Variations of Point Model Analysis in Neutron Multiplicity Counting and Ongoing Theoretical Developments

Fissioning material emits neutrons in temporally correlated bursts corresponding to the length of the fission chains. Measuring the degree of correlation can be used to find certain properties of the material. In most neutron multiplicity applications, the average rate of neutron detection (called singles), and the detection rates for correlated doubles and triples are measured. In a passive assay a simplified analysis model called the point model is used to relate the three measured parameters (singles, doubles and triples) to three unknown material properties; (1) the effective mass of spontaneously fissioning isotopes, (2) the neutron leakage self-multiplication in the material, and (3) the ratio of the number of neutrons produced from (?,n) reactions to those originating from spontaneous fission. The detection efficiency along with other parameters of the measurement chamber are assumed to be known. We review previous modifications to the point model to mitigate the errors from inherent model assumptions, including weighted point models and the inclusion of more refined physical parameters in the model. We also review our current progress in on-going work for developing new modifications to the point model, including extensions for utilizing measured quadruples to better inform the measurement process.