Optimization of the Gamma Reaction History Diagnostic for Double-Shell Campaigns on the National Ignition Facility

The Gamma Reaction History (GRH) diagnostic on the National Ignition Facility (NIF) is a 4-cell gas Cherenkov detector with each of these cells independently energy-thresholded to measure the temporal emission history of gamma-ray producing reactions from NIF implosion experiments. The diagnostic has historically been used to measure the deuterium-tritium (D-T) fusion reaction history, as well diamond ablator areal density for single-shell implosions by measuring the
${ }^{12} \mathrm{C}(\text{n,n}^{\prime}+\gamma[4.4 \text{MeV}]){ }^{12 \mathrm{m}} \mathrm{C}$
reaction. Optimization of the detector for ablator areal density measurements thus far has only been performed for the 4.4 MeV C-12 gamma ray and assumes a monoenergetic source of DT neutrons. The high-Z pushers used in double-shell capsules create gamma rays with a more complex continuum energy spectrum which makes the prediction of the gamma spectrum, and gamma yield, more complex. The high fuel areal density also alters DT neutron energy spectrum significantly which increases uncertainty in the cross-section data used in the monoenergetic model. Therefore, a forward model of the double-shell capsule was developed in MCNP6.2 which uses a continuous energy monte-carlo method to predict the gamma yield and energy distribution. This gamma production model was folded with known GRH detector response functions to find the optimum GRH (ell energy thresholds for measuring the areal density of double-shell pushers such as W, Mo, and Cr, with and without the presence of a cylindrical Au holhraum. The model design and methodology will be presented along with predicted optimized GRH settings.