Constraining statistical-model parameters for symmetric and asymmetric fission, D. Mancusi1, R. J. Charity2, and J. Cugnon3 , [1] CEA, Centre de Saclay, IRFU/Service de Physique Nuclaire, F-91191 Gif-sur-Yvette, France, [2] Department of Chemistry, Washington University, St. Louis, Missouri 63130, USA, [3] University of Liège, AGO Department, allée du 6 Août 17, bât. B5, B-4000 Liège 1,Belgium  − The de-excitation of compound nuclei has been successfully described for several decades by means of statistical models. However, it is not yet possible to make quantitatively accurate predictions without some fine-tuning of the model parameters.  This is partly due to the vastness of the amount of nuclear data that must be fed into the models, and partly to the uncertainties in the fundamental ingredients of de-excitation, such as level densities and emission barriers.  Even the choice of the mathematical formalism is not devoid of confusion.

 

The degeneracy of the model ingredients can be partially lifted by studying compound nuclei with different masses, charges, excitation energies and spins. Equivalently, one can apply the model to several entrance channels, which populate different regions of the parameter space of the compound nucleus.  One should then be able to identify the essential physical ingredients for the description of the de-excitation. 

 

Fusion reactions play an important role in this strategy because they fix three out of four of the compound-nucleus parameters (mass, charge and excitation energy).  If the incomplete-fusion component is negligible, the only uncertainty on the compound nucleus comes from the spin distribution. Thus, fusion reactions minimize the uncertainty on the entrance channel and allow the formulation of global systematics.  However, some de excitation channels, such as fission, are quite sensitive to spin.  Other entrance channels can then be used to discriminate between equivalent parameter sets.

 

We have focused our interest on fission and intermediate-mass-fragment emission cross sections of compound nuclei with 70 ≤ A ≤ 240.  The statistical de-excitation model is GEMINI++.  The choice of the observables is natural in the framework of GEMINI++, which describes IMF emission using a fission-like formalism.  Equivalent parameter sets for fusion-fission reactions can be resolved using another entrance channel (spallation). IMF emission can be constrained in a similar way.  This promising strategy can lead to the identification of a minimal set of physical ingredients necessary for a unified quantitative description of nuclear de-excitation.