Multiple ion counting measurement strategies by SIMS – a case study from nuclear safeguards and forensics

A primary tool of nuclear safeguards authorities for detecting undeclared nuclear activities is environmental sample analysis containing uranium-bearing aerosol particles. SIMS (Secondary Ion Mass Spectrometry) has been established as a powerful technique for performing uranium isotope analysis on safeguards samples. Typically, a screening of the sample for locating the particles of interest is followed by micro-beam measurements of individual particles in order to obtain their precise isotopic composition. The uncertainty of these measurements, in particular for the minor isotopes 234U and 236U, is mainly limited by the counting statistics as the total amount of uranium available is very low, usually less than 1 pg. This uncertainty can be reduced when recording several isotopes simultaneously using a multi-collection system. This has been available commercially for many years and has been used for different applications in geosciences on Large Geometry-SIMS (LG-SIMS) instruments. This paper will describe and discuss how isotope measurements of low content uranium materials can be optimized using a multi-ion counting system consisting of five discrete dynode electron multiplier (EM) detectors. The results obtained for uranium isotope ratios are presented for different measurement protocols: static multi-collector, dynamic multi-collector and single collector measurements. Some of the fundamental properties of the detector system that affect the overall measurement uncertainty are discussed. Best practices are also suggested based on the current experience.