Étude de la structure des noyaux riches en neutrons autour de la fermeture de couches N=28 par spectroscopie gamma en ligne.

Abstract : For a few years now, a loss of magicity in neutron-rich nuclei near the neutron drip-line at N=28 has been suggested and observed. Deformation in these nuclei has been observed. The deformation was explained in S isotopes as being due to a moderate reduction of the N=28 shell closure together with a proton induced collectivity originating from the near degeneracy of the proton d$_{3/2}$ and s$_{1/2}$ orbitals. As a consequence, the observed deformation seems to result from a subtle interplay between neutron and proton excitations. Since the proton configuration in the Si isotopes is expected to be more stable due to the Z=14 sub-shell gap, $^{42}$Si was considered as a key nucleus in order to distinguish the different effects responsible for the structural changes observed at N=28. Even if it is at the limits of our technical possibilities, an in-beam gamma-spectroscopy experiment using two-step fragmentation and one or several nucleons knockout reaction mechanisms was performed at GANIL. The measurement of the energy of the first excited state in $^{42}$Si, combined with the observation of $^{38,40}$Si and the spectroscopy of $^{41,43}$P, has given evidence for the loss of magicity at N=28 far from stability. Modifications of the effective interaction used in modern shell model calculations have been completed following this investigation, increasing its predictive character. This study conforms the role of the tensor force and the density dependence of the spin-orbit interaction in the collapse of the N=28 shell closure.
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B. Bastin. Étude de la structure des noyaux riches en neutrons autour de la fermeture de couches N=28 par spectroscopie gamma en ligne.. Physique Nucléaire Théorique [nucl-th]. Université de Caen, 2007. Français. ⟨tel-00269886⟩

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