The Astrophysical $^{187}$Re$^{187}$Os Ratio: Measurement of the $^{187}$Re(n, 2n $\gamma$)$^{186m}$Re Destruction Cross Section*

We have initiated a program to measure neutron-induced cross sections on $^{187}$Re using monoenergetic neutron beams and an array of HPGe $\gamma$-ray detectors at TUNL. Our emphasis is on measuring transitions in the $^{187}$Re(n, 2n ) reaction that populate the long-lived isomeric state, $^ {186m}$Re. These data are needed to decrease uncertainties in the $^{187} $Re/$^{187}$Os cosmochronometer, which dates the $r$-process nucleosynthesis. Results from a first run using a pulsed 12 MeV neutron beam will be presented, and an overview of planned measurements that will measure the excitation function making use of both prompt $\gamma$-ray detection and activation techniques will be given.