On the secondary X-ray emission induced by electron irradiation in thin samples

Abstract

Résumé. 2014 Une expression générale pour l'émission secondaire de rayons-X induite par électrons dans des composés binaires est dérivée. Des simulations sur ordinateur utilisant cette expression ont été faites pour des échantillons cunéiformes, cette géometrie étant la plus fréquente en microscopie en transmission conventionelle et à balayage. Ceci a permit d'étudier la fluorescence caractéristique en fonction de l'épaisseur de l'échantillon tout en changeant la géometrie de la cible (l'angle du coin) ainsi que le potentiel d'accélération du microscope. Les influences de l'orientation de l'échantillon dans le faisceau d'électrons et de la direction de détection des rayons-X ont aussi été analysées. Ces calculs ont permis de constater que la géometrie de l'échantillon influence fortement la fluorescence caractéristique alors que les autres paramètres mentionnés plus haut n'ont qu'une importance secondaire.Abstract. 2014 In the present paper earlier results in the field of secondary X-ray emission in binary compounds are reviewed and extended. A general expression for the X-ray fluorescence emission in electron excited (scanning) transmission electron microscopy ((S) TEM) specimens is derived. Based on this expression computer simulations are carried out for wedge shaped targets, so as to investigate characteristic fluorescence emission as a function of specimen thickness, wedge angle and primary accelerating voltage and this for various electron impact directions and X-ray detection directions. The general conclusion is that a small change in specimen geometry can have a serious influence on the secondary emission intensity.

The parameterless correction method in X-ray microanalysis

“…Computer simulations of this secondary radiation in wedge shaped specimens revealed that this polynomial in T without constant term, is smooth and that the first or linear term is directly proportional with the wedge angle a [4,5]. To evaluate this influence of the specimen shape on the secondary emission, simulations for a Cu-50 at% Cr alloy are shown in figure 1 for diflerent wedge angles (a = 0°, 10°, 20°, 30° and 40°).…”

“…2 Equation (3) can also be expanded in a power series in T and in the case of wedge shaped specimens, computer simulations showed that the linear term is proportional to the wedge angle [5].…”

“…The total intensity Ix of element X is given by the sum of (2) and (4) (11) and (12) it is clear that strictly speaking the extrapolated value of the uncorrected mass concentration ratio (11) is not equal to the exact ratio (12 It has been shown that for a perfect plane parallel foil the linear term of fluorescence emission equals zero (11 = 0) [5]. Hence If = fB = 0 so that from (11) and (12) it is clear that for T or Ix approaching zero (CA/CB)' == (CA/CB) without any assumption.…”

“…The secondary radiation of element X generated in dV and emitted towards the detector reads : Since this ratio is independent of T it equals the limit as T approaches zero. To evaluate the magnitude of equation (44) numerical data acquired with computer simulations of fluorescence radiation in wedge shaped targets will be used [5]. For The ionization cross section ratio 03C8iY (Eo) /03C8jX (Eo) can be approximated with the BETHE for- (9).…”