Angular anisotropy in the ternary nuclear fission. Study of the angular distribution of the fission fragments in the reaction 238U(n,f)

Most theories of ternary fission predict ternary particles to be formed in the final stage of the fission process, i.e. at scission. A way to confirm this assumption is to analyse the angular distribution of fission fragments. Since it is well established that this distribution is determined near the saddle point of fission, the angular distributions of ternary and binary fission should not differ. Parity violating angular asymmetries of fragments from fission reactions induced by cold polarised neutrons have been compared in recent years for binary and ternary fission. Within experimental errors no difference in the asymmetries could be detected. On the other hand, parity conserving angular anisotropies of fragments from several fission reactions induced by MeV neutrons have been under study starting even in the early seventies. Results, however, were conflicting. One of these experiments was performed at the 3.5 MeV van de Graaff accelerator at the University of Tuebingen. Measuring fragment anisotropies relative to the neutron beam for the reaction 238U(n,f) with neutrons of 1.6 MeV, L. Bondarenko et al. obtained surprisingly a ratio of A3/A2=1.18±0.08 for the anisotropies for ternary and binary fission, A3 and A2, respectively. To clarify the situation, a reinvestigation of the reaction with a completely new and improved experimental approach was carried out. Fission fragments were measured by means of an ionisation chamber, while ternary particles were detected by surface barrier semiconductor diodes. Measurements were performed at two different neutron energies at 1.6 and 1.8 MeV. For binary fission the anisotropies are reported in the literature to be 0.58 and 0.32, respectively. Therefore, different saddle point configurations are involved. In the present experiment the anisotropies (A2) for binary fission are measured to be 0.58±0.03 and 0.35±0.03, respectively. The ratios of the angular anisotropy for ternary and binary fission are found in the present study to be A3/A2=1.04±0.09 and 0.97±0.11 for En=1.6 and 1.8 MeV, respectively. Both results are compatible with A3/A2=1. Hence, the conclusion is drawn that ternary particles are not yet detectable at the saddle point. They are formed in the very last stage at scission