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Register a new userDeformation of Ne isotopes in the island-of-inversion region
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The deformation of Ne isotopes in the island-of-inversion region is determined by the double-folding model with the Melbourne $g$-matrix and the density calculated by the antisymmetrized molecular dynamics (AMD). The double-folding model reproduces, with no adjustable parameter, the measured reaction cross sections for the scattering of $^{28-32}$Ne from $^{12}$C at 240MeV/nucleon. The quadrupole deformation thus determined is around 0.4 in the island-of-inversion region and $^{31}$Ne is a halo nuclei with large deformation. We propose the Woods-Saxon model with a suitably chosen parameterization set and the deformation given by the AMD calculation as a convenient way of simulating the density calculated directly by the AMD. The deformed Woods-Saxon model provides the density with the proper asymptotic form. The pairing effect is investigated, and the importance of the angular momentum projection for obtaining the large deformation in the island-of-inversion region is pointed out.
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The properties of nuclei in the ``island of inversion (IOI) around Z=10 and N=20 are the focus of current nuclear physics research. Recent studies showed that $^{28}$F has a negative-parity ground state (g.s.) and thus lies within the southern shore of the IOI, and $^{29}$F presents a halo structure in its g.s., but it is unclear which effects, such as deformation, shell evolution due to tensor forces, or couplings to the continuum, lead to this situation. We investigate the role of quadrupole deformation and continuum effects on the single-particle (s.p.) structure of $^{28,29,31}$F from a relativistic mean-field (RMF) approach, and show how both phenomena can lead to a negative-parity g.s. in $^{28}$F and halo structures in $^{29,31}$F. We solve the Dirac equation in the complex-momentum (Berggren) representation for a potential with quadrupole deformation at the first order obtained from RMF calculations using the NL3 interaction, and calculate the continuum level densities using the Greens function method. We extract s.p. energies and widths from the continuum level densities to construct Nilsson diagrams, and analyse the evolution of both the widths and occupation probabilities of relevant Nilsson orbitals in $^{28}$F and find that some amount of prolate deformation must be present. In addition, we calculate the density distributions for bound Nilsson orbitals near the Fermi surface in $^{29,31}$F and reveal that for a quadrupole deformation $0.3 leq beta_2 leq 0.45$ (prolate), halo tails appear at large distances. We also demonstrate that while in the spherical case the $pf$ shells are already inverted and close to the neutron emission threshold, a small amount of quadrupole deformation can reduce the gap between $fp$ shells and increase the role of the continuum, ultimately leading to the negative parity in the g.s. of $^{28}$F and the halo structures in $^{29,31}$F.
Isotope-dependence of measured reaction cross sections in scattering of $^{28-32}$Ne isotopes from $^{12}$C target at 240 MeV/nucleon is analyzed by the double-folding model with the Melbourne $g$-matrix. The density of projectile is calculated by the mean-field model with the deformed Wood-Saxon potential. The deformation is evaluated by the antisymmetrized molecular dynamics. The deformation of projectile enhances calculated reaction cross sections to the measured values.
First results are reported on the ground state configurations of the neutron-rich $^{29,30}$Na isotopes, obtained via Coulomb dissociation (CD) measurements as a method of the direct probe. The invariant mass spectra of those nuclei have been obtained through measurement of the four-momentum of all decay products after Coulomb excitation on a $^{208}Pb$ target at energies of 400-430 MeV/nucleon using FRS-ALADIN-LAND setup at GSI, Darmstadt. Integrated Coulomb-dissociation cross-sections (CD) of 89 $(7)$ mb and 167 $(13)$ mb up to excitation energy of 10 MeV for one neutron removal from $^{29}$Na and $^{30}$Na respectively, have been extracted. The major part of one neutron removal, CD cross-sections of those nuclei populate core, in its ground state. A comparison with the direct breakup model, suggests the predominant occupation of the valence neutron in the ground state of $^{29}$Na${(3/2^+)}$ and $^{30}$Na${(2^+)}$ is the $d$ orbital with small contribution in the $s$-orbital which are coupled with ground state of the core. The ground state configurations of these nuclei are as $^{28}$Na$_{gs (1^+)otimes u_{s,d}$ and $^{29}$Na$_{gs}(3/2^+)otimes u_{ s,d}$, respectively. The ground state spin and parity of these nuclei, obtained from this experiment are in agreement with earlier reported values. The spectroscopic factors for the valence neutron occupying the $s$ and $d$ orbitals for these nuclei in the ground state have been extracted and reported for the first time. A comparison of the experimental findings with the shell model calculation using MCSM suggests a lower limit of around 4.3 MeV of the sd-pf shell gap in $^{30}$Na.
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.
We report on the first spectroscopic study of the N=22 nucleus 32Ne at the newly completed RIKEN Radioactive Ion Beam Factory. A single gamma-ray line with an energy of 722(9) keV was observed in both inelastic scattering of a 226 MeV/u 32Ne beam on a Carbon target and proton removal from 33Na at 245 MeV/u. This transition is assigned to the de-excitation of the first J^pi = 2+ state in 32Ne to the 0+ ground state. Interpreted through comparison with state-of-the-art shell model calculations, the low excitation energy demonstrates that the Island of Inversion extends to at least N=22 for the Ne isotopes.
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