It is well known that the nuclear shell structure changes for the most exotic nuclei. One of the consequences of this 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 is needed to better constrain the theoretical models. In these proceedings, we report on the discovery and the experimental study by precise spectroscopy experiments of the 02+ state in 34Si and 44S⁠. The 34Si is located between the magic spherical 36S and the deformed 32Mg⁠, member of the so-called island of inversion, whereas 44S is located between the magic spherical 48Ca and the deformed 42Si⁠. Therefore, the structure of these nuclei, and in particular the phenomenon of shape coexistence, is of crucial importance to understand how the intruder configurations progressively dominate the ground state structure of the most exotic nuclei at both N = 20 and N = 28.

It is well known that the nuclear shell structure changes for the most exotic nuclei. One of the consequences of this 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 is needed to better constrain the theoretical models. In these proceedings, we report on the discovery and the experimental study by precise spectroscopy experiments of the 0(2)(+) state in Si-34 and S-44. The Si-34 is located between the magic spherical S-36 and the deformed Mg-32, member of the so-called island of inversion, whereas S-44 is located between the magic spherical Ca-48 and the deformed Si-42. Therefore, the structure of these nuclei, and in particular the phenomenon of shape coexistence, is of crucial importance to understand how the intruder configurations progressively dominate the ground state structure of the most exotic nuclei at both N = 20 and N = 28.

The ${}^{8}$He($d,p$) reaction was studied in inverse kinematics at 15.4$A$ MeV using the MUST2 Si-CsI array in order to shed light on the level structure of ${}^{9}$He. The well known ${}^{16}$O($d,p$)${}^{17}$O reaction, performed here in reverse kinematics, was used as a test to validate the experimental methods. The ${}^{9}$He missing mass spectrum was deduced from the kinetic energies and emission angles of the recoiling protons. Several structures were observed above the neutron-emission threshold and the angular distributions were used to deduce the multipolarity of the transitions. This work confirms that the ground state of ${}^{9}$He is located very close to the neutron threshold of ${}^{8}$He and supports the occurrence of parity inversion in ${}^{9}$He.

In order to solve the nature of 9He ground state, additional information on this unbound nucleus with extreme N/Z ratio was needed. The present study was performed via the (d,p) reaction, a standard tool for determination of neutron single-particle distribution.

The low-lying spectroscopy of 6He was investigated via the 2-neutron transfer reaction p(He8,t) with the 8He beam delivered by the SPIRAL facility at 15.4AMeV. The light charged particles produced by the direct reactions were measured using the MUST2 Si-strip telescope array. Above the known 2+ state, two new resonances were observed: at E⁎=2.6±0.3MeV (width Γ=1.6±0.4MeV) and at 5.3±0.3MeV with Γ=2±1MeV. Through the analysis of the angular distributions, they correspond to a 2+ state and to an L=1 state, respectively. These new states, challenging the nuclear theories, could be used as benchmarks for checking the microscopic inputs of the newly improved structure models, and should trigger development of models including the treatments of both core excitation and continuum coupling effects.

The quenching of the $N=20$ shell gap in neutron-rich nuclei is investigated by studying the single-particle structure of ${}^{27}$Ne via neutron transfer using a ${}^{26}$Ne beam. Two low-lying negative-parity intruder states have been observed, the lowest of which is identified as ${J}^{\ensuremath{\pi}}=3/{2}^{\ensuremath{-}}$, confirming earlier speculations. A level identified as 7/2${}^{\ensuremath{-}}$ is observed higher in energy than the 3/2${}^{\ensuremath{-}}$, contrary to the ordering at $\ensuremath{\beta}$-stability and at an energy significantly different from the predictions of previous shell-model calculations. The measured energies and deduced spectroscopic factors are well reproduced in full (0,1)-$\ensuremath{\hbar}\ensuremath{\omega}$ $0s$-$0p$-$0d$-$1s$-$0f$-$1p$ calculations in which there is a significant ad hoc reduction ($\ensuremath{\sim}0.7$ MeV) in the $N=20$ shell gap.

This is the publisher's version, also available electronically from http://journals.aps.org/prc/abstract/10.1103/PhysRevC.85.011302.

Fusion-evaporation in the ^124Sn+^136Xe system is studied using a high intensity xenon beam provided by the Ganil accelerator and the LISE3 wien filter for the selection of the products. Due to the mass symmetry of the entrance system, the rejection of the beam by the spectrometer was of the order of 5times10^8. We have thus performed a detailed statistical analysis to estimate random events and to infer the fusion-evaporation cross sections. No signicant decay events were detected and upper limit cross sections of 172 pb, 87 pb and 235 pb were deduced for the synthesis of ^257Rf, ^258Rf and ^259Rf, respectively.

L. Cáceres, D. Sohler, S. Grévy, C. Force, O. Sorlin Zs. Dombrádi, L. Gaudefroy, N.L. Achouri, J.C. Angélique F. Azaiez, D. Bayborodin , B. Bastin, R. Borcea, C. Bourgeois A. Buta, A. Bürger, R. Chapman, J.C. Dalouzy, Z. Dlouhy A. Drouard, Z. Elekes, S. Franchoo, S. Iacob, B. Laurent M. Lazar, X. Liang, E. Liénard, J. Mrazek , L. Nalpas, F. Negoita N.A. Orr, Y. Penionzhkevich, Zs. Podolyák, F. Pougheon P. Roussel-Chomaz, M.G. Saint-Laurent, M. Stanoiu, I. Stefan F. Nowacki, A. Poves

The results obtained from electron and in-beam spectroscopy experiments reveal that the S-44 nucleus is located in a transitional region between the spherical Ca-48 and the oblate Si-42. The comparison of the results with Large Scale Shell Model calculations points towards prolate-spherical shape coexistence where the ground state becomes the intruder configuration due to quadrupole excitations across the Z = 14 and N = 28 shell gaps.

