CNRS Université Bordeaux

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Densités des niveaux et sections d’efficaces dans les Actinides

Jonathan Wilson, IPN Orsay
Vendredi 29 Octobre 2010 à 10h30 Salle des Séminaires

The nuclear level density is an essential quantity in the determination of the thermodynamic properties of nuclei at increasing excitation energies. From the level density it is possible to extract entropy, temperature and heat capacity - quantities that describe the many-particle behaviour of the system. In addition, accurate level density information is crucial for the calculation of nuclear reaction cross sections, which in turn can be important for applications such as simulation of innovative nuclear reactor cores. This presentation will show the first results from a new program of experiments to measure nuclear level densities in actinide nuclei. These experiments, which have never been attempted before, have three important goals touching on both fundamental and applied physics : 1) To provide systematic level density information for improving cross section calculations for nuclei where direct measurements are difficult or impossible. 2) The simultaneous measurements of compound nuclear decay probabilities and hence n,gamma cross sections using the surrogate method. 3) The exploration of fine structure in actinide level densities and gamma ray strength functions, of particular interest for fundamental physics reasons. The recent experiments were carried out at the Oslo cyclotron using 232Th(d,x) and 232Th(3He,x) transfer reactions to populate several actinide nuclei. n,gamma cross sections below the fission threshold have been extracted using the surrogate method for two important thorium cycle nuclei, 231Pa, and 230Th, where there is no direct experimental information. Furthermore, an indirect measurement of the 232Th(n,gamma) cross section has been made and compared with the direct neutron-induced measurements from the nTOF facility, thus providing a stringent test of the validity of the surrogate method. In addition level densities in 231,232,233Th up to the binding energy can be extracted using the Oslo method.