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Dynamics of Fusion-Fission and Fission-Scission Phenomena

Obiettivo

In spite of 60 years of intense research in nuclear fission there are basic questions still open, which are of interest in a context beyond standard fission. In particular, the main characteristics of fission in the super heavy nuclei only start to be unravelled and the scission phase of fission is poorly understood. On one side, very recently, the synthesis at the FLNR, Dubna, of new elements has considerably boosted the interest for capture reactions induced by stable n-rich heavy ions (48Ca, 58Fe) leading to the super heavy region. Indeed, the knowledge of the excitation functions for capture reactions (fusion-fission and quasifission) is crucial for the prediction of the optimum road to the synthesis of new isotopes and elements. The long-standing problem of disentangling these two processes which, present quite similar characteristics, has lately found a solution. Thanks to both, high quality data (for Mass-TKE Distributions and Pre-scission Neutron Multiplicities) obtained in previous INTAS Projects on one hand, and to a new data analysis protocol (Improved Backtracing) on the other hand, the ratio of the cross sections for fusion-fission and quasifission can be deduced allowing an unbiased comparison with dynamical models. The study of the fission characteristics of superheavy nuclei can now be investigated more properly and in particular the description of the fission modes for these nuclei can be put on stage and compared with the experimental situation in more standard heavy nuclei. A comparison with improved dynamical models can give access to the dissipation parameter and to its evolution with the entrance channel conditions (temperature, angular momentum and asymmetry) for both capture reaction channels addressed. As regards scission, the fission process becomes non-adiabatic at this stage and there is no generally accepted theoretical model available. It is here proposed to investigate experimentally scission phenomena in low energy (n,f) and (sf) reactions. Several experimental approaches to the problem have been explored in a previous Project. Keywords are the study of some fine but revealing details in ternary and quaternary decays by exploiting P-even, P-odd and T-odd correlations between fragments and light charged particles born at scission. It has become evident, however, that a better understanding of scission cannot be reached without taking into account neutrons. Even the question whether neutrons are ejected at all at scission is experimentally virtually unsettled. It is put forward that, besides the measurement of neutron-fragment angular correlations, also neutron-neutron correlations should carry relevant information. It is even speculated that possibly the analogue to the famous Brown-Twiss experiment, with fission neutrons replacing the gammas in astronomy, could contribute to our knowledge on scission neutrons and, more generally, on scission. "On-line' support by theory is integrated.

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Universit Libre de Bruxelles
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Avenue F.D. Roosevelt 50
1050 Brussels
Belgio

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Ultimo aggiornamento: 6 Settembre 2004

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Permalink: https://cordis.europa.eu/project/id/INTAS2003-51-6417/it

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