THE SECONDARY FLUORESCENCE CORRECTION FOR X-RAY-MICROANALYSIS IN THE ANALYTICAL ELECTRON-MICROSCOPE

A general formulation for the secondary fluorescence correction is presented. It is intended to give an intuitive appreciation for the various factors that influence the magnitude of the secondary fluorescence correction, the specimen geometry in particular, and to serve as a starting point for the derivation of quantitative correction formulae. This formulation is primarily intended for the X-ray microanalysis of electron-transparent specimens in the analytical electron microscope (AEM). The fluoresced intensity, l(x)(y) is expressed relative to the primary intensity of the fluorescing element, I-y, rather than to that of the fluoresced element, I-x, as has been customary for microanalysis. The importance of this choice of I-y as a reference intensity for the electron-transparent specimens examined in the AEM is discussed. The various factors entering the secondary fluorescence correction are grouped into three factors, representing the dependencies of the correction on specimen composition, X-ray fluorescence probability and specimen geometry. In principle, an additional factor should be appended to account for the difference in detection efficiencies of the fluoresced and fluorescing X-rays; however, this factor is shown to be within a few per cent of unity for practical applications of the secondary fluorescence correction, The absorption of secondary X-rays leaving the specimen en route to the detector is also accounted for through a single parameter. In the limit that the absorption of secondary X-rays is negligible, the geometric factor has the simple physical interpretation as the fractional solid angle subtended by the fluoresced volume from the perspective of the analysed volume, Studies of secondary fluorescence in the published literature are compared with this physical interpretation. It is shown to be qualitatively consistent with Reed's expression for secondary fluorescence in the electron probe microanalyser and with the specimen-thickness dependence of the Nockolds expression for the parallel-sided thin foil, This interpretation is also used to show that the 'sec alpha' dependence on specimen tilt in the latter expression is erroneous and should be omitted. The extent to urhich extrapolation methods can be used to correct for secondary fluorescence is also discussed, The notion that extrapolation methods, by themselves, can be used to correct for secondary fluorescence is refuted, However, extrapolation methods greatly facilitate secondary fluorescence correction for wedge-shaped specimens when used in conjunction with correction formulae.