Fissile Material Detection using Neutron Coincidence Counters

TL;DR

This paper explores neutron coincidence counting techniques for detecting fissile materials, highlighting methods to determine assay mass with varying degrees of calibration and accuracy depending on the counter type and neutron interrogation source.

Contribution

It introduces the use of multiplicity counting for assay mass determination without calibration curves and compares passive and active well counters for fissile material detection.

Findings

Active counters achieve 4% mass error for 1kg HEU

Passive counters are suitable for 240Pu mass measurement

Thermal neutron sources yield smaller errors in small assays

Abstract

A paper study is conducted on the detection of fissile material using neutron coincidence counters. Three unknown independent properties of assays of fissile material assays are identified to be the effective mass, the (alpha,n) production rate and the neutron multiplication rate. Singlet counting can only be implemented for very well-known assay samples. Doublet counting may be employed with calibration curves which reduce the number of unknowns from three to one. Multiplicity counting theoretically enables assay mass determination without calibration curves. Coincidence counting is most widely applied via active and passive well coincidence counters. Passive neutron well counters are well suited for determination of 240 Pu masses. Active neutron well counters are well suited for determination of fissile material masses. Active well counters are found to have relative mass errors of 4% for a highly enriched uranium (HEU) sample of mass of 1kg and 8% for a mass of 50g. Thermal neutron interrogation sources feature smaller relative errors in small assays than fast neutron interrogation sources.