ISSN:
0049-8246
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Physics
Notes:
Coherent and incoherent scattering processes are commonly ignored, or at best estimated only crudely, in X-ray fluorescence experimently designed either to measure physical constants or to effect nondestructive chemical analysis of mixtures and alloys. The influence of this scattering is of second order, but is nevertheless large enough (generally 2-5%) to overshadow inherent errors in experimental measurement. In this paper, we develop a formalsim for calculating scattering corrections applicable to experiments involving either thin polycrystalline or amorphus samples studied in transmission or thick samples of a similar kind examined using reflection geometry. Our principal approximations are physical; we assume (i) that crystal line structure and texture in the sample serve merely to modulate the angular distribution of coherent scattering can be ignored. Mathermatical development of the model is essentially exact. Adequacy of approxination (i) is supported by previously published experimental and theoretical work; as for approximation (ii), we have found that results do not depend crucially upon the particular angular distribution function assumed for scattering. We believe, at a conservative estimate, that overll errors in our scattering corrections are in almost all cases of interest less than 25%; data to which they are applied, therefore are, potentially accrate to better than 0.5-1.25% Three important effects have to be considered: (a) the generation of fluorescence radiation which has been scattered within the sample,(b)the scattering of fluorescent radiation away from the acceptance aperture of lthe detector, and (c)the scattering of fluorescent radiation into this aperture. In our formalism, only (a)and (c)need be calculated explicityly. Corrections are expressed simply as products of quotients σ/μ with certain enhancement terms (μ being a total attenuation constant and σ an attenuation constant for scattering). In transmission geometry, the enhancement terms are complicated functions of transmission factors for exciting and fluorescent radiations only and, in reflection geometry, they are simpler functions of angles specified by the experimental arrangement used and the total attenuation constants for exciting and fluorescent radiations. It turns out in many cases that (c) above is the predominant effect and, commonly, the positive contribution to measured fluorescent intensity made by scatting into the detector outweighs substantially the negative contribution made by scattering out of the detector. Results are also expressed in simple graphical form in terms of effective attenuation constants for exciting and fluorescent radiations; in this form they are readily available for use by the reader without the necessity for further calculation. Surprisingly, effective attenuation constants are generally smaller, often consideraby smaller, than constants representing photoelectric absorption alone.
Additional Material:
8 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/xrs.1300070410
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