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SIMS21, Poland 2017 - Laure Sangely abstract

Laure Sangely oral presentation (FN3-Fri1-2-3)

Dead-Time and Instrumental Mass Fractionation Effects during Uranium Isotopic Analysis of Micrometre-Sized Particles

Laure Sangely, Matvey Aleshin, Thippatai Tanpraphan, Olivier Bildstein, Jane Poths

International Atomic Energy Agency - Safeguards Analytical Services, Wagramerstrasse 5, 1400 Vienna, Austria

Since 2010, CAMECA IMS 1280 instruments have been consistently used in the frame of the International Atomic Energy Agency’s Network of Analytical Laboratories (NWALs) in order to analyse the cotton swipes taken in the environment of inspected facilities. Collected dataset aim at resolving the uranium isotopic signatures present in the sample at the scale of individual micrometre-sized particles in order to verify the completeness of a State's declaration of nuclear activities. An exact assessment of measurement uncertainties is a key for the ability to draw Safeguards conclusions based on the reported results.

A recent study pointed out the influence of Electron Multiplier (EM) detector drift effects on the overall uncertainty during uranium isotopic measurements using multi-collector CAMECA IMS 1280 [1]. In this presentation, mechanisms for other uncertainty sources (related to the correction for EM detector dead-time and instrumental mass fractionation) will be discussed.

During the analysis of micrometre-sized particles, a focused primary ion beam is commonly rastered over a small, square area (typically 5-20 µm) around the particle. As a result, the uranium signal is generally emitted from an area smaller than the rastered area. This so-called gate effect might cause a systematic positive bias on U-235/238, if the dead-time correction is applied to average signals.

In addition, it is common practice to entirely sputter the smallest particles over the course of an analysis. Due to the kinetic isotopic effects related to sputter-ionization process, U-235/238 in the remaining reservoir (particle residue) is likely to decrease as the sputtering of the particle gets closer to completion. This is likely to result in a negative bias on U-235/238, given that instrumental mass fractionation is generally calibrated on much larger particles.

In this study, measurements were performed on reference materials in order to evaluate the influence of those effects on measurement accuracy. In light of the results, three approaches to ensure measurement quality will be discussed: i) the integration of a conservative error term to the uncertainty budget, ii) the application of a refined correction scheme, provided that the effects can be precisely monitored during each analysis, iii) the adaptation of analytical conditions in order to minimize the extent of the correction.

[1] P. M. L. Hedberg, P. Peres, F. Fernandes, L. Renaud., J. Anal. At. Spectrom. 30, 2015, 2516-2524