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

Abstract

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 ∼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 fluence. Averaged over 4 implosions1^–4 at the NIF, the ¹⁶⁷Tm/¹⁶⁸Tm 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.