A Continuous Energy Instantaneous Point Source Model for Implosion Ablator Areal Density Measurements on National Ignition Facility

Recent work on Inertial Confinement Fusion (ICF) implosions have focused on measuring the gamma ray emission history from nuclear reactions inside of DT fuel capsules capsule utilizing the gamma reaction history (GRH) diagnostic. The ablator areal density from carbon based ablators <ρR>_c has been measured by thresholding the GRH gas cells to resolve the emission from the 16 MeV DT gamma rays and 4.4 MeV gamma rays from metastable carbon through the ¹²C(n,n’)^12mC reaction. These measurements relied on the assumptions of Monoenergetic Instantaneous Point-Source (MIPS) model. The error introduced by this model was found to be acceptable for simple capsule geometries and low fuel <ρR> values found in experiments on the OMEGA-60 laser (100 mg/cm²). For the National Ignition Facility (NIF) ICF experiments however, the larger fuel <ρR> value (500-600 mg/cm²) leads to more down scattered neutrons incident on the ablator. Furthermore, the presence of a holhraum creates a more complex scattering geometry where the DT neutrons interact with the holhraum walls producing more gamma rays incident on the detector. The assumption of the monoenergetic 14.1 MeV energy spectrum in the MIPS model was identified to be one of the largest sources of error in the interpretation of the data. Therefore, a Continuous-energy, Instantaneous, Point-Source model (CIPS) was developed using the MCNP6 code by simulating the down-scattered neutrons for various fuel and carbon <ρR> values for the NIF geometries. This model will allow for more accurate <ρR>_c values that have been previously obtained and allow for the identification of trends in capsule performance with <ρR> for NIF shots.