Nuclear octupole deformation and the mechanism of fission

Abstract

The stability of nuclei against an octupole deformation has been calculated. It is found that the interaction of levels of opposite parity makes the nuclei in certain regions very soft to octupole deformation. The negative parity collective states in heavy nuclei and in the rare earth region can be interpreted as octupole vibrations. The predicted A-dependence of the energy of these states agrees well with experimental data. For very elongated shapes, which are of interest in fission, it is found that stable octupole deformation occurs. The potential surface of the fission barrier will therefore possess two asymmetric valleys. There is a linear relationship between the mass ratio of the fission fragments and the degree of octupole deformation at the saddle point. The occurrence of symmetric fission in the lighter elements is explained. The influence of the octupole deformation on the fission barrier also provides a basis for understanding the variation in fission threshold. The experimental data agree with the calculated values. The peak to valley ratio in the mass distribution is calculated, with the assumption of statistical equilibrium at the saddle point. Good agreement is found for excitation energies below 10 MeV. For higher energies a symmetric component appears which rises with increasing energy.