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Structure evolution of ground and excited states in the exotic nucleus $^{22}$Al

Z. C. Xu, H. Y. Shang, S. M. Wang, Y. G. Ma

TL;DR

This work applies the Gamow Shell Model, rooted in chiral effective field theory, to the proton-rich nucleus $^{22}$Al and its mirror $^{22}$F to explore continuum effects and mirror-symmetry breaking near the proton dripline. By deriving valence-space Hamiltonians and operators from EM1.8/2.0 forces via MBPT and treating bound, resonant, and non-resonant continuum within a Berggren basis, the study predicts a $4^+$ ground state for $^{22}$Al and a nearby $3^+$ excitation, with only small $s$-wave components and negligible Thomas–Ehrman shifts for these states. In contrast, the excited $1^+_1$ state exhibits a pronounced halo-like structure due to a large $s$-wave occupancy and strong coupling to the continuum, while other low-lying states remain compact. The results reproduce observed separation energies and beta-decay strengths, highlight the role of continuum coupling in shaping near-threshold states, and provide predictions for $^{22}$Si to probe shell evolution and isospin-symmetry breaking in this region.

Abstract

Recent experimental studies on proton-rich nuclei in the $sd$ shell have revealed intriguing near-threshold phenomena, including exotic structures associated with mirror-symmetry breaking. In particular, a halo-like structure has been suggested for the $1^+$ state of $^{22}$Al based on the large isospin asymmetry observed in the $^{22}$Si/$^{22}$O mirror Gamow-Teller transitions. Recent mass measurements further indicate that the ground state of $^{22}$Al is weakly bound, with a single-proton separation energy of about 100 keV. To investigate how the continuum affects the structure and decay properties of this proton-dripline nucleus, we employ the state-of-the-art Gamow shell model. This approach utilizes valence-space effective interactions and operators derived from chiral forces. Our calculations identify the ground state of $^{22}$Al as a $4^+$ state, with a $3^+$ state as the first excitation. Despite their diffuse nature under weak binding, the Thomas-Ehrman shift for these states is found to be negligible due to their small $s$-wave components. In contrast, the excited $1_1^+$ state possesses a significantly larger $s$-wave component, resulting in a more pronounced halo-like structure.

Structure evolution of ground and excited states in the exotic nucleus $^{22}$Al

TL;DR

This work applies the Gamow Shell Model, rooted in chiral effective field theory, to the proton-rich nucleus Al and its mirror F to explore continuum effects and mirror-symmetry breaking near the proton dripline. By deriving valence-space Hamiltonians and operators from EM1.8/2.0 forces via MBPT and treating bound, resonant, and non-resonant continuum within a Berggren basis, the study predicts a ground state for Al and a nearby excitation, with only small -wave components and negligible Thomas–Ehrman shifts for these states. In contrast, the excited state exhibits a pronounced halo-like structure due to a large -wave occupancy and strong coupling to the continuum, while other low-lying states remain compact. The results reproduce observed separation energies and beta-decay strengths, highlight the role of continuum coupling in shaping near-threshold states, and provide predictions for Si to probe shell evolution and isospin-symmetry breaking in this region.

Abstract

Recent experimental studies on proton-rich nuclei in the shell have revealed intriguing near-threshold phenomena, including exotic structures associated with mirror-symmetry breaking. In particular, a halo-like structure has been suggested for the state of Al based on the large isospin asymmetry observed in the Si/O mirror Gamow-Teller transitions. Recent mass measurements further indicate that the ground state of Al is weakly bound, with a single-proton separation energy of about 100 keV. To investigate how the continuum affects the structure and decay properties of this proton-dripline nucleus, we employ the state-of-the-art Gamow shell model. This approach utilizes valence-space effective interactions and operators derived from chiral forces. Our calculations identify the ground state of Al as a state, with a state as the first excitation. Despite their diffuse nature under weak binding, the Thomas-Ehrman shift for these states is found to be negligible due to their small -wave components. In contrast, the excited state possesses a significantly larger -wave component, resulting in a more pronounced halo-like structure.
Paper Structure (4 sections, 2 equations, 2 figures, 3 tables)

This paper contains 4 sections, 2 equations, 2 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Method
  3. Results
  4. Summary

Figures (2)

  • Figure 1: The spectrum (upper) and occupation (lower) of $^{22}$Al/$^{22}$F mirror pairs. In the upper panel, the spectrum calculated by standard SM with USDC and EM1.8/2.0, and by GSM with EM1.8/2.0, compared with experimental data Lee2020Basunia2015. In the lower panel, the valence proton/neutron occupation of states in $^{22}$Al/$^{22}$F mirror pairs calculated by GSM.
  • Figure 2: Valence proton density $r^2\rho$ of $^{22}$Al with respect to the $^{16}$O core, obtained from SM (dashed lines) and GSM (solid lines) calculations using the chiral interaction EM1.8/2.0. The contributions of the $d_{3/2}$, $s_{1/2}$, and $d_{5/2}$ partial-wave proton densities in the GSM calculations are shown in the lower part of each panel and are indicated by the gray, orange, and blue bands, respectively. The results for the $1^+_2$, $1^+_1$, $3^+_1$, and $4^+_1$ states are displayed in panels (a), (b), (c), and (d), respectively.