The spin-isospin dependence of the nuclear force affects the neutron’s effective single-particle energies in such a way that the expected shell closures are eroded. Nuclei in these regions, a priori expected to be spherical, become deformed.  The experimental observation of the deformed Islands of Inversion at N=8, 20, and 40 are prime examples of such changes.

The study of odd-A nuclei, with a valence nucleon coupled to a core, provides a sensitive tool to disentangle the interplay of single-particle and collective motion. The effects of rotation on the single-particle motion are well understood in the Particle Rotor Model, which successfully explains the observed structure and decay properties in deformed nuclei. The rotational dynamic is controlled by the ratio of the Coriolis coupling term to the intrinsic level spacings H_c/∆E. In the limit of  H_c/∆E ≫1, the Coriolis force aligns the particle angular momentum j with the core and a decoupled band, with spins I=j,j+2,j+4,…  and energy spacings equal to that of the core, develops.

Figure 2. Consistent solution for the energy of the effective 28O core and the energy of the first excited, ½+, state in 29F, given by the crossing of the blue and black lines.

Recent results from the RIBF facility in Japan [1,2] showed unexpected features on the structure of ^25,29 F, which were explained by NSD’s Nuclear Structure group within the scenario discussed above [3,4]. The observed 5/2⁺ ground states correspond to the lowest energy state of a decoupled band resulting from the coupling of a d_5/2 proton to the oxygen cores, while the excited 1/2⁺ states must arise from the anti-parallel coupling of j with the core rotation, R (Fig. 1). The analysis indicates that the additional proton polarizes ^24,28O to a moderate deformation, ϵ₂≈ 0.16, and place the 2⁺ states at  ≈ 3 MeV and  ≈ 2.5 MeV, respectively (Fig. 2).  Coulomb excitation experiments with GRETA and the S800 spectrometer at FRIB, sensitive to electromagnetic transition probabilities, will test model predictions and shed further light on the nature of the effective oxygen cores in ^25,29F.

References

[1] P. Doornenbal, et al., Phys. Rev. C 95 041301(R), (2017).
[2] T. L. Tang, et al. Phys. Rev. Lett. 124, 212502 (2020).
[3] A. O. Macchiavelli, et al., Phys. Lett. B 775, 160 (2017).
[4] A. O. Macchiavelli, et al., Phys. Rev. C 102, 041301(R) (2020).