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A useful new technique for mineralogy; the backscattered-electron/low vacuum mode of SEM operation

1979; Mineralogical Society of America; Volume: 64; Linguagem: Inglês

1945-3027

Brian W. Robinson, Ernest H. Nickel,

Geochemistry and Geologic Mapping

Operation of a scanning electron microscope in the backscattered-electron/low-vacuum mode instead of the conventional secondary-electron'/high-vacuum ode transforrns the SEM into a powerful general-purpose tool for mineralogy and the other earth sciences. The technique his two major advantages over conventional SEM operation: (l) High-quality atomic-number contrast images are produced, providing information that is much more useful than that normally obtained from an SEM. Compositional information dominates topographic information in the images so that most phases are clearly differentiated. (2) The technique is very simple. No conductive coating is required, even at high accelerating voltages. Drill core, hand specimens, mill products, polished sections, or porous wet samples can all be imaged directly as received, and it is possible to display an funage and an X-ray spectrum within a minute of receipt of the specimen. Introduction creased useful inage information available from a The purpose of this paper is to bring rc the attenBE image when compared with a SE image of the tion of mineralogists and geologists a very useful desame rough area. velopment in scanning electron microscopy, namely Many SEMs now in use produce BE images which the backscattered-electron/low-vacuum mode of opare inferior in quality when compared with SE imeration. This technique was developed by Dr. V. N. E. ages produced by the same instrument. This is due Robinson at the University of New South Wales not to any intrinsic advantage of SEs over BEs, but to (Robinson, 1975, 1976) and has been in use in our thecommonuseoflow-eftciencyBEdetectors(often laboratory since mid-1977. Our experience with the the BE detector is merely the SE detector with the technique has been most encouraging, and we feel it bias voltage removed). This may explain why the has an assured future in mineralogical research. SEM has, to date, found only limited application in Backscattered-electron (BE) images are far more mineralogy and has been used chiefly to illustrate useful for mineralogy than are the secondary electron mineral morphology. (SE) images normally used in scanning electron miHowever, at least two types of BE detectors are croscopes (SEM$. This is because BE images obnow available, capable of producing images of the tained with a detector placed above the specimen same high quality and resolution as those produced contain a great deal of compositional information with SE detectors. Instead of the more common (atomic-number contrast) which dominates the toposemiconductor BE detector, the detector chosen for graphic information. [See, for example, Blaschke and this work is of the wide-angle scintillator-photoHeywood (1977), Kiss and Brinkies (1976), Wells multiplier type which replaces the normal SE detec(1977).1Thus the primary feature of the BE image is tor in the SEM and requires no additional electronics the distribution of phases of different average atomic (Robinson, 1975). This type of detector allows the number, while the topography of the surface is only a production of high-quality TV images, which is a superimposed secondary feature. On the other hand, major advantage for investigative mineralogy where in SE images, compositional information is usually large areas of sample need to be examined quickly to completely masked by the dominant topographic search for phases of interest. An example of the TVcontrast. Figures lA and lB illustrate the greatly inimage quality is shown in Figure 2. w3-n4x/'t9/lrr2-r122$o2.oo 1322 Fig. l. (A) Backscattered-electron, and (B) secondary_electron images of the same area of a rough section of banded iron forma_ tion. The hematite (bright) is clearly distinguished from the quartz (dark) on the BE image. On the SE image, however, the compositional information is almost completely masked by topographic information of no mineralogical interest. Carbon coated, 20kV. ETEC Autoscan semiconductor BE detector. ln addition to providing a more useful image than SE detectors, BE detectors allow the use of an ..environmental cell or low-vacuum modification to the specimen chamber (Robinson, 1976). This is simply the creation of a pressure differential between the specimen chamber and the column so that, while the electron gun runs at a normal .,high,, vacuum, the specimen chamber is maintained at a ,.low vacuum of about 0.1 torr. The pressure differential is maintained by the constricting effect of the small final aperture in the electron column. The relatively high gas pressure in the specimen chamber allows the dissipation of charge from the specimen surface and prevents the occurrence of charging artefacts in BE images. Thus no conductive coating is required even ROBINSON AND NICKEL: SEM OPERATION 1323 Fig. 2. Photograph of TV screen showing high-quality BE image at TV scanning rates. Polished section ofgossan clearly shows a gold grain (bright); uncoated,30kV. for insulating specimens at high accelerating voltages. This essentially eliminates any specimen preparation and greatly increases ample throughput: Wet, dirty, and porous samples may be quickly examined without fear of contamination of the electron gun by gases or volatile compounds. An SEM used in the BEllow-vacuum mode is a most useful tool for many mineralogical tasks, especially when it is equipped with an energy-dispersive X-ray (EDX) detector, and the technique is as easy to use as optical microscopy. As no conductive coating is required, subsequent use of other techniques on the same specimen is not hampered by the presence T o d i t l u s i o n p u m p