Abstract
A long-lived J ¼ 4þ
1 isomer, T1=2
¼ 2:2ð1Þ ms, has been discovered at 643.4(1) keV in the weakly
bound 26
9 F nucleus. It was populated at Grand Acce´le´rateur National d’Ions Lourds in the fragmentation of
a 36S beam. It decays by an internal transition to the J ¼ 1þ
1 ground state [82(14)%], by decay to 26Ne,
or -delayed neutron emission to 25Ne. From the -decay studies of the J ¼ 1þ
1 and J ¼ 4þ
1 states,
new excited states have been discovered in 25;26Ne. Gathering the measured binding energies of the
J ¼ 1þ
1
4þ 1 multiplet in 26 9 F, we find that the proton-neutron 0d5=20d3=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 1þ 1 , 2þ 1 , 4þ 1 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.
