Effective proton-neutron interaction near the drip line from unbound states in $^{25,26}$F
Creators: name: Vandebrouck, M.
name: Lepailleur, A.
name: Sorlin, O.
name: Aumann, T.
name: Caesar, C.
name: Holl, M.
name: Panin, V.
name: Wamers, F.
name: Stroberg, S. R.
name: Holt, J. D.
name: Santos, F. De Oliveira
name: Alvarez-Pol, H.
name: Atar, L.
name: Avdeichikov, V.
name: Beceiro-Novo, S.
name: Bemmerer, D.
name: Benlliure, J.
name: Bertulani, C. A.
name: Bogner, S. K.
name: Boillos, J. M.
name: Boretzky, K.
name: Borge, M. J. G.
name: Caamano, M.
name: Casarejos, E.
name: Catford, W.
name: Cederkäll, J.
name: Chartier, M.
name: Chulkov, L.
name: Cortina-Gil, D.
name: Cravo, E.
name: Crespo, R.
name: Pramanik, U. Datta
name: Fernandez, P. Diaz
name: Dillmann, I.
name: Elekes, Z.
name: Enders, J.
name: Ershova, O.
name: Estrade, A.
name: Farinon, F.
name: Fraile, L. M.
name: Freer, M.
name: Galaviz, D.
name: Geissel, H.
name: Gernhauser, R.
name: Gibelin, J.
name: Golubev, P.
name: Göbel, K.
name: Hagdahl, J.
name: Heftrich, T.
name: Heil, M.
name: Heine, M.
name: Heinz, A.
name: Henriques, A.
name: Hergert, H.
name: Hufnagel, A.
name: Ignatov, A.
name: Johansson, H. T.
name: Jonson, B.
name: Kahlbow, J.
name: Kalantar-Nayestanaki, N.
name: Kanungo, R.
name: Kelic-Heil, A.
name: Knyazev, A.
name: Kröll, T.
name: Kurz, N.
name: Labiche, M.
name: Langer, C.
name: Bleis, T. Le
name: Lemmon, R.
name: Lindberg, S.
name: Machado, J.
name: Marganiec, J.
name: Marques, F. M.
name: Movsesyan, A.
name: Nacher, E.
name: Najafi, M.
name: Nikolskii, E.
name: Nilsson, T.
name: Nociforo, C.
name: Paschalis, S.
name: Perea, A.
name: Petri, M.
name: Pietri, S.
name: Plag, R.
name: Reifarth, R.
name: Ribeiro, G.
name: Rigollet, C.
name: Röder, M.
name: Rossi, D.
name: Savran, D.
name: Scheit, H.
name: Schwenk, A.
name: Simon, H.
name: Syndikus, I.
name: Taylor, J.
name: Tengblad, O.
name: Thies, R.
name: Togano, Y.
name: Velho, P.
name: Volkov, V.
name: Wagner, A.
name: Weick, H.
name: Wheldon, C.
name: Wilson, G.
name: Winfield, J. S.
name: Woods, P.
name: Yakorev, D.
name: Zhukov, M.
name: Zilges, A.
name: Zuber, K.
Publisher: name: arXiv
Year: 2017
Subjects: subject: Nuclear Experiment (nucl-ex)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 $^{26}$F nucleus, composed of a deeply bound $\pi0d\_{5/2}$ proton and an unbound $\nu0d\_{3/2}$ neutron on top of an $^{24}$O core, is particularly adapted for this purpose. The coupling of this proton and neutron results in a $J^π = 1^{+}\_1 - 4^{+}\_1$ multiplet, whose energies must be determined to study the influence of the proximity of the continuum on the corresponding proton-neutron interaction. The $J^π = 1^{+}\_1, 2^{+}\_1,4^{+}\_1$ bound states have been determined, and only a clear identification of the $J^π =3^{+}\_1$ is missing.Purpose: We wish to complete the study of the $J^π = 1^{+}\_1 - 4^{+}\_1$ multiplet in $^{26}$F, by studying the energy and width of the $J^π =3^{+}\_1$ unbound state. The method was firstly validated by the study of unbound states in $^{25}$F, for which resonances were already observed in a previous experiment.Method: Radioactive beams of $^{26}$Ne and $^{27}$Ne, produced at about $440A$\,MeV by the FRagment Separator at the GSI facility, were used to populate unbound states in $^{25}$F and $^{26}$F via one-proton knockout reactions on a CH$\_2$ target, located at the object focal point of the R$^3$B/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 $^{25}$F and two in $^{26}$F. Results: Based on its width and decay properties, the first unbound state in $^{25}$F is proposed to be a $J^π = 1/2^-$ arising from a $p\_{1/2}$ proton-hole state. In $^{26}$F, the first resonance at 323(33)~keV is proposed to be the $J^π =3^{+}\_1$ member of the $J^π = 1^{+}\_1 - 4^{+}\_1$ multiplet. Energies of observed states in $^{25,26}$F 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 of implementing the role of the continuum in theoretical descriptions, or to a wrong determination of the atomic mass of $^{26}$F.
description: 14 pages, 7 figures, 3 tables, submitted to PRC