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

Abstract

This paper describes and discusses 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.

Latest improvements in isotopic uranium particle analysis by large geometry–secondary ion mass spectrometry for nuclear safeguards purposes

“…1). This allows the 235 U/ 238 U ratio to be calculated from the measurement on the L1 detector, 7 or as is done in this paper, the 235 U/ 238 U ratio is based on the static measurements (sequence 1), but corrected with the average relative detector yield obtained from the 238 U measurements on L1 and H1 detectors (sequences 1 and 2). The typical variation of relative L1/H1 detector yield within the 100 cycles of a given microbeam measurement is shown in Fig.…”

“…7 Previously, only an average DT correction was available for isotope ratio measurements in the CAMECA software. The DT correction was applied to the average count rate over a measurement cycle and was therefore imprecise in the case of rastered beam and nonuniform signal, as it is typically the case for microbeam measurements on small particles.…”

“…Static multicollector protocol can also be selected, but then the EM detectors yield intercalibration needs to be handled carefully, in particular for L1/H1 as these detectors receive the higher secondary ion doses and thus are expected to have more substantial gain drift. 7 The instrumental mass bias effect in uranium isotope analysis requires a correction in the order of $0.1-0.4% per amu. 6 This mass bias is mainly the result of different ionization yields at sample emission, nonuniform distribution over a cross section of the secondary ion beam, as well as different detector responses for isotopes with different masses.…”

“…7 For optimized accuracy and precision for both minor and major U isotopes, the multidynamic mode is preferred as it combines advantages of single collector and static multicollector methods. H signals in static mode, and a second one after switching the 238 U signal from detector H1 to L1 (Fig.…”

“…Recently, the performance of SIMS measurements for this application has been greatly improved using large geometry-SIMS (LG-SIMS) instruments that provide both automated screening capabilities to locate particles of interest as well as high precision isotopic measurements using a multi-ion counting detector system. [6][7][8][9] Low detection limits and low uncertainties are generally required for isotopic analysis of these micrometer to submicrometer particles. Samples taken from uranium enrichment facilities allow to investigate the production details and the used feed material by analyzing the 234 U/ 235 U and 236 U/ 235 U isotope ratios.…”