Effective proton-neutron interaction near the drip line from unbound states in $^{25,26}$F

M. Vandebrouck (1) , A. Lepailleur (1) , O. Sorlin (1) , T. Aumann , C. Caesar , M. Holl , V. Panin , F. Wamers , S.R. Stroberg , J.D. Holt , Francois de Oliveira Santos (1) , H. Alvarez-Pol , L. Atar , V. Avdeichikov , S. Beceiro-Novo , D. Bemmerer , J. Benlliure , C. A. Bertulani , S.K. Bogner , J.M. Boillos , K. Boretzky , M.J.G. Borge , M. Caamano , E. Casarejos , W. Catford , J. Cederkäll , M. Chartier , L. Chulkov , D. Cortina-Gil , E. Cravo , R. Crespo , U. Datta Pramanik , P. Diaz Fernandez , I. Dillmann , Z. Elekes , J. Enders , O. Ershova , A. Estrade , F. Farinon , L.M. Fraile , M. Freer , D. Galaviz , H. Geissel , R. Gernhauser , J. Gibelin (2) , P. Golubev , K. Göbel , J. Hagdahl , T. Heftrich , M. Heil , M. Heine (3) , A. Heinz , A. Henriques , H. Hergert , A. Hufnagel , A. Ignatov , H.T. Johansson , B. Jonson , J. Kahlbow , N. Kalantar-Nayestanaki , R. Kanungo , A. Kelic-Heil , A. Knyazev , T. Kröll , N. Kurz , M. Labiche , C. Langer , T. Le Bleis , R. Lemmon , S. Lindberg , J. Machado , J. Marganiec , F.M. Marqués (2) , A. Movsesyan , E. Nacher , M. Najafi , E. Nikolskii , T. Nilsson , C. Nociforo , S. Paschalis , A. Perea , M. Petri , S. Pietri , R. Plag , R. Reifarth , G. Ribeiro , C. Rigollet , M. Röder , D. Rossi , D. Savran , H. Scheit , A. Schwenk , H. Simon , I. Syndikus , J. Taylor , O. Tengblad , R. Thies , Y. Togano , P. Velho , V. Volkov , A. Wagner , H. Weick , C. Wheldon , G. Wilson , J.S. Winfield , P. Woods , D. Yakorev , M. Zhukov , A. Zilges , K. Zuber

1 GANIL - Grand Accélérateur National d'Ions Lourds
2 LPCC - Laboratoire de physique corpusculaire de Caen
3 IPHC - Institut Pluridisciplinaire Hubert Curien

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 $J^{\pi} = 1^{+}_1 - 4^{+}_1$ multiplet, whose energies must be determined to study the influence of the proximity of the continuum on the corresponding proton-neutron interaction. The $J^{\pi} = 1^{+}_1, 2^{+}_1,4^{+}_1$ bound states have been determined, and only a clear identification of the $J^{\pi} =3^{+}_1$ is missing. Purpose: We wish to complete the study of the $J^{\pi} = 1^{+}_1 - 4^{+}_1$ multiplet in $^{26}$F, by studying the energy and width of the $J^{\pi} =3^{+}_1$ 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 $\gamma$-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 $J^{\pi} = 1/2^-$ arising from a $p_{1/2}$ proton-hole state. In $^{26}$F, the first resonance at 323(33)~keV is proposed to be the $J^{\pi} =3^{+}_1$ member of the $J^{\pi} = 1^{+}_1 - 4^{+}_1$ 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.

Mots clés: 21.10.-k; 27.30.+t; 25.60.-t; 29.30.Hs

Domaines: Physique Nucléaire Expérimentale [nucl-ex]

M. Vandebrouck, A. Lepailleur, O. Sorlin, T. Aumann, C. Caesar, et al.. Effective proton-neutron interaction near the drip line from unbound states in $^{25,26}$F. Physical Review C, 2017, 96, pp.054305. ⟨10.1103/PhysRevC.96.054305⟩. ⟨in2p3-01568314⟩