Results of Three Neutron Diagnosed Subcritical Experiments

Neutron diagnosed subcritical experiments (NDSEs) aim to measure the fission chain decay of subcritical test objects initiated by neutrons from an external source. The ultimate goal of future NDSEs is to make such measurements on dynamic subcritical objects as they are imploded. As a step toward that goal, we made measurements on three static subcritical assemblies containing highly enriched uranium. Specifically, we measured the die-off of both fission gamma rays and neutrons, with nanosecond resolution over several hundred microseconds, relative to the emission time of neutrons from a source in close proximity to the subcritical objects. As simulations are expected to play a key role in interpreting future NDSEs, we compared our data to detailed MCNP (R) calculations. There was good agreement between the data and MCNP over die-off times expected to be most important to future NDSEs, but there were significant differences at both earlier and later times. We attempted to reconcile these differences by several changes to the simulations. In particular, we found that reducing the U-235(n,n') cross section to 80% of the current ENDF7.1 evaluation resulted in much better agreement.