Shape coexistence at N=20 and N=28 : study of 0+2 states in 34^{34}Si and 44^{44}S

It is well known that the nuclear shell structure changes for the most exotic nuclei. One of the consequences ofthis phenomenon is the modification of the "classical" magic numbers, as experimentally observed at N = 20 and N = 28.Nevertheless, the mechanisms responsible for such changes are still under discussion and more experimental information isneeded to better constrain the theoretical models. In these proceedings, we report on the discovery and the experimental studyby precise spectroscopy experiments of the 0+2 state in 34Si and 44S. The 34Si is located between the magic spherical 36S andthe deformed 32Mg, member of the so-called island of inversion, whereas 44S is located between the magic spherical 48Caand the deformed 42Si. Therefore, the structure of these nuclei, and in particular the phenomenon of shape coexistence, isof crucial importance to understand how the intruder configurations progressively dominate the ground state structure of themost exotic nuclei at both N = 20 and N = 28

Study of neutron rich nucleus 25^{25}F via single-step fragmentation

Expérience GANIL/SPEGInternational audienceThe structure of nucleus 25F was studied by use of in-beam -ray spectroscopy of the fragmentation of a 36S beam. The emitted rays were detected by BaF2 detectors. In the -ray spectrum obtained a wide bump between 3 and 4.5 MeV energy was observed corresponding to a set of rays, including their first and second escape peaks. In order to resolve the peaks and to determine the high-energy structure of 25F, the experimental spectra were compared to Geant4 simulations taking the complex line shape into account. The observed decomposition of the bump is in good agreement with the results of the two-step fragmentation experiment [Zs. Vajta et al., submitted to Phys. Rev. C]. Furthermore, an additional transition was observed at 2140(30) keV

Observation of bound excited states in 15^{15}B

NESTERInternational audienceThe structure of the A/Z = 3 nucleus $^{15}$B has been investigated using the in-beam $\gamma$ -spectroscopy technique with a fragmentation reaction of a $^{36}$S beam on a $^{9}$Be target at 77.5 MeV. The fragments were identified and selected by their energy loss and time of flight using the SPEG spectrograph. $\gamma$ -ray energies and intensities have been measured in coincidence with the projectile-like fragments. From this information as well as from the $\gamma\gamma$ -coincidence relationships a level scheme is proposed for $^{15}$B up to the neutron separation energy. The experimental results have been interpreted using shell model calculations in the psd valence space. Effects of the weakly bound nature of the valence neutrons have been observed

Collapse of the N=28 shell closure in the Si isotopes

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Search for neutron excitations across the N=20 shell gap in 25−29^{25-29}Ne

Expérience GANIL, équipement SISSINuclear structure of the neutron rich $^{25-29}$Ne nuclei has been investigated through the in-beam $\gamma$-ray spectroscopy technique using fragmentation reactions of both stable and radioactive beams. Level schemes have been deduced for these Ne isotopes. In order to examine the importance of intruder $fp$ configurations, they are compared to shell model calculations performed either in the restricted sd or in the larger $sdpf$ valence space. The $^{25,26}$Ne and ${27}$Ne nuclei were found to be in agreement with the $sd$ shell model calculations, whereas $^{28}$Ne exhibits signatures of the intruder $fp$ shell contribution

Publisher's Note: In-beam spectroscopic studies of the 44S nucleus [Phys. Rev. C 85, 024311 (2012)]

This paper was published online on 15 February 2012 with errors in Figs. 2 and 4. The figures has been replaced as of 28 February 2012. The figures are correct in the printed version of the journal