A Delayed Neutron Counting System for the Analysis of Special Nuclear Materials

by Madison Theresa Sellers

Institution: Royal Military College of Canada
Department: Chemistry and Chemical Engineering/Chimie et génie chimique
Degree: MASc (Master of Applied Science/Maîtrise ès sciences appliquées)
Year: 2015
Keywords: delayed neutron; special nuclear material; activation analysis; nuclear forensics
Record ID: 2060852
Full text PDF: http://hdl.handle.net/11264/502


Nuclear forensic analysis is a modern science that uses numerous analytical techniques to identify and attribute nuclear materials in the event of a nuclear explosion, radiological terrorist attack or the interception of illicit nuclear material smuggling. The Canadian Department of National Defence has participated in recent international exercises that have highlighted the Nation’s requirement to develop nuclear forensics expertise, protocol and capabilities, specifically pertaining to the analysis of special nuclear materials (SNM). A delayed neutron counting (DNC) system has been designed and established at the Royal Military College of Canada (RMC) to enhance the Government’s SNM analysis capabilities. This analytical technique complements those already at RMC by providing a rapid and non-destructive method for the analysis of the fissile isotopes of both uranium (U) and plutonium (Pu). The SLOWPOKE-2 reactor at RMC produces a predominately thermal neutron flux. These neutrons induce fission in the SNM isotopes 233U, 235U and 239Pu releasing prompt fast neutrons, energy and radioactive fission fragments. Some of these fission fragments undergo β- decay and subsequently emit neutrons, which can be recorded by an array of sensitive 3He detectors. The significant time period between the fission process and the release of these neutrons results in their identification as ‘delayed neutrons’. The recorded neutron spectrum varies with time and the count rate curve is unique to each fissile isotope. In-house software, developed by this project, can analyze this delayed neutron curve and provides the fissile mass in the sample. Extensive characterization of the DNC system has been performed with natural U samples with 235U content ranging from 2 – 7 μg. The system efficiency and dead time behaviour determined by the natural uranium sample analyses were validated by depleted uranium samples with similar quantities of 235U resulting in a typical relative error of 3.6%. The system has accurately determined 235U content over three orders of magnitude with 235U amounts as low as 10 ng. The results have also been proven to be independent of small variations in total analyte volume and geometry, indicating that it is an ideal technique for the analysis of samples containing SNM in a variety of different matrices. The Analytical Sciences Group at RMC plans to continue DNC system development to include 233U and 239Pu analysis and mixtures of SNM isotopes. L’analyse nucléaire légale est une science moderne qui utilise plusieurs techniques d’analyse pour identifier et attribuer des matériaux nucléaires dans le cas d’explosion nucléaire, d’attaque terroriste radiologique ou de la découverte de contrebande illicite de matériaux nucléaires. Le Ministère de la défense nationale du Canada a participé récemment à des exercices internationaux qui ont mis en lumière le besoin de notre pays de développer le savoir-faire en analyse nucléaire légale, ainsi que les protocoles et les capacités, plus spécifiquement pertinents à l’analyse…