Fission-mode calculations for U-239, a revision of the multi-modal random neck-rupture model

In the course of an experimental study of the fragment characteristics afte r neutron-induced fission of U-238 With incident neutron energies between 1 .2 and 5.8 MeV fission-mode calculations in the frame of the multi-modal ra ndom neck-rupture model have been performed. During these calculations some technical parts of the model have been revised. The identification of fiss ion modes is now based on unequivocal and reproducible criteria. The Raylei gh criterion has consequently been applied to determine each possible sciss ion configuration. The most remarkable new results are that all physically relevant fission modes branch off in the second potential minimum exhibitin g different outer barriers, and that the total kinetic energy distribution of the fission fragments is a direct consequence of the Rayleigh criterion, The fragment characteristics as the mean mass, the mean total kinetic ener gy and the corresponding width obtained from the fission-mode calculations compare reasonably well with the experimental findings. For the first time the weighted fission cross sections through each particu lar fission mode have been analyzed simultaneously using a Hill-Wheeler typ e expression for the transmission through a double-humped fission barrier, The results support the picture of individual outer barriers with slightly different penetrabilities and a slightly lower inner barrier. (C) 1998 Else vier Science B.V.