Advances in fission-track detection and analysis for nuclear forensics and safeguards investigations

A major task in nuclear forensics and safeguards investigations is the detection and analysis of small particles containing fissile isotopes that may be present at low concentration within particulate environmental samples. These radioactive particles-of-interest (POIs) are usually in the micro-metric size range, and intermixed within large populations of other particles, such as dust, soil, or industrial exhaust pollutants. A commonly used method for the detection of particles of single POIs containing fissile isotopes is to embed a dispersion of
particles within a thin polymer layer (“catcher”), attach a “detector" sheet (for example, LEXAN®) to the catcher, irradiate this structure with thermal neutrons, and then separate the two and chemically etch the detector to “develop” fission track (FT) clusters. The resulting FT clusters are visible under light microscopy,
and indicate the presence of 235U or other fissile isotopes within the POI. Measuring the coordinates of the FT cluster in the detector image enables the POI to be retrieved from the corresponding location in the catcher for further analysis. The main challenge in this procedure is imaging a large area detector (typically 100-400 mm² ), at a resolution ≤1 µm, and automatically scan and locate the FT clusters while rejecting artifacts. This paper reports a novel approach which enhances the ability of the Particle Analyst to accurately identify and locate FT clusters.