X-ray microscopy

An X-ray microscope uses electromagnetic radiation in the soft X-ray band to produce magnified images of objects. Since X-rays penetrate most objects, there is no need to specially prepare them for X-ray microscopy observations.

Unlike visible light, X-rays do not reflect or refract easily, and they are invisible to the human eye. Therefore, an X-ray microscope exposes film or uses a charge-coupled device (CCD) detector to detect X-rays that pass through the specimen. It is a contrast imaging technology using the difference in absorption of soft X-rays in the water window region (wavelengths: 2.34-4.4 nm, energies: 280-530 eV) by the carbon atom (main element composing the living cell) and the oxygen atom (main element for water).

Microfocus X-ray also achieves high magnification by projection. A microfocus X-ray tube produces X-rays from an extremely small focal spot (5µm down to 0.1µm). The X-rays are in the more conventional X-ray range (20 to 300 kV), and they are not re-focused.

Early X-ray microscopes by Paul Kirkpatrick and Albert Baez used grazing incidence reflective X-ray 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 1940s.

In the 1950s Sterling 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.

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