No abstract

The energies of the excited states in very neutron-rich (42)Si and (41,43)P have been measured using in-beam gamma-ray spectroscopy from the fragmentation of secondary beams of (42,44)S at 39A MeV. The low 2(+) energy of (42)Si, 770(19) keV, together with the level schemes of (41,43)P, provides evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that (42)Si is best described as a well-deformed oblate rotor.

PACS numbers: to be definedMany properties of the atomic nucleus, such as vibrations, rotations and incompressibility can be interpreted as due to a two-component quantum liquid of protons and neutrons. Electron scattering measurements on stable nuclei demonstrate that their central densities are saturated, as for liquid drops. In exotic nuclei near the limits of mass and charge, with large imbalances in their proton and neutron numbers, the possibility of a depleted central density, or a "bubble" structure, was discussed in a recurrent manner since the seventies. Here we report first experimental evidence that points to a depletion of the central density of protons in the short-lived nucleus 34 Si. The proton-to-neutron density asymmetry in 34 Si offers the possibility to place constraints on the density and isospin dependence of the spin-orbit force -on which nuclear models have disagreed for decades-and on its stabilizing effect towards limits of nuclear existence.Microscopic systems composed of atoms or clusters can exhibit intrinsic structures that are bubble-like, with small or depleted central densities. For example, the fullerene molecules, composed of C atoms, are structures with extreme central depletion [1]. In nuclear physics, depletions also arise in nuclei with well-developed cluster structures when clusters are arranged in a triangle or ring-like structure -such as in the triple-α Hoyle state [2,3]. Unlike such a non-homogeneous, clustered system, central density depletions or bubble-like structures would be much more surprising in homogeneous systems, such as typical atomic nuclei with properties characteristic of a quantum liquid [4].This hindrance of bubble formation in atomic nuclei is inherent in the nature of the strong force between nucleons, which is strongly repulsive at short distances (below 0.7 fm), attractive at medium range (≈1.0 fm) and vanishes at distances beyond 2 fm. In a classical picture, the medium-ranged attraction of nuclear forces implies that nucleons interact strongly and attractively only with immediate neighbors, leading to a saturation of the nuclear central density, ρ 0 . Quantum mechanically, the delocalization of nucleons [5] leads to a further homogeneity of the density. Extensive precision electron scattering studies from stable nuclei [6] confirm that their central densities are essentially constant, with ρ 0 ≈ 0.16 fm −3 , independent of the number of nucleons A. As a consequence, like a liquid drop, the nuclear radii and volumes increase as A 1/3 and as A, respectively. Thus, a priori, bubble-like nuclei with depleted central densities are unexpected.Historically, the possibility of forming bubble nuclei was investigated theoretically in intermediate-mass [7][8][9][10], superheavy [11] and hyperheavy systems [12]. In general, central depletions will arise from a reduced occupation of single particle orbits with low angular momentum . These wave functions extend throughout the nuclear interior whereas those with high-are more excluded by centrifugal forces. For...

The 0 þ 2 state in 34 Si has been populated at the GANIL-LISE3 facility through the decay of a newly discovered 1 þ isomer in 34 Al of 26(1) ms half-life. The simultaneous detection of e þ e À pairs allowed the determination of the excitation energy Eð0 þ 2 Þ ¼ 2719ð3Þ keV and the half-life T 1=2 ¼ 19:4ð7Þ ns, from which an electric monopole strength of 2 ðE0Þ ¼ 13:0ð0:9Þ Â 10 À3 was deduced. The 2 þ 1 state is observed to decay both to the 0 þ 1 ground state and to the newly observed 0 þ 2 state [via a 607(2) keV transition] with a ratio Rð2Gathering all information, a weak mixing with the 0 þ 1 and a large deformation parameter of ¼ 0:29ð4Þ are found for the 0 þ 2 state, in good agreement with shell model calculations using a new SDPF-U-MIX interaction allowing np-nh excitations across the N ¼ 20 shell gap.