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

We report on the first observation of tertiary reaction-in-ight (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-ight") 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∼5 keV, the presence of neutrons with kinetic energies greater than 15 MeV is a unique signature for RIF neutron production. The reaction ¹⁶⁹Tm(n,3n) ¹⁶⁷Tm 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 167Tm/168Tm observed in a ¹⁶⁹Tm foil, where the reaction ¹⁶⁹Tm(n,2n) ¹⁶⁸Tm samples the primary neutron uence. Averaged over 4 implosions^1-4 at the NIF, the ¹⁶⁷Tm/¹⁶⁸Tm ratio is measured to be 1.5± 0.3× 10^-5, leading to an average ratio of RIF to primary neutron ratio of 1.0± 0.2× 10^-4. These ratios are consistent with the predictions for charged particle stopping in a quantum degenerate plasma.

Duke Scholars

Author Anton P. Tonchev Physics