Use of thermal-neutron time-correlated counting to analyze multiplying assemblies of HEU

A series of experiments and numerical simulations using thermal-neutron time-correlated measurements has been performed to determine the neutron multiplication, M, of assemblies of highly-enriched uranium available at Idaho National Laboratory. The experiments used up to 14.4 kg of highly-enriched uranium, including bare assemblies and assemblies reflected with high-density polyethylene, carbon steel, and tungsten. A small
²⁵²
Cf source was used to initiate fission chains within the assembly. Both the experiments and the simulations used 6-channel and 8-channel detector systems, each consisting of
³
He proportional counters moderated with polyethylene; data was recorded in list mode for analysis. `True' multiplication values for each assembly were empirically derived using basic neutron production and loss values determined through simulation. A total of one-hundred sixteen separate measurements were performed using fifty-seven unique measurement scenarios, the multiplication varied from 1.75 to 10.90. This paper presents the results of these comparisons and discusses differences among the various cases.