Impact of afterglow correction uncertainty on down-scattered neutron imaging at the National Ignition Facility

Down-scattered neutron imaging (6-12 MeV) at the National Ignition Facility diagnoses cold fuel assembly in inertial confinement fusion experiments. Residual primary neutron signal (afterglow) from scintillator decay must be subtracted from down-scattered measurements. With afterglow correction uncertainty comparable to the signal itself (both at the 1%-4% level relative to primary), reconstructions are highly sensitive to correction errors. We examine this sensitivity by perturbing the afterglow correction factor from -75% to +75% in 5% increments for shot N250126. Legendre P0 contour analysis at 17%, 50%, 80%, and 90% intensity levels reveals that outer contours (17%, 50%) show <10% variation across the full perturbation range, while inner contours (80%, 90%) exhibit steep sensitivity to over-subtraction, with >40% variation as the correction factor increases. The sensitivity is asymmetric: over-subtraction causes rapid degradation and peak location shifts due to the oblate primary geometry, while under-subtraction produces near-linear changes. For analyses of internal hotspot features, under-subtraction is preferable to over-subtraction to avoid reconstruction artifacts.