Year
2010
Abstract
Passive multiplicity measurements of fissile metal assemblies are well established for measuring the multiplication, fissile content, and detection efficiency. In plutonium metal assemblies, most of the neutrons are unambiguously observed from fission chains because of the high spontaneous fission rate. However, uranium metal assemblies have low spontaneous fission rate such that the observed neutrons are a mixture of fission chain neutrons and cosmic-ray spallation neutrons. As a result the multiplication, fissile content, and detection efficiency are difficult to measure without accurate quantification of the single neutron events unrelated to the fission chains. However, studying fission chains is simplified by knowing when each chain starts, which can be accomplished using a time-tagged active source such as a 252Cf ionization chamber or a deuterium-tritium (D-T) neutron generator with an alpha detector. This paper presents time correlation measurements that have been performed with up to five 18 kg highly enriched uranium (HEU) storage castings and a 252Cf ionization chamber placed near the center of each assembly. The measurements included liquid scintillators with pulse shape discrimination operating on the timescale of fission chains and 3 He proportional counters whose traditional use in multiplicity counting is well understood within a point-kinetics framework. Neutron time correlation distributions have been measured to characterize each assembly and are compared with the equivalent distributions collected passively.