Spectroscopy of 18Na: Bridging the two-proton radioactivity of 19Mg

Abstract

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-

Mg19two-proton decay lifetime

“…scale (t < ∼10 −15 s) well below our experimental sensitivity, contributed to the observed 17 Ne yield: the 2p1n breakup of the 20 Mg secondary beam, the 1p1n breakup followed by prompt proton emission from unbound 18 Na [16,21,22], and sequential 1p emissions of 19 Mg excited states through broad resonances in 18 Na. Furthermore, the 20 Mg beam had sufficient energy to pass unreacted through the target and undergo knockout reactions on the DSSD.…”

“…A radioactive 20 Mg secondary beam was selected from the fragmentation reaction products by the A1900 Fragment Separator [19]. Three significant beam contaminants- 16 O (29%), 17 F (11%), and 18 Ne (19%)-were distinguished from 20 Mg (36%) by their time of flight between two thin plastic scintillators at the A1900 focal plane and the S800 object position, as illustrated in Fig. 1.…”

“…Both models favored a dominant d 2 configuration for the 19 Mg valence protons. However, spectroscopy of the unbound intermediary nucleus 18 Na uncovered unexpected low-lying broad s-wave resonances; 19 Mg lifetime estimates via sequential 1p decays through these states were as low as 1 ps [16].…”

Excited states of19Mg

“…for a number of different inputs: potential Set 1 vs Set 2, S from sm vs S from sm + weak coupling, and calculated energies in 18 Na vs recently reported [11] experimental ones. Results are listed in Table I.…”

“…Almost simultaneously, results appeared from an experiment [11] to measure energies in 18 Na. We used these to recalculate the g.s.…”