A. Ya. Rusanov

Asymmetric fission of the pre-actinide nuclei

The fragment mass and energy distributions have been measured for fission of213At,212Po,210Po,208Po,209Bi,207Bi,205Bi,204Pb and201Tl in (α, f) and (p, f) reactions. The yield of the asymmetric component has been found to decrease with decreasingZ andN of the fissile nucleus and vanish at201Tl. The analysis has shown the two main fission modes (symmetric and asymmetric) to be determined by the two distinct valleys in the deformation potential energy, which had been theoretically predicted by Pashkevich. The boundaries of validity of the hypothesis of two distinct fission modes have been estimated at 200 <A < 232 and 82 <Z < 92. The two-mode hypothesis is strongly substantiated in the paper.

New experimental data on the formation of the symmetric fission mode

The variances of the fission-fragment mass distributions,σM2, were measured for (α,f) and (3He,f) reactions in the interval between Os and At. The analysis of the experimental data suggests that the properties of symmetric fission aroundx-0.7 are mainly determined by the barrier passing stage. This conclusion is in discrepancy with results of dynamic calculations.

213At fission asymmetry

Abstract Measurements of fission fragment mass and energy distributions were carried out on the 209 Bi (α, f ) reaction for the α -particle energy range 34.7–50.0 MeV. Asymmetric fission of 213 At was observed, the contribution of which was equal to some tenths of a percent, slightly dependent on excitation energy. The hypothesis of two independent fission modes is applied for the interpretation of the experimental results.

Stability of heated nuclei against mass-asymmetric deformations in the fission process

The fission-fragment mass distributions for3He-induced reactions on162Dy,169Tm,176Hf,181Ta, and180W were measured at a bombarding energy of 60–65 MeV. The parameters of stability of the saddle-point configurations with respect to the mass-asymmetric deformationsQ have been estimated from the data on the distribution variances and compared with the LDM predictions. Quantitative analysis of the fission barriers andQ(X)-dependence has shown inconsistencies in the modern description of the mass and energy macroscopic component.