X-ray microscope

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. 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 (5 µm down to 0.1 µm). The X-rays are in the more conventional X-ray range (20 to 300 kV), and they are not re-focused. The history of X-ray microscopy can be traced back to the early 20th century. After the German physicist Rontgen discovered X-rays in 1895, scientists soon illuminated an object using an X-ray point source and captured the shadow images of the object with a resolution of several microns. In 1918, Einstein pointed out that the refractive index for X rays in most mediums should be just slightly less than 1, which means refractive optical parts would be difficult to use for X-ray applications. 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. After a silent period in the 1960s, X-ray microscopy regained people's attention in the1970s. In 1972, Horowitzand Howell built the first synchrotron-based X-ray microscope at the Cambridge Electron Accelerator. This microscope scanned samples using synchrotron radiation from a tiny pinhole and showed the abilities of both transmission and fluorescence microscopy. Other developments in this period include the first holographic demonstration by Sadao Aoki and Seishi Kikuta in Japan, the first TXMs using zone plates by Schmahl et al., and Stony Brook’s experiments in STXM. The uses of synchrotron light sources brought new possibilities for X-ray microscopy in the 1980s. However, as new synchrotron source-based microscopes were built in many groups, people realized that it was difficult to perform such experiments due to insufficient technological capabilities at that time, such as poor coherent illuminations, poor quality x-ray optical elements, and user-unfriendly light sources. Entering the 1990s, new instruments and new light-sources greatly fueled the improvement of X-ray microscopy. Microscopy methods including tomography, cryo, and cryo-tomography were successfully demonstrated. With rapid development, X-ray microscopy found itsnew applications in soil science, geochemistry, polymer sciences, and magnetism. The hardware was also miniaturized so that researchers could perform experiments in their own laboratories.