Angular distributions in quasi-fission reactions |
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| Institution: | 1. Graduate School, China Academy of Engineering Physics, Beijing 100193, China;2. Institute of Applied Physics and Computational Mathematics, Beijing 100088, China;3. Laboratory of Computational Physics, Institute of Applied Physics and Computational Mathematics, Beijing 100088, China;1. Department of Nuclear Physics, Research School of Physics, Australian National University, Canberra, ACT 2601, Australia;2. Department of Theoretical Physics, Research School of Physics, Australian National University, Canberra, ACT 2601, Australia;1. Department of Physics, University of Jyväskylä, P.O. Box 35, FI-40014, Jyväskylä, Finland;2. Centre d''Etudes Nucléaires de Bordeaux Gradignan, UMR 5797 CNRS/IN2P3 - Université de Bordeaux, 19 Chemin du Solarium, CS 10120, F-33175, Gradignan Cedex, France;3. National Research Center “Kurchatov Institute” - KCTEP, B. Cheremushkinskaya 25, 117218, Moscow, Russia;4. Natural Resources Institute Finland, Yliopistokatu 6B, FI-80100, Joensuu, Finland;5. Finnish Institute for Educational Research, University of Jyväskylä, P.O. Box 35, FI-40014, Jyväskylä, Finland;6. Center for Theoretical Physics, Sloane Physics Laboratory, Yale University, New Haven, CT 06520-8120, USA;7. Institute for Theoretical and Experimental Physics, NRC “Kurchatov Institute”, B. Cheremushkinskaya 25, 117218, Moscow, Russia |
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| Abstract: | Angular distributions for fission-like fragments were measured in the systems 50Ti, 56Fe + 208Pb by applying an off-line K X-ray activation technique. The distributions d2σ/dθdZ exhibit forward-backward asymmetries that are strongly Z-dependent. They result from a process (quasi-fission) which yields nearly symmetric masses in times comparable to the rotational period of the composite system. A method for obtaining the variance of the tilting angular momentum, K02, from these skewed, differential angular distributions is described. The results indicate that the tilting mode is not fully excited in quasi-fission reactions. The results are compared to the sum of the variances of all statistical spin components, measured via γ-multiplicities. Integration of the angular distributions d2σ/dθdZ over all values of Z yields the integral angular distributions dσ/dθ and dσ/dΩ symmetric around 90°. The associated unusually large anisotropies do not at all provide an adequate basis for tests or modifications of the transition-state theory. A deconvolution of d2σ/dθdZ is performed with gaussian distributions depending on rotational angles Δθ extending over a range of up to 540°. From the mean values 〈Δθ〉 a time scale for the evolution of K0 is calculated. |
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