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Register a new userProton decays in $^{16}$Ne and $^{18}$Mg and isospin-symmetry breaking in carbon isotopes and isotones
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Proton-rich nuclei possess unique properties in the nuclear chart. Due to the presence of both continuum coupling and Coulomb interaction, phenomena such as halos, Thomas-Ehrman shift, and proton emissions can occur. Experimental data are difficult to be obtained therein, so that theoretical calculations are needed to understand nuclei at drip-lines and guide experimentalists for that matter. In particular, the $^{16}$Ne and $^{18}$Mg isotopes are supposed to be one-proton and/or two-proton emitting nuclei, but associated experimental data are either incomplete or even unavailable. Consequently, we performed Gamow shell model calculations of carbon isotones bearing $A=15text{-}18$. Isospin-symmetry breaking occurring in carbon isotones and isotopes is also discussed. It is hereby shown that the mixed effects of continuum coupling and Coulomb interaction at drip-lines generate complex patterns in isospin multiplets. Added to that, it is possible to determine the one-proton and two-proton widths of $^{16}$Ne and $^{18}$Mg. Obtained decay patterns are in agreement with those obtained in previous experimental and theoretical works. Moreover, up to the knowledge of authors, this is the first theoretical calculation of binding energy and partial decay widths of $^{18}$Mg in a configuration interaction picture.
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Proton-proton correlations were observed for the two-proton decays of the ground states of $^{19}$Mg and $^{16}$Ne. The trajectories of the respective decay products, $^{17}$Ne+p+p and $^{14}$O+p+p, were measured by using a tracking technique with microstrip detectors. These data were used to reconstruct the angular correlations of fragments projected on planes transverse to the precursor momenta. The measured three-particle correlations reflect a genuine three-body decay mechanism and allowed us to obtain spectroscopic information on the precursors with valence protons in the $sd$ shell.
The density profile of exotic nuclei can be a rich source of information on the nuclear surface. In particular, the nuclear surface diffuseness parameter is correlated with the occupation probability of nucleons indistinct nuclear orbits, especially those with low angular momenta. The aim of this paper is to investigate the relationship between the nuclear surface diffuseness and spectroscopic information of neutron rich Ne and Mg isotopes both at the cusp and inside the island of inversion. We use the microscopic antisymmetrized molecular dynamics model to calculate these densities and other spectroscopic information. A two-parameter Fermi density distribution is then used to define the diffuseness parameter. To relate them with observables, the nucleon-nucleus elastic scattering differential cross section is calculated with the demand that they reproduce the first peak position and its magnitude. A marked increase in the occupation of neutrons in the pf-orbit is noted in Ne and Mg isotopes from N=19 onwards. We observed that the nuclear diffuseness is strongly correlated with the nuclear deformation, in the island of inversion, and gradually increases with the occupation of neutrons in the 1p3/2 orbit. This result is also confirmed by a single-particle estimate of the valence neutron density distribution. An exception is noted for 35-37Mg, where the filling up of the holes in the sd-shell partially compensates the increase in diffuseness due to filling up of the 1p3/2 orbit. Information on nuclear density profile of neutron rich medium mass nuclei can be reliably extracted by studying the first diffraction peak of the nucleon-nucleus elastic scattering differential cross section. The enormous surface diffuseness of Ne and Mg isotopes, in the island of inversion, could be attributed to the increasing neutron occupation of the 1p3/2 orbit.
This paper examines the $^{18}$Ne($alpha, p_{0}$)$^{21}$Na cross-section relevant in X-ray bursts. The study was performed with the K600 magnetic spectrometer in coincidence with the CAKE, a silicon detector array, at iThemba LABS in Cape Town, South Africa. A 100-MeV proton beam was impinged on a $^{24}$Mg target to study the $^{24}$Mg($p,t$)$^{22}$Mg reaction. The triton ejectiles were momentum-analysed with the magnetic spectrometer and proton decays from the $^{22}$Mg recoil nucleus to the ground state of $^{21}$Na and various excited states thereof were detected with the CAKE. In doing so, we were able to compare our results to previous direct and indirect measurements of the $^{18}$Ne($alpha, p$)$^{21}$Na reaction.
Low-energy dipole excitations in neon isotopes and N=16 isotones are calculated with a fully consistent axially-symmetric-deformed quasiparticle random phase approximation (QRPA) approach based on Hartree-Fock-Bogolyubov (HFB) states. The same Gogny D1S effective force has been used both in HFB and QRPA calculations. The microscopical structure of these low-lying resonances, as well as the behavior of proton and neutron transition densities, are investigated in order to determine the isoscalar or isovector nature of the excitations. It is found that the N=16 isotones 24O, 26Ne, 28Mg, and 30Si are characterized by a similar behavior. The occupation of the 2s_1/2 neutron orbit turns out to be crucial, leading to nontrivial transition densities and to small but finite collectivity. Some low-lying dipole excitations of 28Ne and 30Ne, characterized by transitions involving the neutron 1d_3/2 state, present a more collective behavior and isoscalar transition densities. A collective proton low-lying excitation is identified in the 18Ne nucleus.
We discuss the role of pairing anti-halo effect in the observed odd-even staggering in reaction cross sections for $^{30,31,32}$Ne and $^{36,37,38}$Mg isotopes by taking into account the ground state deformation of these nuclei. To this end, we construct the ground state density for the $^{30,31}$Ne and $^{36,37}$Mg nuclei based on a deformed Woods-Saxon potential, while for the $^{32}$Ne and $^{38}$Mg nuclei we also take into account the pairing correlation using the Hartree-Fock-Bogoliubov method. We demonstrate that, when the one-neutron separation energy is small for the odd-mass nuclei, a significant odd-even staggering still appears even with finite deformation, although the degree of staggering is somewhat reduced compared to the spherical case. This implies that the pairing anti-halo effect in general plays an important role in generating the odd-even staggering in reaction cross sections for weakly bound nuclei.
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