General Features of Electron Microprobe Analysis
In his thesis (Paris 1951) R. CASTAING (1) published not only the conception of a new instrument for x-ray emission spectrochemical microanalysis, but also a survey of physical principles involved in this promising, non destructive analytical method.
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R. Theisen, Quantitative Electron Microprobe Analysis © Springer-Verlag Berlin Heidelberg 1965
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§ 1 General Features of Electron Microprobe Analysis
tion in depth of the direct emission involving the first factor of the atomic number effect. Further research has proved that: a) The derivation of a theoretical solution for evaluation of the distribution in depth of direct emission is indirecdy (through a modified LENARD coefIicient) and directly dependent on the difference (V - ~), on the electron acceleration voltage (V) and the critical excitation potential of the primary radiation (~). This effect is specially important at low accelerating voltages. At initial electron energies higher than 25 kilovolts a compensation of the ( V-v,: ) dependent factor according to the combination of the LENARD and BOTHE laws occurs, as will be seen below. b) The intensity ratio of backscattered electrons with energies higher than V-v,: (loss of x-ray excitation) emitted by the reference element and the specimen, deviates strongly from unity, especially for analysis of alloys with components of widely differing atomic number and the use of high electron accelerations. Following the development of POOLE and THOMAS ( 5) many microprobe users have tried to improve the correction formulae of PHILIBERT (3) or THEISEN (4) by introducing a supplementary atomic number correction for electron stopping power of the anticathode and for electron backscatter. However, this approach does not give complete satisfaction as it introduces a twofold electron deceleration factor. For unresolved problems, and to test the application and extension of empiricallaws, valuable assistance could be given by electronic computers. The method descrtbed by ARCHARD and MULVEY (6) for numerical computations of the electron path and the emission of x-rays from a given specimen is likely to gain in importance if a better model for electron trajectories is found. 1.2 Constructional Elements of the Electron Microanalyser It is out of the scope of this volume to detail extensively the general structure of the
microprobe for which reference may be made to publications, by CASTAING, BIRKS, DUNCUMB (7-9). The important elements of an electron microprobe are the following: (Fig. 1)
a) Eleetron opneal system The electron be am is generated by a hot filament electron gun working in a vacuum of the order of W- 5 torr and an electron acceleration field of generally 2 to 50 kilovolts. The reduced image of the electron cross-over is focalised by one or several electromagnetic lenses to produce on the specimen surface an electron probe from 0,2 firn up to several firn in diameter. The geometrical resolution of microanalysis is however limited to the electron range in the specimen, from V, the incident electron energy up to the critical excitation potential of the measured characteristic radiation. From LANGMUIRS theory (10) for the brightness of an image and z\\oRYKINS (11) equation for spherical aberration of magnetic lenses, the incident theoretical electron density as a
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