Spectroscopy of ${}^{26}$F to probe proton-neutron forces close to the drip line

Abstract

A long-lived $J^{\ddot{I}}€=4_1^{+}$ isomer, $T_{1/2}=2.2(1)$ms, has been discovered at 643.4(1) keV in the weakly-bound ${}_9^{26}$F nucleus. It was populated at GANIL in the fragmentation of a ${}^{36}$S beam. It decays by an internal transition to the $J^{\ddot{I}}€=1_1^{+}$ ground state (82(14)%), by ${\hat{I}}^2$-decay to ${}^{26}$Ne, or beta-delayed neutron emission to ${}^{25}$Ne. From the beta-decay studies of the $J^{\ddot{I}}€=1_1^{+}$ and $J^{\ddot{I}}€=4_1^{+}$ states, new excited states have been discovered in ${}^{25,26}$Ne. Gathering the measured binding energies of the $J^{\ddot{I}}€=1_1^{+}-4_1^{+}$ multiplet in ${}_9^{26}$F, we find that the proton-neutron $\ddot{I}€0d_{5/2}\hat{I}½0d_{3/2}$ effective force used in shell-model calculations should be reduced to properly account for the weak binding of ${}_9^{26}$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.

5 pages, 3 figures, accepted for publication in Physical Review Letters