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Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

  • Nuclei
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Zeitschrift für Physik A Atoms and Nuclei

Abstract

Emission of4He in the reaction 334 MeV40Ar+238U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the4He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed4He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the4He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average4He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.

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Author notes

  1. N. N. Ajitanand

    Present address: Bhabha Atomic Research Centre, 400085, Trombay, Bombay, India

  2. D. Guerreau

    Present address: GANIL, B.P. 5027, F-14021, Caen Cedex, France

Authors and Affiliations

  1. Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York, USA

    E. Duek, N. N. Ajitanand, John M. Alexander, Louis C. Vaz & D. Guerreau

  2. Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA

    D. Logan, M. Kildir, Morton Kaplan & D. J. Moses

  3. Department of Physics and Geoscience, Montclair State College, Upper Montclair, New Jersey, USA

    L. Kowalski

  4. Lawrence Berkeley Laboratory, Berkeley, California, USA

    M. S. Zisman

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  1. E. Duek
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Duek, E., Ajitanand, N.N., Alexander, J.M. et al. Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U. Z Physik A 317, 83–100 (1984). https://doi.org/10.1007/BF01420452

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