  We present analytic calculations and results from computational simulations
showing that reaction-in-flight (RIF) neutrons act as a robust indicator for
mixing of the ablator shell material into the fuel in DT capsules designed for
the National Ignition Facility. The sensitivity of RIF neutrons to
hydrodynamical mixing arises through the dependence of RIF production on
charged-particle stopping lengths in the mixture of DT fuel and ablator
material. Since the stopping power in the plasma is a sensitive function of the
electron temperature and density, it is also sensitive to mix. RIF production
scales approximately inversely with the degree of mixing taking place, and the
ratio of RIF to downscattered neutrons provides a measure of the mix fraction
and/or the mixing length. For sufficiently high-yield capsules, where spatially
resolved RIF images may be possible, neutron imaging could be used to map RIF
images into detailed mix images.
