The Spatially Distributed Neutron Activation Diagnostic FNADs at the National Ignition Facility

In need of a spatially resolved neutronic measurement to better understand the implosion physics of inertial-confined fusion, the National Ignition Facility (NIF) developed a distributed Flange-mounted Neutron Activation Diagnostic system (FNAD). FNAD measures primary deuterium-tritium (D-T) fusion neutron fluence at 20 points surrounding the target chamber using the ⁹⁰Zr(n,2n)⁸⁹Zr reaction, utilizing the 12.1-MeV reaction threshold to minimize signal from spurious neutron sources. Through careful design of the measurement systematics, the relative ratios of fluence at those 20 points are measured to within 2%. This precision is sufficient to allow interpretation of the resulting neutron sky as a map of scattering mass areal density (ρR) of the cold compressed D-T fuel surrounding the nuclear burn. Controlling the shape of this fuel during assembly is essential to achieving optimal implosion performance. This paper details the system design and locational deployment, measurement techniques, and calibration procedure. It also outlines data analysis and reduction, and data presentation methods used during the National Ignition Campaign and High-Foot Campaign.