No abstract
The variances of the fission-fragment mass distributions, cry, were measured for (0c,f) and (3He,J) reactions in the interval between Os and At. The analysis of the experimental data suggests that the properties of symmetric fission around x-0.7 are mainly determined by the barrier passing stage. This conclusion is in discrepancy with results of dynamic calculations.The combination of the bulk assumptions of the liquid-drop model with the nucleon shell concepts had led to generally consistent pattern of the nuclear fission process, which is still qualitative to some extent. A number of substantial gaps in it is concerned with the descriptions of the fragment mass distributions, which are actually phenomenological up to day. If the two characteristics of a fissioning nucleus be pointed out, i.e. the excitation energy E and the fissility parameter X=2a 2 1-k (1) with the coefficients c3, a2, k as obtained in [1], one can classify the properties of the mass distributions as follows. At x>0.7 and E<40 50 MeV, the asymmetric mode prevales in fission, and moreover, the nuclear shell structure decisively affects the formation of asymmetric fission. In the rest regions of x-and E-values, structureless symmetric fission predominates; in that case, the liquid-drop model description is applied as usual.Whereas the dependence of the fission-fragment mass distributions on E is a simple phenomenon, which can be explained in terms of shell destruction with energy, even the qualitative description of x-dependence has a number of ambiguous points. One of them is connected to the stage of the fission process which is mainly responsible for the formation of mass division. In this connection, the two phase of a fissioning nucleus are usualy opposed, i.e. the saddle point and the scission point. For saddle and scission, the liquid-drop model predicts a substantial difference between x-dependences of their variances of the mass distributions ~ [2]. That fact may be used for testing of various hypotheses, which account for the mass distribution formation in the fission process. In this connection, of special interest is the present study of pre-actinide nuclei (x ~ 0.7) for the certain reasons:i) Symmetric fission predominates for pre-actinide nuclei, which can be an appropriate objects for the liquid-drop model descriptions.ii) It is the region of x-~0.7, where one may expect the most favourable correlation between the saddle and the scission configurations, i.e. the difference between them is sufficient to make a choice between the two hypotheses mentioned above, while it is not too large to cause substantial distortions in the effect under investigations during the descent from the fission barrier. Present experiments were performed using the AlmaAta isochronous cyclotron. The range of nuclei from T1 to At was investigated, making use of 50 MeV a-particle beam. That range was expanded later towards smaller values of x to Os, using (3He,J) reaction at a bombarding energy of 60 MeV. The procedure of complementary fragment energy spect...
The fission-fragment mass distributions for 3He-induced reactions on t62Dy, 169Tm, 176Hf~ 181Ta, and 18~ were measured at a bombarding energy of 60-65 MeV. The parameters of stability of the saddle-point configurations with respect to the massasymmetric deformations Q have been estimated from the data on the distribution variances and compared with the LDM predictions. Quantitative analysis of the fission barriers and Q(X)-dependence has shown inconsistencies in the modern description of the mass and energy macroscopic component.
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