Vandebrouck, M.; Lepailleur, A.; Sorlin, O.; Aumann, T.; Caesar, C.; Holl, M.; Panin, V.; Wamers, F.; Stroberg, S. R.; Holt, J. D.; de Oliveira Santos, F.; Kanungo, R.
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 π0d5/2 proton and an unbound ν0d3/2 neutron on top of an 24O core, is particularly adapted for this purpose. The coupling of this proton and neutron results in a Jπ = 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 Jπ = 11+, 21+, 41+ bound states have been determined, and only a clear identification of the Jπ = 31+ is missing.
Purpose: We wish to complete the study of the Jπ = 11+ − 41+ multiplet in 26F, by studying the energy and width of the Jπ = 31+ unbound state. The method was first validated by the study of unbound states in 25F, for which resonances were already observed in a previous experiment.
Method: Radioactive beams of 26Ne and 27Ne, produced at about 440A MeV 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 rays 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 Jπ = 1/2− arising from a p1/2 proton-hole state. In 26F, the first resonance at 323(33) keV is proposed to be the Jπ = 31+ member of the Jπ = 11+ − 41+ multiplet. Energies of observed states in 25,26F 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.