X-ray microscope
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An X-ray microscope uses electromagnetic radiation in the soft X-ray band to produce images of very small objects. Unlike visible light microscopes, X-rays do not reflect or refract easily, and they are invisible to the human eye. Therefore the basic process of an X-ray microscope is to expose film or use a charge-coupled device (CCD) detector to detect X-rays that pass through the specimen, rather than light which bounces off the specimen.
Early X-ray microscopes by Kirkpatrick and Baez used grazing-incidence reflective optics to focus the X-rays, which grazed X-rays off parabolic curved mirrors at a very high angle of incidence. An alternative method of focusing X-rays is to use a tiny fresnel zone plate of concentric gold or nickel rings on a silicon dioxide substrate. Sir Lawrence Bragg produced some of the first usable X-ray images with his apparatus in the late 1940's.
In the 1950's Newberry produced a shadow X-ray microscope which placed the specimen between the source and a target plate, this became the basis for the first commercial X-ray microscopes from the General Electric Company.
In the present Berkeley's XM-1 uses an X-ray lens to focus X-rays on a CCD, in a manner similar to an optical microscope.
Sources of soft X-rays suitable for microscopy, such as synchrotron radiation sources, have fairly low brightness of the required wavelengths, so an alternative method of image formation is scanning transmission soft X-ray microscopy. Here the X-rays are focused to a point and the sample is mechanically moved in a raster fashion, with the transmitted X-rays at each point measured with a detector such as a proportional counter or an avalanche photodiode.
The resolution of X-ray microscopy lies between that of the optical microscope and the electron microscope. It has an advantage over electron microscopes in that it can view biological samples in their natural state.
Additionally, X-rays cause fluorescence in most materials, and these emissions can be analyzed to determine the chemical elements of an imaged object. Another use is to generate diffraction patterns, a process called X-ray crystallography. By analyzing the internal reflections of a diffraction pattern (usually with a computer program), the three-dimensional structure of a crystal can be determined down to the placement of individual atoms within its molecules. X-ray microscopes are sometimes used for these analyses because the samples are too small to be analyzed in any other way.