The spectroscopy of 21O has been investigated using a radioactive 20O beam and the (d,p) reaction in inverse kinematics. The ground and first excited states have been determined to be Jpi=5/2+ and Jpi=1/2+ respectively. Two neutron unbound states were observed at excitation energies of 4.76 +- 0.10 and 6.16 +- 0.11. The spectroscopic factor deduced for the lower of these interpreted as a 3/2+ level, reveals a rather pure 0d3/2 single-particle configuration. The large energy difference between the 3/2+ and 1/2+ states is indicative of the emergence of the N=16 magic number. For the higher lying resonance, which has a character consistent with a spin-parity assignment of 3/2+ or 7/2-, a 71% branching ratio to the first 2+ state in 20O has been observed. The results are compared with new shell model calculations.

The spectroscopy of 21 O has been investigated using a radioactive 20 O beam and the (d,p) reaction in inverse kinematics.The ground and first excited states have been determined to be J π = 5/2 + and 1/2 + , respectively.Two neutron unbound states were observed at excitation energies of 4.77(10) and 6.17(11) MeV.The spectroscopic factor deduced for the lower of these, interpreted as a 3/2 + level, reveals a relatively pure (60%) 0d 3/2 singleparticle configuration, in good agreement with shell-model calculations that predict 26 O to be unbound.The large energy difference between the 3/2 + and 1/2 + states is indicative of the emergence of the N = 16 shell gap, which is estimated to be 5.1(11) MeV.For the higher-lying resonance, which has a character consistent with a spin-parity assignment of 3/2 + or 7/2 -, a 0.71(22) branching ratio to the first 2 + state in 20 O has been observed.

The structure of 44S has been studied by using delayed γ and electron spectroscopy. The decay rates of the 02+ isomeric state to the 2(1)+ and 0(1)+ states, measured for the first time, lead to a reduced transition probability B(E2: 2(1)+→0(2)+)=8.4(26) e(2) fm4 and a monopole strength ρ2(E0: 0(2)+→0(1)+)=8.7(7)×10(-3). Comparisons to shell model calculations point towards prolate-spherical shape coexistence, and a two-level mixing model is used to extract a weak mixing between the two configurations.

New experimental results on S-43 and S-44 reveal that these nuclei are located in a transitional region of shape coexistence between the spherical Ca-48 and the oblate Si-42. The origin of the deformation is discussed in terms of the evolution of the single particle energy levels leading to the compression of the energy difference of the orbitals in the sd and pf shells for protons and neutrons, respectively. Therefore, due to quadrupole excitations across the Z = 14 and N = 28 gaps, the intruder configuration in the neutron rich S isotopes became the ground state.

We investigated the low-lying spectroscopy of 6He via the 2-neutron transfer reaction induced by the 8He SPIRAL beam at 15.4 A.MeV on a proton-rich target. The light charged recoil particles produced by the direct reactions were measured using theMUST2 Si-strip telescope array. Two new resonances were observed above the known 2+ state in 6He, and the angular momentum transfer was deduced through the analysis of the angular distributions. Results are discussed in comparison with the recent calculations of various nuclear structure theories which include the coupling to the continuum technique and to the ones which give an understanding of the cluster correlations in the light weakly-bound nuclei.

The structure of $^{44}$S has been studied using delayed $\gamma$ and electron spectroscopy at \textsc{ganil}. The decay rates of the 0$^+_2$ isomeric state to the 2$^+_1$ and 0$^+_1$ states have been measured for the first time, leading to a reduced transition probability B(E2~:~2$^{+}_1$$\rightarrow$0$^{+}_2)$= 8.4(26)~e$^2$fm$^4$ and a monopole strength $\rho^2$(E0~:~0$^{+}_2$$\rightarrow$0$^{+}_1)$ =~8.7(7)$\times$10$^{-3}$. Comparisons to shell model calculations point towards prolate-spherical shape coexistence and a phenomenological two level mixing model is used to extract a weak mixing between the two configurations.

We report on the g-factor measurement of the first isomeric state in (16)43S27 [Ex=320.5(5) keV, T1/2=415(5) ns, and g=0.317(4)]. The 7/2- spin-parity of the isomer and the intruder nature of the ground state of the nucleus are experimentally established for the first time, providing direct and unambiguous evidence of the collapse of the N=28 shell closure in neutron-rich nuclei. The shell model, beyond the mean-field and semiempirical calculations, provides a very consistent description of this nucleus showing that a well deformed prolate and quasispherical states coexist at low energy.

The reaction O-20(d,t) has been studied in inverse kinematics using a secondary radioactive beam produced with the SPIRAL facility at GANIL. Fragments, light charged particles and gamma rays were measured with the TIARA, MUST2, VAMOS and EXOGAM detectors and preliminary results are reported. The level scheme of O-19 is built and the spin and parity of one state is tentatively assigned using the one-neutron transfer angular distribution.

We report on the g-factor measurement of the first isomeric state in 43 16 S 27 [E x = 320.5(5) keV, T 1/2 = 415(5) ns, and g = 0.317(4)]. The 7/2 - spin-parity of the isomer and the intruder nature of the ground state of the nucleus are experimentally established for the first time, providing direct and unambiguous evidence of the collapse of the N = 28 shell closure in neutron-rich nuclei. The shell model, beyond the mean-field and semiempirical calculations, provides a very consistent description of this nucleus showing that a well deformed prolate and quasispherical states coexist at low energy.