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.
Categories: nucl-ex