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HK: Physik der Hadronen und Kerne

HK 33: Heavy Ions III

HK 33.7: Vortrag

Mittwoch, 10. März 2004, 16:00–16:15, E

Angular Anisotropy in Binary and Ternary Fission of ²³⁸U Induced by MeV Neutrons — •S. Dilger¹, W. Rochow¹, F. Gönnenwein¹, and M. Mutterer² — ¹Physikalisches Institut, U Tübingen, Germany — ²IKP, TU Darmstadt, Germany

Most theories of ternary fission predict the 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 investigate the angular distribution of fission fragments. Since 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. No difference in the asymmetries could be detected. Parity conserving angular anisotropies were studied even since the early seventies. Results, however, were conflicting. Experiments were also performed at the accelerator at the University of Tübingen. For the reaction ²³⁸U(n,f) with neutrons at 1.6 MeV, an unexpected ratio of A₃/A₂ = 1.18 ± 0.08 for the anisotropies for ternary and binary fission, A₃ and A₂ respectively, was obtained. To clarify the situation, a reinvestigation with an improved experimental approach was carried out. Measurements were performed at two different neutron energies. The ratios of the angular anisotropy for ternary and binary fission are found to be A₃/A₂ = 1.06 ± 0.10 and 0.98 ± 0.11 for E_n = 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.