IsomerRe188m

Two new transitions in ${\mathrm{Re}}^{188m}$ were found which are responsible for the 18.7-min half-life of this isomer. The energies and branching ratios of these transitions which originate from a 6-state at 171.9 keV, are 2.4 keV (57%) and 15.9 keV (43%). Their multipole orders are $M3\mathrm{}$. Each transition is followed by an $M1\mathrm{}$ transition, 105.9 and 92.4 keV, respectively, both terminating in the first excited (2-) state at 63.58 keV, which decays by an $M1\mathrm{}$ transition to the ground state. The ground state and the first two excited states appear to be members of a rotational band with $K=1-$, whereas the 3rd and 4th excited states can be interpreted as composed of the same proton orbit as that of the ground state, but of different neutron orbits. The thermal neutron activation cross section of ${\mathrm{Re}}^{187}$ leading to the isomer was found to be 1.25±0.25 b.