Spectroscopy of ²⁶F to probe proton-neutron forces close to the drip line

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A long-lived Jπ=4₁+ isomer, T₁/₂=2.2(1)ms, has been discovered at 643.4(1) keV in the weakly-bound ²⁶₉F nucleus. It was populated at GANIL in the fragmentation of a ³⁶S beam. It decays by an internal transition to the Jπ=1₁+ ground state (82(14)%), by β-decay to ²⁶Ne, or beta-delayed neutron emission to ²⁵Ne. From the beta-decay studies of the Jπ=1₁+ and Jπ=4₁+ states, new excited states have been discovered in ²⁵,²⁶Ne. Gathering the measured binding energies of the Jπ=1₁+-4₁+ multiplet in ²⁶₉F, we find that the proton-neutron π0d₅/₂ ν0d₃/₂ effective force used in shell-model calculations should be reduced to properly account for the weak binding of ²⁶₉F. Microscopic coupled cluster theory calculations using interactions derived from chiral effective field theory are in very good agreement with the energy of the low-lying 1₁+,2₁+,4₁+ states in ²⁶F. Including three-body forces and coupling to the continuum effects improve the agreement between experiment and theory as compared to the use of two-body forces only.