ISSN 1063-7788, Physics of Atomic Nuclei, 2007, Vol. 70, No. 11, pp. 1846–1858. c  Pleiades Publishing, Ltd., 2007. Original Russian Text c  M.V. Borunov, P.N. Nadtochy, G.D. Adeev, 2007, published in Yadernaya Fizika, 2007, Vol. 70, No. 11, pp. 1897–1909. NUCLEI Theory Dynamical Description of the Moments of the Energy Distribution of Fission Fragments and Scission of a Fissile Nucleus M. V. Borunov * , P. N. Nadtochy, and G. D. Adeev Omsk State University, pr. Mira 55A, Omsk, 644077 Russia Received August 7, 2006; in final form, December 25, 2006 Abstract—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 <Z 2 /A 1/3 < 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 0.3R 0 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. PACS numbers: 25.85.-w, 05.10.Gg, 24.75.+i DOI: 10.1134/S106377880711004X INTRODUCTION The problem of pinpointing the condition under which a fissile nucleus undergoes scission into frag- ments inevitably comes to the fore in any theoretical treatment and simulation of the fission process, where the breakup of a primary compound nucleus into predominantly two fragments appears to be one of the characteristic features. By scission, one typically means a transition from a continuous nuclear con- figuration, which becomes unstable for a number of reasons, to a configuration in which the system being considered is formed by already separated fragments. The problem of the scission of the neck between would-be fragments was repeatedly addressed in the past [1–6], but it has yet to be solved conclusively, remaining one of the unclear problems in the physics of scission. The problem is aggravated substantially by the fact that neither the instant of nuclear scission into fragments nor the nuclear shape immediately before scission is observable experimentally, so that information about them can be deduced from a com- parison of several experimentally observable quanti- ties with the results of calculations performed within one model or approach or another that describes the nuclear-fission process. In the present study, we take * E-mail: bmv@opsb.ru the energy distribution of fission fragments—more precisely, its first four moments—for such experimen- tally observable quantities. It should be noted from the outset that two scis- sion criteria (conditions), which are obviously lim- iting cases with respect to each other, are basically used in fission theory. For example, the condition requiring the vanishing of the neck radius, R N =0, is taken for the scission criterion in numerous studies of Nix and his colleagues [7–9] and in some studies of other groups [10, 11]. Although this condition is self-consistent within the liquid-drop model involving a sharp nuclear boundary [1, 12], it is unsatisfac- tory from the conceptual point of view [2], since the treatment of a nucleus within the liquid-drop model is meaningless if the neck radius becomes commen- surate with the nucleon–nucleon distance [2]. From the physical point of view, it is appealing to define the scission condition on the basis of the criterion of instability of a nucleus against variations in the thick- ness of its neck [2], in which case the ridge between the fission valley and the valley of separated fragments disappears. This scission condition corresponds to prescission fissile-nucleus configurations involving a finite neck radius equal, on average, to 0.3R 0 (where R 0 is radius of the primary spherical nucleus) [1, 2, 13–16]. The nuclear-scission condition based on 1846