Low Energy Neutron Measurements in High Energy Density Plasmas using the National Ignition Facility

摘要

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).