Measurement of reaction-in-flight neutrons using thulium activation at the National Ignition Facility

We report on the first observation of tertiary reaction-in-flight (RIF) neutrons produced in compressed deuterium and tritium filled capsules using the National Ignition Facility at Lawrence Livermore National Laboratory, Livermore, CA. RIF neutrons are produced by third-order, out of equilibrium ("in-flight") fusion reactions, initiated by primary fusion products. The rate of RIF reactions is dependent upon the range of the elastically scattered fuel ions and therefore a diagnostic of Coulomb physics within the plasma. At plasma temperatures of similar to 5 keV, the presence of neutrons with kinetic energies greater than 15 MeV is a unique signature for RIF neutron production. The reaction Tm-169(n,3n)Tm-167 has a threshold of 15.0 MeV, and a unique decay scheme making it a suitable diagnostic for observing RIF neutrons. RIF neutron production is quantified by the ratio of Tm-167/Tm-168 observed in a Tm-169 foil, where the reaction Tm-169(n,2n)Tm-168 samples the primary neutron fluence. Averaged over 4 implosions(14) at the NIF, the Tm-167/Tm-168 ratio is measured to be 1.5 +/- 0.3 x 10(-5), leading to an average ratio of RIF to primary neutron ratio of 1.0 +/- 0.2 x 10(-4). These ratios are consistent with the predictions for charged particle stopping in a quantum degenerate plasma.