Microscopic multiphonon approach to spectroscopy in the neutron-rich oxygen region

Abstract

Background: A fairly rich amount of experimental spectroscopic data have disclosed intriguing properties of the nuclei in the region of neutron rich oxygen isotopes up to the neutron dripline. They, therefore, represent a unique laboratory for studying the evolution of nuclear structure away from the stability line.Purpose: We intend to give an exhaustive microscopic description of low and high energy spectra, dipole response, weak, and electromagnetic properties of the even $^{22}\mathrm{O}$ and the odd $^{23}\mathrm{O}$ and $^{23}\mathrm{F}$.Method: An equation of motion phonon method generates an orthonormal basis of correlated $n$-phonon states ($n=0,1,2,\ensuremath{\cdots}$) built of constituent Tamm-Dancoff phonons. This basis is adopted to solve the full eigenvalue equations in even nuclei and to construct an orthonormal particle-core basis for the eigenvalue problem in odd nuclei. No approximations are involved and the Pauli principle is taken into full account. The method is adopted to perform self-consistent, parameter free, calculations using an optimized chiral nucleon-nucleon interaction in a space encompassing up to two-phonon basis states.Results: The computed spectra in $^{22}\mathrm{O}$ and $^{23}\mathrm{O}$ and the dipole cross section in $^{22}\mathrm{O}$ are in overall agreement with the experimental data. The calculation describes poorly the spectrum of $^{23}\mathrm{F}$.Conclusions: The two-phonon configurations play a crucial role in the description of spectra and transitions. The large discrepancies concerning the spectra of $^{23}\mathrm{F}$ are ultimately traced back to the large separation between the Hartree-Fock levels belonging to different major shells. We suggest that a more compact single particle spectrum is needed and can be generated by a new chiral potential which includes explicitly the contribution of the three-body forces.