first four moments of the energy distribution of fission fragments over a broad range of Coulomb parameter values (700 < Z2/ A1/3 < 1700). For the scission of a fissile nucleus into fragments, use was made of various criteria traditional in modern fission theory: the vanishing of the neck radius at the scission instant and the equality of the neck radius to about 0.3 R0 at this instant. In calculating the energy distribution, both of the criteria used lead to a fairly good description of experimental data on the first two moments and to a satisfactory description of data on the third and fourth moments of the distribution. However, the quality of the description of available experimental data is insufficiently good for giving preference to any of these criteria. Within three-dimensional Langevin dynamics, it is shown that the vanishing-radius criterion leads to unexpectably good agreement with experimental data on the first four moments of the energy distribution. A modified version of one-body dissipation where the coefficient that takes into account the reduction of the wall-formula contribution was set to k s = 0.25 was used in the calculations. [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 based on three-dimensional Langevin equations was applied to detailed study of fission dynamics in fusion-fission reactions. The reduction coefficient in surface-plus-window dissipation is deduced by analyzing the available experimental data on various observable quantities in fusion-fission reactions. Analysis of the results shows that not only characteristics of the mass-energy distribution of fission fragments but also the mass and kinetic-energy dependence of the prescission and postscission neutron multiplicities, the angular anisotropy, and fission probability can be reproduced using surface-plus-window dissipation with the reduction coefficient from a wall formula k[SUBs] = 0.25-0.5 for compound nuclei lighter than [SUP224]Th. The analysis performed reveals that the coordinate-independent reduction Coefficient k[SUBs] is not compatible with the simultaneous description of the main fission characteristics for fissioning systems heavier than [SUP224]Th. [ABSTRACT FROM AUTHOR]

A probabilistic criterion is proposed for the scission of a fissile nucleus into fragments. The probability of the rupture of the neck between would-be fragments is estimated by considering scission as a fluctuation. The energy of the prescission configuration and the energy of the separated-fragment configuration are computed on the basis of a macroscopic model that takes into account a finite range of nuclear forces and the di3useness of the nuclear surface. The e3ect of the probabilistic criterion of nuclear scission on fission-process observables, such as the moments of the mass-energy distribution of fission fragments, the mean multiplicity of prescission neutrons, and mean fission times, is demonstrated. It is shown that the Strutinsky criterion, according to which nuclear scission occurs at a finite neck radius of 0.3 R[SUB0], is a rather good approximation to the probabilistic scission criterion in Langevin dynamical calculations employing the one-body nuclear-viscosity mechanism modified in such a way that the wall-formula contribution is reduced, the reduction factor satisfying the condition K[SUBs] < 0.(5). [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]

Two-dimensional mass-energy distributions of fission fragments are calculated for the first time on the basis of three-dimensional stochastic Langevin equations. In these calculations, the emission of light prescission particles is taken into account within the statistical model. The results demonstrate that calculations within three-dimensional Langevin dynamics make it possible to describe most comprehensively the properties of the mass-energy distribution of fission fragments and the mean multiplicity of prescission neutrons. [ABSTRACT FROM AUTHOR]

A stochastic approach to fission dynamics is proposed. The approach, which is based on Langevin equations, is used to calculate the mass distributions of fragments originating from the fission of excited nuclei. The effect of viscosity and light-particle emission on the variance of mass distributions is studied. The results of the calculations based on the above approach reveal that, in order to obtain a simultaneous description of mass-distribution parameters and the multiplicities of prescission particles, it is necessary to use sufficiently large values of nuclear viscosity both for the one-body and for the two-body viscosity mechanism, anomalously large values of the viscosity coefficient being required in the latter case. [ABSTRACT FROM AUTHOR]