The Use of the Point Model Equations in Understanding Active Neutron Measurements

Volume: 68, December 2026, pages 45 - 54

Authors:  Stephen Croft^a, Andrea Favalli^b, Robert D. McElroy Jr.^c, and Melissa Styth^a
aLancaster University, Bailrigg, Lancaster, LA1 4YW, UK 

^bEuropean Commission, Joint Research Centre (JRC), Ispra, Italy
^cOak Ridge National Laboratory, Oak Ridge, TN, USA (Ret.)
 

Abstract: The familiar point model equations used to interpret passive neutron correlation counting for the assay of spontaneous fission effective mass are adapted in this work to interpret and guide active neutron interrogation measurements with a steady-state time random neutron source. We show that two distinct formulations of the problem are mathematically equivalent, despite emphasising different physical interpretations and item characteristics. The first is where the fission rate induced by the interrogating neutrons is used as the primary source term. This views active interrogation as a means of creating a signal. It is the intuitive perspective when considering the assay of dilute non-multiplying items. The second viewpoint is where the interrogating neutrons entering the item are treated as the assay process initiator, focusing instead on estimating multiplication. For bulk items where self-multiplication is significant, the response becomes non-linear in fissile mass, and this formulation helps visualise and explain that behaviour. The distinction between these two equivalent approaches is pedagogically important depending on one’s background; specifically, it bridges the conceptual gap between nuclear physics experiments which commonly use thin targets (i.e. a thin-target accelerator experiment) to simplify data analysis, and practical safeguards measurements which routinely deal with non-linear calibration curves in large reactor assemblies.
 

Keywords:
Active Neutron Interrogation; Point Model; Neutron Correlation Counting; Nuclear Safeguards

Reference guideline: Croft, S., Favalli, A., McElroy, R.D., Styth, M., (2026). The Use of the Point Model Equations in Understanding Active Neutron Measurements, ESARDA Bulletin - The International Journal of Nuclear Safeguards and Non-proliferation, 68, 45-55.  https://doi.org /10.3011/ESARDA.IJNSNP.2026.5