Intense and purified radioactive beam of post-accelerated 14 O was used to study the low-lying states in the unbound 15 F nucleus. Exploiting resonant elastic scattering in inverse kinematics with a thick target, the second excited state, a resonance at E R =4.757(6)(10) MeV with a width of Γ=36(5)(14) keV was measured for the first time with high precision. The structure of this narrow above-barrier state in a nucleus located two neutrons beyond the proton drip line was investigated using the Gamow Shell Model in the coupled channel representation with a 12 C core and three valence protons. It is found that it is an almost pure wave function of two quasi-bound protons in the 2s 1/2 shell.
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-
The decay of19 O(β − ) and 19 Ne(β + ) implanted in niobium in its superconducting and metallic phase was measured using purified radioactive beams produced by the SPIRAL/GANIL facility. Half-lives and branching ratios measured in the two phases are consistent within one-sigma error bar. This measurement casts strong doubts on the predicted strong electron screening in superconductor, the so-called superscreening. The measured difference in screening potential energy is 110 (90) The cloud of electrons surrounding the atomic nucleus is known to act as a screening potential. The effect of this screening on a charged projectile is to increase or reduce the penetrability of the projectile through the Coulomb barrier of the positively charged nucleus. Such an electron screening is known to play an important role in astrophysics since it modifies the nuclear cross sections at low energies [1]. When measuring the cross sections at energies well below the Coulomb barrier, it is important to take into account the composition of the target since insulator, metallic or alloys materials have different electron screening effects [2,3]. In superconductors, electrons are organized in Cooper pairs obeying the Bose-Einstein statistics. Due to their different nature, Stoppini [4] predicted that the local density of electrons around the nuclei could be higher than in the normal phase, a strong electron screening effect, called "superscreening", could happen in these materials. It is of high interest to study this predicted superscreening effect because of the possible theoretical implications and because of its potential use in the management of nuclear waste disposal [5].Electron screening also affects decays of radioactive nuclei [5,6]. From the theoretical point of view, the effects of the environment on the radioactive decay can be reduced to the effect of one parameter called the screening potential energy U e . In most cases, it is difficult to calculate the exact value of U e since it depends on several effects, including the electron screening and atomic bonding. Since these environmental effects do not modify the nuclear matrix element, the product of the β-decay Fermi function f (Z, Q) and the half-life t remains constant. Hence, the change of the β-decay rate can be calculated should be used for each individual branch i in order to calculate the new partial half-lives t ′(i) . It results in a new half-life 1/t ′ = i 1/t ′(i) and new branching ratios BR ′(i) = t ′ /t ′(i) . Hence, the environment can modify the half-life of the nucleus and also the β-decay branching ratios (BR). This last effect had not been previously discussed.
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