Averaging of electron backscatter and x-ray continuum intensities in multi-element compounds (Abstract only)

(published in IUMAS 2000, Kona, HI, Conference Proceedings)

John J. Donovan¹ and Andrew Westphal²

¹Department of Geology and Geophysics, The University of California, Berkeley, California 94720-4767, USA

²Department of Physics, The University of California, Berkeley, California 94720-7300, USA

For multi-element samples, backscatter and x-ray continuum intensity averages are traditionally predicted by summing the mass fractions of the pure element intensities [1, 2]. In spite of the success of this assumption, there is no physical reason why mass averaging should apply to either of these properties, both of whose productions are determined by physical processes involving only electron and proton charges.

This is demonstrated by the following measurements (Fig. 1 and 2), where in a comparison of absorbed current (reciprocally equivalent to backscatter yield) and x-ray continuum production, it can be seen that stable isotope pairs of natural and enriched materials of identical Z yield essentially equal intensities with a precision of approximately 0.3% for the absorbed current measurements and 1% for the x-ray continuum measurements. Demonstrably and unsurprisingly, the presence or absence of neutrons in the nuclei of the target atoms does not influence backscatter or x-ray continuum productions at levels detectable in the electron microprobe.

Fig 1. Absorbed current measurements (two different sample splits), for the three isotope pairs. The absorbed current precision levels were typically 0.3% or less, while the natural abundance and isotopes samples differed from each other by 2.2 to 4% in mass.

Since mass is not a factor in these productions, we suggest that the efficacy of traditional mass averaging in predicting these properties is simply due to the approximate constancy of A/Z. We suggest that calculations of these properties in a physically based theory of backscatter and continuum production should depend only on Z, and be independent of mass. This hypothesis is supported by the following measurements (Fig 3) in which x-ray continuum intensities are plotted versus predictions calculated from pure element end-member measurements, by assuming either mass or electron fraction averaging. We suggest further precision tests which could be carried out to constrain calculations.

Fig 2. Continuum intensities for the three isotope pairs, uncorrected for continuum absorption and anisotropy, measured on the high theta side (+ 0.01 sinf) of the Fe and Al Ka emission line positions. Continuum intensity precision levels were typically 1%, although the averaged values for each isotope pair usually differed by considerably less than this.

Fig

Fig 3. Predicted "property" averaged x-ray continuum intensities from measurements on the NIST Au-Ag-Cu SRM (20 keV, 100 nA, 180 seconds counting time per point, average of 5 measurements per point), corrected for continuum absorption and anisotropy, acquired at 0.01 sinf above the actual line position for Fe Ka, using both mass and electron fraction [3] averaging.

References

[1] Duncumb P and Reed SJB 1968 Quantitative Microanalysis, ed KFJ Heinrich (National Bureau of Standards Spec. Pub. 298) p 133

[2] Goldstein JI, Newbury DE, Echlin P, Joy DC, Fiori C and Lifshin E 1981 Scanning Electron Microscopy and X-ray Microanalysis (New York and London: Plenum Press) p 96

[3] Pingitore NE, Donovan JJ and Jeanloz R 1999 Jour. App. Phys. 86:5 p 2790-2794