The primary objective of the NIF Gamma Reaction History (GRH) diagnostic is to provide bang time and burn width information in order to constrain implosion simulation parameters such as shell velocity and confinement time. This is accomplished by measuring deuterium-tritium (DT) fusion gamma-rays with energy-thresholded Gas Cherenkov detectors that convert MeV gamma-rays into UV/visible photons for high-bandwidth optical detection. Burn-weighted CH ablator areal density is also inferred based on measurement of the 12C(n,n') gammas emitted at 4.44 MeV from DT fusion neutrons inelastically scattering off carbon nuclei as they pass through the plastic ablator. This requires that the four independent GRH gas cells be set to differing Cherenkov thresholds (e.g., 2.9, 4.5, 8 & 10 MeV) in order to be able to unfold the primary spectral components predicted to be in the gamma ray energy spectrum (i.e., DT?; 27Al & 28Si (n,n')? from the hohlraum thermo-mechanical package (TMP); and 12C(n,n')? from the ablator). The GRH response to 12C(n,n')? is calibrated in-situ by placing a known areal density of carbon in the form of a puck placed ~6 cm from a DT exploding pusher implosion. Additional puck materials are used to make nuclear cross section measurements for 14.1 MeV DT fusion neutrons. Comparisons between inferred gamma fluences and simulations based on the nuclear cross sections database will be presented.

Prompt gamma-ray measurements in inertial confinement fusion experiments

248th National Meeting (2014)

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