Estimating propagation of uncertainty in the instrumental function into reconstructions of neutron sources in ICF implosions at NIF

   Neutron imaging is an important component of the nuclear diagnostics suite at the National Ignition Facility (NIF). The Nuclear Imaging System (NIS) provides a means to measure neutron sources in inertially confined fusion (ICF) implosions. Two- and three-dimensional neutron source reconstructions of an ICF implosion are key indicators of its overall performance.

   The estimation of uncertainty in the shape of the reconstructed neutron sources is critical to the interpretation of the results provided by the diagnostic. There are two main sources of errors in the reconstructions. First, the limited statistics of the neutrons leads to random errors in the reconstructed images; and second, the incomplete knowledge of the instrumental function (a pinhole-dependent point-spread function) of the recording device results in shape distortions in the reconstructed images. While statistical errors are the leading source of uncertainty in reconstructed images for low-yield shots, they are dominated by errors associated with the instrumental function for high-yield shots on the order of 1e16 neutrons.

   This work proposes a method to estimate errors in reconstructed images associated with the incomplete knowledge of the instrumental function. Additional reconstructions are built from the fixed collection of pinhole images best aligned with the neutron source. Statistical variations in the shapes of these reconstructions originate solely from uncertainties in the instrumental function. This additional uncertainty estimate accounts for the reconstruction shape variations in high-yield shots where statistical uncertainty is insufficient.