A Platform for Measuring Neutron Capture Cross Sections in a Plasma Environment

The National Ignition Facility (NIF) laser is capable of producing a plasma environment with temperatures ~10 keV, particle densities ~10^32 m^-3, and neutron fluxes of up to 10^34 m^-2 s^-1. These features make it uniquely suitable for carrying out experiments to investigate interactions between Plasma Physics and Nuclear Physics. A NIF Discovery Science experiment has recently been commissioned to measure the neutron capture cross section of Thulium-171 (171Tm) at a neutron energy of 2.45 MeV. The experiment will include trace amounts of 171Tm and other monitor isotopes in a deuterium capsule. Therefore, the 171Tm isotopes undergoing neutron capture will have a temperature of ~10 keV, and so a significant population of nuclear excited states of 171Tm will be created (first excited state is at 5.036 keV). Initial capture cross section calculations indicate that the ground state and excited state cross sections are similar. Although the experiment will yield a combined cross section that shows little dependence on the excited-state population, it will provide a baseline for future excited-state measurements where significant differences are expected. Measurements of the plasma temperature, density, size and duration will also be used for calculating the populations of nuclear excited states and for modelling excitation processes which affect population rates. This data will support the development of a reliable platform on the NIF for measuring capture cross sections of excited state nuclei.