Electron-probe microanalysis of low atomic number elements

Abstract

It is shown that electron-probe microanalysis for the low atomic number elements such as oxygen and carbon may be achieved with a commercial instrument by the addition of simple pulse-height analysis equipment. The necessary apparatus comprises a network unit, ratemeters and pulse-height analysers, as described by Dolby, together with a thin-window proportional counter. Results are presented which demonstrate the detection and separation of carbon, nitrogen and oxygen X radiations when present with other X rays; the scanning pictures included show the distribution of carbides and oxides in the metals beryllium, iron and uranium. Some problems concerning contamination of the sample during irradiation by the electron probe and specimen oxidation are discussed with relation to light element microanalysis. The network method may be readily applied to quantitative analysis although difficulties were experienced in obtaining suitable reference standards in light element work. Results are presented for analysis of the respective carbides of silicon, iron and uranium and are subsequently discussed with relation to the correction procedures proposed by Philibert and by Archard and Mulvey. The absorption of carbon X-radiation is shown to be high in all three compounds, but atomic number effects may differ considerably, being large for carbon-uranium and carbon-iron and small for carbon-silicon. In general, atomic number effects may compensate for X-ray absorption; thus at an operating voltage characteristic for each system, it is possible to measure the correct carbon concentration.