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arXiv 2108.11222nucl-th
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四极形变和连续效应在“反转岛”核$^{28,29,31}$ F中的作用

Role of quadrupole deformation and continuum effects in the "island of inversion'' nuclei $^{28,29,31}$F

Yu-Xuan Luo, K. Fossez, Quan Liu, Jian-You Guo

摘要:

在Z=10和N=20附近的“反转岛”(IOI)中的核性质是当前核物理研究的焦点。最近的研究表明,$^{28}$ F具有负宇称基态(g.s.)因此位于IOI的南岸内,并且$^{29}$ F在其g. s.中呈现晕结构,但目前还不清楚是什么效应,如形变、张量力引起的壳层演化或与连续介质的耦合,导致了这种情况。我们研究了四极形变和连续效应对单粒子(s.p.)结构的$^{28,29,31}$ F从相对论平均场(RMF)的方法,并显示如何这两种现象可以导致一个负宇称g. s。和$^{28}$ F中的晕结构。我们解决了狄拉克方程的复动量(Berggren)表示的潜在的四极形变在一阶从RMF计算获得使用NL3相互作用,并计算连续能级密度使用格林函数方法。我们从连续能级密度中提取出s.p.能量和宽度来构造Nilsson图,并分析了$^{28}$ F中相应Nilsson轨道的宽度和占据几率的演化,发现一定量的扁长形变必然存在.此外,我们计算的密度分布附近的费米面的束缚Nilsson轨道在$^{29,31}$ F和揭示,对于四极形变$0.3 \leq β_2 \leq 0.45$(长),晕尾出现在大的距离。我们还表明,虽然在球形的情况下,$pf$壳层已经倒置,接近中子发射阈值,少量的四极形变可以减少$fp$壳层之间的间隙,并增加连续体的作用,最终导致在g. s的负宇称。和$^{28}$ F中的晕结构。

英文摘要:

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 Green's 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 $0.3 \leq β_2 \leq 0.45$ (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.

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