[en] We investigate the shell evolution in unstable nuclei with large-scale shell-model calculations. Since the tensor force has been pointed out to be promising in accounting for the shell evolution, we construct a new interaction in the sd-pf shell region whose tensor force in the cross-shell part is determined with attention. The resulting strength is much stronger than the Millener-Kurath (MK) interaction which has been often used as the cross-shell interaction, giving rise to a significant difference from that by MK in the proton shells in the N = 28 region and the nuclear structure of ⁴²Si

[en] Excited states in the nuclei "3"8","4"0","4"2Si have been studied using in-beam γ-ray spectroscopy following multi-nucleon removal reactions to investigate the systematics of excitation energies along the Z=14 isotopic chain. The most probable candidates for the transition from the yrast 4"+ state were tentatively assigned among several γ lines newly observed in the present study. The energy ratios between the 2_1"+ and 4_1"+ states were obtained to be 2.09(5), 2.56(5) and 2.93(5) for "3"8","4"0","4"2Si, respectively, indicating a rapid development of deformation in Si isotopes from N=24 to, at least, N=28. (authors)

No abstract available

[en] Complete text of publication follows. The persistence of the N=28 shell closure at Z=14 was studied by in-beam ray spectroscopy of the very neutron-rich ⁴²Si. Since ⁴²Si is next to the heaviest bound Si nucleus, the search for its excited states is an experimental challenge. To determine the energy of the 2₁⁺ state in ⁴²Si the method of two-step-fragmentation was used. The experiment was carried out at GANIL in Caen, France. A 4μA ⁴⁸Ca beam at 60 MeV/u was fragmented in the SISSI device. The primary reaction products were selected by the α spectrometer the magnets of which were optimised for ⁴⁴S. The beam particles were identified event by event using standard energy loss and time-of-flight method. The cocktail beam impinged onto a secondary ⁹Be target of 185 mg/cm². The nuclei produced in the second fragmentation were transmitted through the SPEG spectrometer the Bρ value of which was adjusted to maximise the collection of ⁴²Si. The reaction products were unambiguously identified through their energy loss in an ionisation chamber and their mass over charge (A/Q) ratio deduced from time-of-flight and Bρ measurements. The ⁴²Si nuclei were produced exclusively from the ⁹Be(⁴⁴S,⁴²Si) reaction. The total number of ⁴²Si nuclei observed is 132, which corresponds to a cross section of 102 ± 26 μbarn. To detect γ rays the secondary target was surrounded by the 4π array Chateau de Crystal consisting of 74 BaF₂ scintillators. The ray spectra were obtained by selecting event-by-event the incoming nuclei and the ejectiles after the secondary target. Since the momentum of the reaction was measured directly with the SPEG spectrograph, a precise Doppler correction could be performed. As it can be seen in figure 1 a clear peak on a low background at 770 ± 15 keV energy appears in the spectrum extracted in coincidence with ⁴²Si nuclei. The low energy of the transition is a strong indication for the collapse of the N=28 spherical shell closure at Z=14 and the deformed nature of ⁴²Si. The small production cross section of ⁴²Si and the weak population of its excited states, on the other hand, can be interpreted as signatures of a closed proton shell. These findings together suggest that ⁴²Si is a deformed closed shell nucleus in agreement with the predictions of recent mean field calculations. (author)

[en] Measurements of the beta-decay half-lives of neutron-rich nuclei (Mg-Ar) in the vicinity of the N=28 shell closure are reported. Some 22 half-lives have been determined, 12 of which for the first time. Particular emphasis is placed on the results for the Si isotopes, the half-lives of which have been extended from N=25 to 28. Comparison with QRPA calculations suggests that ⁴²Si is strongly deformed. This is discussed in the light of a possible weakening of the spin-orbit potential

[en] The study of various magic nuclei with 28 neutrons: ⁴⁸Ca, ⁴²Si, ⁴³S and ⁴⁴S show that they are more and more deformed as they are more away from the stability valley. ⁴⁸Ca is spherical while ⁴²Si is so deformed that it looks like a disk and its first energy level is only at 0.770 MeV. This study shows also that some nuclei with 28 neutrons have not the features of magic nuclei. It seems that N=28 is no longer a magic number in areas away from the stability valley. (A.C.)

[en] The study of the evolution of the N = 28 magic shell has been started about 15 years ago. A lot of experimental studies aimed at determining whether the N = 28 shell closure is eroded in very neutron-rich nuclei through complementary methods. The very first hints of the vanishing of the N = 28 gap were obtained by -βdecay [1], mass measurements [2] and Coulomb-excitation [3,4] experiments. Theoretical studies came progressively to the conclusion that the erosion of the N = 28 shell gap should lead to shape coexistence in 4^4S and deformation in 4^2Si (e.g. ). For this latter, it was however not clear whether spherical shape would be preferred to deformation. Recent experimental campaigns were carried out worldwide to study atomic masses [5], β-decay [6], nuclear spectroscopy [7,8,9,10,11,12], neutron single-particle energies [13] to probe the onset of collectivity along the N=28 isotones. With these pieces of information in hand and new unpublished results, the progressive collapse of the N = 28 shell closure is now established. Interestingly this study is also ideal for probing the nuclear forces such as the spin-orbit and tensor forces in nuclei. Experimental highlights will be shown at the N = 28 shell closure, in connection with the state of the art theoretical descriptions. More generally, the shell-breaking mechanism discovered at N=28 should apply to other shell closures, such as the N = 14, N = 50 and N = 82 ones, in which the same spin orbit forces are at play. However they seem to have various consequences with respect to shell erosion...(author)

[en] The effect of the tensor term in the Skyrme interaction has been estimated in calculating the static and dynamic nuclear properties in sd and fp-shell model spaces nuclei. The nuclear shell gaps have been studied with different Skyrme parameterizations; Skxta and Skxtb with tensor interaction, SkX, SkM, and SLy₄ without tensor interaction, and Skxcsb with consideration of the effect of charge symmetry breaking. We have examined the stability of N = 28 for ⁴²Si and ⁴⁸Ca. The results showed that the disappearance of the magicity occurs in the shell closure of ⁴²Si. Furthermore, excitation energy, quadrupole deformation, neutron separation energy, pairing energy, and density profile have also been calculated. Quadrupole deformation indicates a strong pairing correlation between neutrons and protons for ⁴²Si. We found that the effect of the tensor force is most distinct when the proton shell is unoccupied. Consequently, we conclude that the interaction between neutrons depends on a correlation between n-p pairings leading to the disappearance of magic. (author)

[en] Mass measurements of very neutron-rich nuclei near the N=20 and 28 shell closures are presented. Seven masses have been determined for the first time and the precision of 36 masses has been significantly improved. These results are used to investigate the evolution of the odd-even staggering of binding energies with neutron number. Special attention is paid to the evolution of the N=28 shell closure as the neutron dripline is approached. Changes in shell structure are observed around N=28 for the P and S isotopes but not for Si. This may be interpreted as a persistence of the shell closure at N=28 or as the result of a very sudden onset in deformation at ⁴²Si

[en] The structure of exotic nuclei around N = 28 is investigated in the sd-pf shell-model space using a new effective interaction. The cross-shell part of the interaction is provided by the monopole-based universal interaction which has been successful in accounting for single-particle evolution in several mass regions. Focusing on the nuclear structure that is sensitive to the shell evolution, we show successful results for the proton-hole states in K isotopes and large deformation in ⁴²Si. The results demonstrate that the present scheme may be a promising way for constructing an effective interaction for other mass regions.