The root-mean-square (rms) nuclear charge radius of 8 He, the most neutron-rich of all particlestable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of 6 He was measured to be 2.068(11) fm, in excellent agreement with a previous result.The significant reduction in charge radius from 6 He to 8 He is an indication of the change in the correlations of the excess neutrons and is consistent with the 8 He neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations. * Electronic address: pmueller@anl.gov 1

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The unbound nucleus 18 Na, the intermediate nucleus in the two-proton radioactivity of 19 Mg, was studied by the measurement of the resonant elastic scattering reaction 17 Ne(p, 17 Ne)p performed at 4 A.MeV. Spectroscopic properties of the low-lying states were obtained in a R-matrix analysis of the excitation function. Using these new results, we show that the lifetime of the 19 Mg radioactivity can be understood assuming a sequential emission of two protons via low energy tails of 18 Na resonances. From near to beyond the drip-lines, the nuclear force is no longer able to bind the interacting nucleons leading to instability of nuclei with respect to nucleon emission. On the proton-rich side of the chart of nuclides, the pairing force may lead to a situation where a drip-line nucleus is bound with respect to single proton emission but unbound to two-proton emission [1,2]. Several types of two-proton emitters have been observed. On the one hand, there are the short-lived (τ 1/2 ≤ 10 −18 s) light nuclei such as 6 [6] when assuming only d-wave single-particle states in the low-lying structure of 18 Na and thus a dominant d 2 single-particle configuration for 19 Mg. Theoretical calculations of properties of 19 Mg depend strongly on the assumption made about the structure of 18 Na and its mirror nucleus. In this letter, we investigate both experimentally and theoretically the low lying spectrum of 18 Na. We strongly refine the knowledge about the lowlying spectrum of 18 Na and find that there are also lowlying s-wave states. These states should strongly boost the expected two-proton width.On the theoretical side, the structure of 18 Na has been predicted assuming a core of 17 Ne + proton structure [6] or by coupling a neutron hole to the lowest states in 19 Na [7]. In both cases, the low-lying structure of 18 Na is found to have d-wave configuration. In the following, another theoretical approach is described and predicts also low lying s-wave states. The dimensionless reduced widths θ 2 (sometimes called spectroscopic factors) where estimated with the shell model. The values shown in Tab. I were obtained with the OXBASH code [8] and the ZBM interaction [9] in the 1p 1/2 , 1d 5/2 and 2s 1/2 shells space. It predicts that the first six low lying states can be described mainly (with θ 2 > 0.5) by single particle con-