A. Lepailleur, O. Sorlin, L. Caceres, B. Bastin, C. Borcea, R. Borcea, B. A. Brown, L. Gaudefroy, S. Gr évy, G. F. Grinyer, G. Hagen, M. Hjorth-Jensen, G. R. Jansen, O. Llidoo, F. Negoita, F. de Oliveira, M. -G. Porquet, F. Rotaru, M. -G. Saint-Laurent, D. Sohler, M. Stanoiu, J. C. Thomas
A long-lived $J^{\pi}=4_1^+$ isomer, $T_{1/2}=2.2(1)$ms, has been discovered at 643.4(1) keV in the weakly-bound $^{26}_{9}$F nucleus. It was populated at GANIL in the fragmentation of a $^{36}$S beam. It decays by an internal transition to the $J^{\pi}=1_1^+$ ground state (82(14)%), by $\beta$-decay to $^{26}$Ne, or beta-delayed neutron emission to $^{25}$Ne. From the beta-decay studies of the $J^{\pi}=1_1^+$ and $J^{\pi}=4_1^+$ states, new excited states have been discovered in $^{25,26}$Ne. Gathering the measured binding energies of the $J^{\pi}=1_1^+-4_1^+$ multiplet in $^{26}_{9}$F, we find that the proton-neutron $\pi 0d_{5/2} \nu 0d_{3/2}$ effective force used in shell-model calculations should be reduced to properly account for the weak binding of $^{26}_{9}$F. 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 $1_1^+,2_1^+,4_1^+$ states in $^{26}$F. 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.
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http://arxiv.org/abs/1301.5803
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