Study of one cause of error in microanalysis by electron probe: indirect excitation of the matrix

Systematic microprobe measurements on zirconium-graphite diffusion couples demonstrated that parasitic CK X-radiation is emitted from areas in the sample located far away from the electron focusing point. To investigate this cause of error, different targets were used, while the lower face of the objective pole piece was being screened with foils of different atomic numbers, ranging from 4 (Be) to 73 (Ta). From the discussion of results, it appears that these parasitic radiations are mainly due to electrons which have been twice backscattered, firstly at the specimen itself, and secondly at the objective pole piece. The resulting errors in intensity evaluations may be particularly disturbing in the analysis of inclusions if the same elements to be analysed are present both in the inclusion and in the matrix. Several methods for minimizing these errors are proposed, for example coating the pole piece with a material of low atomic number such as beryllium.