Étude de la méthode de substitution à partir de la mesure simultanée des probabilités de fission et d'émission gamma des actinides 236U, 238U, 237Np et 238Np

Abstract : Neutron-induced cross sections of short-lived nuclei are important in various fields such as fundamental physics, astrophysics or nuclear energy. However, these cross sections are often extremely difficult to measure due to high radioactivity of the targets involved. The surrogate-reaction method is an indirect way to determine neutron-induced cross sections of short-lived nuclei. In order to study the validity of the method, we have measured for the very first time in a surrogate-reaction experiment simultaneously fission and gamma-decay probabilities for the actinides 236U, 238U, 237Np and 238Np. This is challenging because one has to remove the gamma rays emitted by the fission fragments. The measurement was performed at the Oslo cyclotron. Our results show that for a given excitation energy, our gamma-decay probabilities are several times higher than neutron-induced probabilities, which can be attributed to differences in spin distribution between the two types of reactions. On the other hand, our fission probabilities are in good agreement with neutron-induced data. Statistical-model calculations applied with standard parameters cannot repro duce the weak spin sensibility to variations of the angular momentum observed for the fission probabilities. However, it is possible to repro duce the experimental observations by considering a stronger increase of the moment of inertia of the fissionning nucleus with deformation. Further theoretical efforts are needed to improve the understanding of our results.
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Q. Ducasse. Étude de la méthode de substitution à partir de la mesure simultanée des probabilités de fission et d'émission gamma des actinides 236U, 238U, 237Np et 238Np. Physique Nucléaire Expérimentale [nucl-ex]. UNIVERISITE DE BORDEAUX, 2015. Français. ⟨tel-01262725⟩

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