Modeling a Graphite Diagnostic System using MCNPX

Inertial Confinement Fusion (ICF) implosions can be characterized by the target areal density ($\rho $R). The $\rho $R of ICF targets in the National Ignition Facility (NIF) target chamber can be determined by tertiary-induced neutron activation of elements with appropriately high thresholds. In such materials as $^{12}$C, neutron activation results in beta decay and the emission of 511-keV coincidences which are detected by a pair of NaI(Tl) detectors. Optimal diagnostic thickness, contamination effects, and detector response have been modeled using MCNPX. Results will be presented.