Effective proton-neutron interaction near the drip line from unbound states in ²⁵̛ ²⁶ F

Abstract

Background: Odd-odd nuclei, around doubly closed shells, have been extensively used to study proton-neutron interactions. However, the evolution of these interactions as a function of the binding energy, ultimately when nuclei become unbound, is poorly known. The ²⁶F nucleus, composed of a deeply bound π 0d_(5/2) proton and an unbound ν0d_(3/2) neutron on top of an ²⁴O core, is particularly adapted for this purpose. The coupling of this proton and neutron results in a J^π= 1₁⁺ - 4₁⁺ multiplet, whose energies must be determined to study the influence of the proximity of the continuum on the corresponding proton-neutron interaction. The J^π = 1₁⁺, 2₁⁺, 4₁⁺ bound states have been determined, and only a clear identification of the J^π = 3₁⁺ is missing.
Purpose: We wish to complete the study of the J^π = 1₁⁺ - 4₁⁺multiplet in ²⁶F, by studying the energy and width of the J^π = 3₁⁺unbound state. The method was first validated by the study of unbound states in ²⁵, for which resonances were already observed in a previous experiment.
Method: Radioactive beams of ²⁶Ne and ²⁷Ne, produced at about 440AMeV by the fragment separator at the GSI facility were used to populate unbound states in ²⁵F and ²⁶F via one-proton knockout reactions on a CH₂ target, located at the object focal point of the R³B/LAND setup. The detection of emitted. γ and neutrons, added to the reconstruction of the momentum vector of the A - 1 nuclei, allowed the determination of the energy of three unbound states in ²⁵F and two in ²⁶F.
Results: Based on its width and decay properties, the first unbound state in ²⁵F, at the relative energy of 49(9) keV, is proposed to be a J^π = 1/ 2ˉ arising from a p_(1/2) proton- hole state. In ²⁶F, the first resonance at 323(33) keV is proposed to be the J^π = 3₁⁺ member of the J^π = 1₁⁺- 4₁⁺multiplet. Energies of observed states in ²⁵ʼ²⁶F have been compared to calculations using the independent-particle shell model, a phenomenological shell model, and the ab initio valence-space in-medium similarity renormalization group method.
Conclusions: The deduced effective proton- neutron interaction is weakened by about 30-40% in comparison to the models, pointing to the need for implementing the role of the continuum in theoretical descriptions or to a wrong determination of the atomic mass of ²⁶F.