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Noyau à halo : spectroscopie de niveaux excités liés par excitation coulombienne et développement d'une méthode originale pour la mesure des fonctions d'excitation

Abstract : This thesis presents the study of halo nuclei (whose last nucleons are weakly bound and able to move far from the core) in two different and original ways: the first one is a search of bound excited levels by means of Coulomb excitation of medium energy radioactive beams. After a brief recall of classical and relativistic Coulomb excitation theory (Winther and Alder's model), we describe experimental set-up. We used Doppler effect as a signature of the de-excitation of the projectile. The data analysis shows that we populated the excited state E1 of 11Be nucleus with the cross-section rather different from theoretical value. Different explanations are possible: -destructive Coulomb-nuclear interferences; - an excitation followed by the dissociation of the projectile. This is for the first time that such a result is obtained for radioactive beams at high energy: it shows that Coulomb excitation is a reliable method for the spectroscopy of halo nuclei. In the case of the other studied nuclei 12,14Be and 8He, we havn't found excited bound state: upper limits for E1 transition probabilities are given. The second part of this report presents an original technique allowing the measurement of fragmentation excitation function with a beam at fixed energy: it is suitable to study dissociation of weakly bound nuclei. The technique uses a telescope of silicon detectors which plays in the same time the role of target, slowing down material and effective detector. The method used for both reactions (triton, deuteron) and 8B, 7Be plus proton proved to be of limited use in the low energy domain due to experimental conditions. However, even if these difficulties are overcome, it seems very difficult to reach the Coulomb barrier domain with our method.
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J.M. Corre. Noyau à halo : spectroscopie de niveaux excités liés par excitation coulombienne et développement d'une méthode originale pour la mesure des fonctions d'excitation. Physique Nucléaire Expérimentale [nucl-ex]. Université de Caen, 1994. Français. ⟨in2p3-00010206⟩

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