AVS 51st International Symposium
    Applied Surface Science Monday Sessions
       Session AS-MoP

Paper AS-MoP6
Application of Time Interpolation to SIMS Isotopic Ratio Measurements

Monday, November 15, 2004, 5:00 pm, Room Exhibit Hall B

Session: Poster Session
Presenter: D. Simons, National Institute of Standards and Technology
Authors: D. Simons, National Institute of Standards and Technology
K.J. Coakley, National Institute of Standards and Technology
A.M. Leifer, National Institute of Standards and Technology
Correspondent: Click to Email
Secondary ion mass spectrometry (SIMS) can be used to perform localized isotopic ratio measurements on a micrometer scale. Such measurements have broad applicability in areas of biology, geology and astronomy. A specific application area of recent interest is nuclear forensics, whereby SIMS has been applied to the search for evidence of uranium enrichment activities through the measurement of the relative abundances of U-235 and U-238 in micrometer-sized particles. In SIMS measurement systems, the count rate of isotopes may vary in time as a limited sample quantity is consumed during the analysis. Since only one isotope at a time is measured in conventional ion counting detection systems, this drift can introduce systematic error into the estimate of the ratio of any two isotopes. Hence, correcting the data for drift is critical to the accurate determination of isotopic ratios and their associated uncertainties. We consider isotopic measurements in a pairwise fashion, with the more abundant isotope of the pair designated as the major isotope and the less abundant one as the minor isotope. We correct the measurements for drift by aligning the major and minor time series of isotopic pairs by use of linear interpolation. We estimate an isotopic ratio for each of two cases. In one case the time series of the more abundant isotope is aligned with respect to the time series of the less abundant isotope. In the second case the less abundant isotope is aligned with respect to the more abundant one. We average both of these estimates to get a drift-corrected estimate. We present an analytical formula for the uncertainty of the isotopic ratio which accounts for correlation introduced by interpolation. We also present an approximate hypothesis test procedure to detect and quantify possible time-dependent drift of the measured isotopic ratio during a single analysis.