Generalized multiplication correlation model for the description of fast and thermal neutron multiplication in neutron coincidence counting and time interval analysis

In safeguard verification measurements, neutron coincidence and multiplicity counting are commonly applied for the non-destructive assay of large amounts of plutonium. A model to describe the multiplication effects in the sample is needed to obtain unbiased measurement results. Assay of plutonium containers with a neutron coincidence collar lined with a cadmiumsheet generally involves only fast multiplication, since the moderating power of oxygen in plutonium oxides or mixed oxides is too low to generate a thermal neutron population in the sample. In this situation, Bohnel's model for fast, instantaneous multiplication is adequate to describe neutron multiplication and its effects on coincidence counting. When however fresh or spent fuel is assayed under water, thermal multiplication plays an important role. In these measurements conditions Bohnel's model is no longer valid. In this paper a new model, called the generalized multiplication correlation model, is presented. This model allows to describe the combined effect of fast and thermal multiplication in neutron coincidence and multiplicity counting techniques. The generalised multiplication correlation model is based on the identification of different time correlation schemes, which are involved in the detection of correlated neutrons leaking from a multiplying medium. The proposed model allows to describe the singles-, doubles- and triples-rates of fast and thermal neutrons in a medium with fast and thermal multiplication. It is also shown how these count rates can experimentally be obtained from the one- and two-dimensional Rossi-alpha distribution in time interval analysis and how the measurement of these distributions for fast and thermal neutrons allows to obtain the ²⁴⁰Pu_eff-mass.