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

doi:10.1016/j.physletb.2012.04.059

Physics Letters B

Volume 712, Issue 3, 6 June 2012, Pages 198-202

https://doi.org/10.1016/j.physletb.2012.04.059 Get rights and content

Abstract

The unbound nucleus ¹⁸Na, the intermediate nucleus in the two-proton radioactivity of ¹⁹Mg, was studied by the measurement of the resonant elastic scattering reaction H(¹⁷Ne,p)¹⁷Ne performed at 4 A.MeV using a radioactive beam from the SPIRAL Facility. Spectroscopic properties of the low-lying states were obtained in an R-matrix analysis of the excitation function.

Introduction

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: short-lived ( $τ_{1 / 2} ⩽ 10^{- 18} s$ ) light nuclei such as ⁶Be, ¹²O and ¹⁶Ne (where most probably the decaying ground state has a large width so that it overlaps with 1p emitter states in the intermediate nucleus), and longer-lived ( $τ_{1 / 2} \sim ms$ ) intermediate mass nuclei such as ⁴⁵Fe, ⁵⁴Zn and ⁴⁸Ni [3] where the decaying ground states has a very narrow width. Understanding decay mechanisms is a general problem in quantum physics, three-body decay being one of the most interesting cases [4]. The mechanism of two-proton emission can either be sequential via intermediate configurations or direct with simultaneous emission of the two protons to the continuum. In both cases, it is essential to know the structure of the intermediate nucleus in order to understand the emission mechanism. In most cases, it is extremely difficult to study the intermediate nucleus since it is located very far from the valley of stability. A new case of two-proton radioactivity, ¹⁹Mg [5], was observed recently. Its half-life of 4.0(15) ps makes ¹⁹Mg an intermediate case between short and long-lived two-proton decaying nuclei. The measured lifetime and p–p angular correlations [6] are well described by the predictions of Ref. [7] when assuming only d-wave single-particle states in the low-lying structure of the intermediate nucleus ¹⁸Na and thus a dominant $d^{2}$ single-particle configuration for ¹⁹Mg. Theoretical calculations of properties of ¹⁹Mg depend strongly on the assumptions made about the structure of ¹⁸Na and its mirror nucleus. The ¹⁸Na nucleus is one of the rare intermediate nuclei which is accessible experimentally. In this Letter, we investigate, both experimentally and theoretically, the low-lying states of ¹⁸Na. We significantly improve knowledge of the low-lying states of ¹⁸Na and find that there are also low-lying s-wave states. These states should increase the expected two-proton width.

Section snippets

Theory

The structure of ¹⁸Na has been calculated by assuming a ¹⁷Ne core plus one proton configuration [7] or, by coupling a neutron hole to the lowest states in ¹⁹Na [8]. In both cases, the low-lying structure of ¹⁸Na is found to have a d-wave configuration. In the following, another theoretical approach is described which also predicts low-lying s-wave states.

The dimensionless reduced widths $θ^{2}$ (sometimes called spectroscopic factors) were estimated with the shell model. The values shown in Table 1

Experiment

Experimentally, the ¹⁸Na nucleus has been studied only once using a stripping reaction [13], [15], but the ground state was not clearly identified. Two peaks were observed, one with a proton separation energy of $S_{p} = 0.41 (16) MeV$ and a width of $Γ = 0.34 (9) MeV$ , and the other with $S_{p} = 1.26 (17) MeV$ and $Γ = 0.54 (13) MeV$ . If the first peak ( $S_{p} = 0.41 MeV$ ) corresponds to the ground state, then its position is in strong disagreement with model predictions. Moreover, in this case, ¹⁹Mg would decay with an

Analysis

The excitation function obtained (see Fig. 2) shows that the Rutherford scattering is dominant at low energy and it also shows at energies higher than 1.5 MeV several resonances reflecting the ¹⁸Na compound nucleus structure. The position of these resonances is related to the energy of the excited states in the compound nucleus and their widths and shapes provide information on lifetimes (and spectroscopic factors) and spin-parity respectively. Spectroscopic properties of the low-lying states

Two-proton decay of ¹⁹Mg

Even though the ground state of ¹⁸Na was not observed directly in this experiment, there are strong indications of a very narrow state located well above the ¹⁹Mg ground state. These are the conditions required for simultaneous two-proton decay of ¹⁹Mg to the continuum, the so-called “true 2p decay” [23].

The properties of ¹⁸Na and ¹⁹Mg were predicted by L. Grigorenko et al. [7] using a three-body model based on the ground state of ¹⁷Ne. Concerning low-lying states of ¹⁸Na, they are not in

Conclusion

The measurement of the resonant elastic scattering reaction p(¹⁷Ne,p)¹⁷Ne allowed us to obtain the spectroscopic properties of four low-lying states in the unbound nucleus ¹⁸Na and suggests the presence of two very narrow states including a ground state located well above the ¹⁹Mg ground state. This measurement has been possible thanks to the development of an intense and pure radioactive beam of ¹⁷Ne at GANIL. Although the experimental results differ from previous theoretical predictions, they

Acknowledgements

We thank the GANIL staff, Dr. L. Grigorenko and the FULIS Collaboration for their help. We acknowledge the support of the European Commission within the Sixth Framework Programme through I3-EURONS (contract No. RII3-CT-2004-506065), the support from the French–Romanian Collaboration agreement IN2P3-IFIN-HH Bucharest No. 03-33, the LEA COPIGAL, the LEA NuAG, the Région Normandie, the support from the UK STFC, and the support of the French–Serbian CNRS/MSCI Collaboration agreement (No. 20505).

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Physics Letters B

Abstract

Introduction

Section snippets

Theory

Experiment

Analysis

Two-proton decay of ¹⁹Mg

Conclusion

Acknowledgements

References (24)

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Probing nuclear forces beyond the nuclear drip line: the cases of <sup>16</sup> F and <sup>15</sup> F: A Tribute to Mahir Hussein

Exploring Beyond the Proton Drip Line

Invariant-mass spectroscopy of Ne 18, O 16, and C 10 excited states formed in neutron-transfer reactions INVARIANT-MASS SPECTROSCOPY OF Ne 18, ... R. J. CHARITY et al.

Invariant-mass spectroscopy of <sup>18</sup>Ne, <sup>16</sup>O, and <sup>10</sup>C excited states formed in neutron transfer reactions.

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

Abstract

Introduction

Section snippets

Theory

Experiment

Analysis

Two-proton decay of 19Mg

Conclusion

Acknowledgements

Nucl. Phys.

Nucl. Phys. A

Phys. Rev. C

Nucl. Instr. and Meth. A

Nucl. Instr. and Meth. B

Rep. Prog. Phys.

Phys. Rev. Lett.

Eur. Phys. J. A

Phys. Rev. Lett.

Phys. Rev. C

Phys. Rev. C

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. C

Nucl. Phys. A

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

Two-proton decay of ¹⁹Mg