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

A general formulation for the secondary fluorescence correction is pre sented. It is intended to give an intuitive appreciation for the vario us factors that influence the magnitude of the secondary fluorescence correction, the specimen geometry in particular, and to serve as a sta rting point for the derivation of quantitative correction formulae. Th is formulation is primarily intended for the X-ray microanalysis of el ectron-transparent specimens in the analytical electron microscope (AE M). The fluoresced intensity, l(x)(y) is expressed relative to the pri mary 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 gro uped into three factors, representing the dependencies of the correcti on on specimen composition, X-ray fluorescence probability and specime n geometry. In principle, an additional factor should be appended to a ccount for the difference in detection efficiencies of the fluoresced and fluorescing X-rays; however, this factor is shown to be within a f ew per cent of unity for practical applications of the secondary fluor escence correction, The absorption of secondary X-rays leaving the spe cimen en route to the detector is also accounted for through a single parameter. In the limit that the absorption of secondary X-rays is neg ligible, the geometric factor has the simple physical interpretation a s the fractional solid angle subtended by the fluoresced volume from t he perspective of the analysed volume, Studies of secondary fluorescen ce in the published literature are compared with this physical interpr etation. It is shown to be qualitatively consistent with Reed's expres sion for secondary fluorescence in the electron probe microanalyser an d with the specimen-thickness dependence of the Nockolds expression fo r the parallel-sided thin foil, This interpretation is also used to sh ow that the 'sec alpha' dependence on specimen tilt in the latter expr ession is erroneous and should be omitted. The extent to urhich extrap olation methods can be used to correct for secondary fluorescence is a lso discussed, The notion that extrapolation methods, by themselves, c an be used to correct for secondary fluorescence is refuted, However, extrapolation methods greatly facilitate secondary fluorescence correc tion for wedge-shaped specimens when used in conjunction with correcti on formulae.