Low energy neutron measurements in high energy density plasmas using the National Ignition Facility

Deuterium-tritium-loaded capsules at the National Ignition Facility (NIF) are now regularly producing a neutron rich high energy density plasma (nHEDP) with a low-energy "ICF-thermal" neutron density > 10²¹ neutrons/cm³. These low-energy neutrons are produced via multiple scatter off of the highly compressed capsule and therefore provide insight into the confinement time (τ_confinement) of the assembled plasma. Neutrons are formed in the center of the 5 m NIF chamber that is well suited for minimizing "room return" thermal capture. This nHEDP environment is befitting for activation-based measurements of the (n, γ) cross sections responsible for the formation of heavy elements in astrophysical settings. These experiments also offer the first opportunity to search for the effects of nuclear-plasma interaction-induced excited state population on (n, x) reaction rates in a stellar-like plasma environment. Unfortunately, no capability currently exists at the NIF to measure the neutron spectrum in a capsule down to the 100 eV level required to enable these new classes of nuclear-plasma experiments. In this paper we will discuss nHEDP-based neutron capture experiments, compare them to accelerator-based (n, γ) measurements, and discuss the requirements for a NIF-based low energy neutron spectrometer (LENS). (author)