Mass and charge distributions in the very asymmetric mass region of the neutron induced fission of Np-238

The mass-separator Lohengrin was used to measure the yields of the light fi ssion products with A = 74-85 and their nuclear charge and kinetic energy d istributions from the odd-Z compound nucleus Np-239* formed by double captu re of thermal neutrons. The mass yield distribution reveals an influence of the fragment shell with N = 50 affecting also the nuclear charge and kinet ic energy distributions. An odd-even effect for protons is found in the ver y asymmetric mass division, increasing from 4% to 35% with increasing fissi on asymmetry. This is in contrast to findings in normal asymmetric fission (region of high fission yields) where no odd-even effect for protons was ob served. An odd-even effect for neutrons is also found comparable in size wi th that for protons. The latter effect exists also in normal asymmetric fis sion and is at least partly attributed to prompt neutron emission from the fragments. Some information on the number of prompt neutrons emitted is als o obtained. From this information and from the energy dependence of the odd -even effect for protons it is concluded that the very light fragments orig inate from a cold and nearly undeformed light sphere of a dumbbell scission configuration characterised by a double shell closure with 28 protons and about 50 neutrons. In contrast to the double shell closure at mass 132 (Z = 50, N approximate to 82) the two shell closures in the light sphere do not coincide at one mass but are realised at A = 70 and 80, respectively. This leads to a layered structure of the light sphere of the dumbbell scission configuration. (C) 2001 Elsevier Science B.V, All rights reserved.