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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Electron probe microanalysis | 3/4 | https://en.wikipedia.org/wiki/Electron_probe_microanalysis | reference | science, encyclopedia | 2026-05-05T10:04:28.363360+00:00 | kb-cron |
=== Application in metallurgy === At the end of the 1950's, Castaing's innovative work was complemented by an instrument that scanned the electron beam and thus enabled the distribution of trace and alloying elements in a sample of metal to be imaged. From a metallurgist's point of view this constituted the biggest advance in metallography since Henry Clifton Sorby had invented the reflected light microscope a hundred years earlier. For while it is helpful to be able to detect the presence of an element on the micron scale, it is even more valuable to be able to image its distribution. This ability to detect for the first time the presence of alloying or trace elements dissolved in a host metal, and image their distribution advanced the science of metallurgy itself. It enabled the identification of non-metallic inclusions, revealed segregation during solidification, and allowed identification of the sources of grain boundary weakness as well as many other problems. The instrument that first did this, the scanning electron probe microanalyzer, emerged from research at Cambridge University, and development work at the nearby laboratories of British engineering firm Tube Investments (TI). It is one of the early examples of a breakthrough borne of the close collaboration between university and industry in what became known as the Cambridge Phenomenon. One of the organizers of the 1949 Delft Electron Microscopy conference had been Vernon Ellis Cosslett at the Cavendish Laboratory at Cambridge University, a center of research on electron microscopy. Concurrently, in the Department of Engineering at Cambridge, Charles Oatley had been working on the related but distinct field scanning electron microscopy, and Bill Nixon on X-ray microscopy. In 1957 Peter Duncumb, then a young physicist and research fellow, combined all three technologies to produce a prototype scanning electron X-ray microanalyzer for his PhD thesis. Meanwhile, ten miles south of Cambridge, British engineering group Tube Investments (TI) had recently opened (1954) a group research laboratory; the Tube Investments Research Laboratory (TIRL) at Hinxton Hall, and in 1957 had recruited David Melford, a metallurgist from Cambridge who had just completed his own PhD. They set him the task of finding the distribution of trace elements dissolved in steel in regions on the scale of microns. Melford was quickly directed to Duncumb, back at the university, and on August 7, 1957, the pair examined a piece of steel in the instrument Duncumb had built. It proved an ideal demonstration of the potential value of this equipment as a research tool.
TIRL at once recruited Duncumb as a consultant and tasked Melford to design whatever it took to embody the demonstrator Duncumb had developed into an instrument for metallurgical use. Melford's pencil sketch, drawn on Christmas Day 1957 and now in the Cambridge University library, defined the layout of the instrument, although no engineering drawings had yet been made. Crucially, the instrument included an optical metallurgical microscope, essential in selection of the field of view, and allowing both optical and X-ray images of the sample to be captured and studied alongside each other. Duncumb and he then produced around a 100 dimensioned sketches which the well-equipped workshop at Hinxton Hall converted into a finished instrument. It was commissioned shortly before Christmas 1958 and is now in the reserve collection of the Science Museum, London. There had been no thought so far of building anything other than a valuable research tool, but, in January 1959, H. C. Pritchard the Managing Director of the Cambridge Instrument Company visited TIRL and saw the instrument in action. In March of that year the Company, with the agreement of TI and the Cavendish Laboratory, decided to build a copy – the first commercial scanning electron probe microanalyzer. With the help of Duncumb and Melford's drawings, they soon started manufacture and the first instrument was on show at the Institute of Physics meeting in January 1960. This early example (pictured at the head of this page) is now in the Cambridge Museum of Technology.
== Operation == A beam of electrons is fired at a sample. The beam causes each element in the sample to emit X-rays at a characteristic frequency; the X-rays can then be detected by the electron microprobe. The size and current density of the electron beam determines the trade-off between resolution and scan time and/or analysis time.