The most accurate absolutely calibrated measurement of the total yield of neutrons from experiments on the National Ignition Facility will be from activation of threshold nuclear reactions. The high-yield neutron activation system is being designed to provide high-accuracy (similar to the ±7% achieved on other fusion experiments) linear measurements over a 9-order-of-magnitude dynamic range from the facility limit of ∼1019 neutrons/shot down to a minimum of ∼3×1010 neutrons/shot. The system design requirements are presented, and a conceptual design to meet those requirements described.