The Legacy of Raimond Castaing: A Perspective at 60 Years

P Carpenter, E Vicenzi, R Gauvin, J Fournelle
2011 Microscopy and Microanalysis  
Raimond Castaing is arguably the most influential individual in the field of x-ray and ion microanalysis (Figs. 1 and 2). In this paper we briefly summarize his remarkable contributions. Castaing's 1951 Ph.D. thesis and the English translation by Duwez and Wittry, "Application of Electron Probes to Local Chemical and Crystallographic Analysis", serve as an important treatise on the birth of electron-probe microanalysis (EPMA) [1, 2] . Castaing adopted a novel approach by nondestructively
more » ... ing quantitative analysis on micron-sized precipitates in metallurgical samples (previously identified only by powder x-ray diffraction). In doing so he developed the electron microprobe instrumentation, the wavelength-dispersive spectrometer (WDS), improved aspects of electron optics including aberration correction, developed the theory of quantitative analysis, and applied the technique to important problems of the day. After these impressive accomplishments he supervised the development of the ion microprobe with his then Ph.D. student Georges Slodzian. A number of his contributions to microanalysis are used either in essentially unmodified form or make up the foundation of modern theory of microanalysis. The instrumentation he used consisted of a commercial electron column to which was added a WDS, a reflected light optical microscope, and a sample stage assembly with xyz motion. Instrumental developments were closely coupled with theoretical aspects of the probe-forming system, beam diameter as a function of probe current, and the correction of astigmatism. The electron column was modified in order to produce higher probe current necessary for x-ray microanalysis. These critical efforts by Castaing illustrate how the basic electron column required necessary improvements to achieve required performance. The importance of a co-axial reflected light microscope was emphasized and a similar system was developed for sample viewing and placement relative to the electron beam. Some instrumentation design aspects ultimately required commercial resources to improve optical microscope and WDS takeoff angle issues. A sample introduction and isolation design was developed and the issue of sample displacement resulting from variable vacuum was recognized. The small depth of field design of the optical microscope allowed location of the sample analysis z-axis position, ensuring X-ray optical focus in the WDS. The effect of electron column design and parameters affecting beam diameter and stigmation were evaluated by Castaing. For this first instrument a high probe current was necessary, which required an assessment of the thermal effects of beam irradiation on samples. Experimental observations were compared to theoretical calculations concerning these factors. The need to have sufficient probe current and analytical spot size of~ 1 micron remain a reference point in the thesis, as any relaxation in instrumental requirements would have failed to adequately solve the problem. A WDS with Johannson geometry was built for x-ray diffraction, and used a quartz diffracting crystal with a Geiger counter for detection. The WDS alignment presented a significant challenge and a systematic procedure was outlined in order to iteratively locate the Cu K peak excited from a Cu sample. The theory of quantitative analysis included the relation between x-ray intensity and concentration, with both theoretical and experimental aspects discussed. Castaing recognized the need to correct for atomic number effects, x-ray absorption, and characteristic x-ray fluorescence, thus laying the foundation for le-546
doi:10.1017/s1431927611003606 fatcat:srzhq7vn5jajneqvygi7kds5ci