Measurement of the 209Bi(n,4n)206Bi and 169Tm(n,3n)167Tm cross sections between 23.5 and 30.5 MeV relevant to reaction-in-flight neutron studies at the National Ignition Facility

At the National Ignition Facility, experiments are being performed to measure charged-particle stopping powers in the previously unexplored warm dense plasma regime. These measurements are done using reaction-in-flight (RIF) neutrons from an inertial confinement fusion system. RIF neutrons are produced with a continuum of energies up to 30MeV. By making activation measurements utilizing threshold reactions for neutrons in the energy range of 15 < E-n < 30 MeV, the number of RIF neutrons can be determined and from this the stopping power of the deuterium and tritium ions that produced the RIF neutrons can be inferred. Currently, the Tm-169(n, 3n)Tm-167 reaction has been used. However, in an effort to provide a secondary complimentary measurement, efforts are underway to make use of the Bi-209(n, 4n)Bi-206 reaction, with a threshold of 22.5 MeV. The cross sections were measured at the 10 MV tandem Van De Graaff accelerator at the Triangle Universities Nuclear Laboratory with quasimonoenergetic neutrons between 23.5 and 30.5 MeV, where few previous measurements have been made. Cross-section data are compared to calculations and other available measurements.