Publication: Effective proton-neutron interaction near the drip line from unbound states in ²⁵̛ ²⁶ F
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2019-11-08
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American Physical Society
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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.
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©2017 American Physical Society.
Articulo firmado por mas de diez autores. Autoria conjunta: Pierre Auger collaboration.
P. Van Isacker and M. Ploszajczak are greatly acknowledged for fruitful discussions and suggestions on how to improve the manuscript. TRIUMF receives funding via a contribution through the National Research Council of Canada. This work was supported in part by NSERC, the NUCLEI SciDAC Collaboration under the US Department of Energy Grants No. DE-SC0008533 and No. DE-SC0008511, the National Science Foundation under Grant No. PHY-1404159, the European Research Council Grant No. 307986 STRONGINT, the Deutsche Forschungsgesellschaft under Grant No. SFB 1245, and the BMBF under Contracts No. 05P15RDFN1 and No. 05P15WOFNA. This work has also been supported by the Spanish MINECO via Projects No. FPA2013-41267-P, No. FPA2015-64969-P, and No. FPA2015-65035-P and by the Portuguese FCT, Project No. PTDC/FIS/103902/2008. Computations were performed with an allocation of computing resources at the Jülich Supercomputing Center, Ohio Supercomputer Center (OSC), and the Michigan State University High Performance Computing Center (HPCC)/Institute for Cyber-Enabled Research (iCER). C. A. Bertulani acknowledges support from US DOE Grant No. DE-FG02-08ER41533 and the US NSF Grant No. 1415656. M. Petri acknowledges support from the Helmholtz International Center for FAIR within the framework of the LOEWE program launched by the State of Hesse.