Number 284 (Story #2), September 3, 1996 by Phillip F. Schewe and Ben Stein
PHASE IMAGING WITH HIGH-ENERGY X-RAYS . Conventional x-ray imaging methods detect how many x rays are absorbed as they pass through a sample. Contrast in the resulting image arises from variations in absorption. But as objects get thicker it is necessary to use high-energy x rays (which are more penetrating) and larger amounts of x rays (to build up a good contrast). But this can cause considerable radiation damage to the sample. An alternative approach, requiring much smaller amounts of x-rays, is to image objects by measuring changes in phase. (If one considers the x ray as a wave with crests and valleys, a phase shift is the amount by which the crests and valleys are shifted when the x ray penetrates the object.) The phase shift can provide information about both the composition of the material and its thickness. This technique normally requires that an x-ray beam be split in two, one part passing through the sample and the other serving as a reference beam. Then the two waves are brought together again to form an interference pattern. Unfortunately, these interferometric methods are difficult to carry out because the beam of x rays must be coherent, and currently lasers are only available at lower (softer) x-ray energies. However, Keith Nugent of the University of Melbourne in Australia (k.nugent@physics.unimelb.edu.au) and his colleagues have demonstrated a phase imaging technique that does not require the use of interferometry. Instead they convert measurements of how the x rays are re-directed through a sample into information about the phase shifts at different points in the sample. This "phase map," in turn, can be transformed into a direct physical image. So far the researchers have used this method to image a carbon grid with lines 330 microns apart. Ultimately, the researchers expect potential resolutions of about 1 micron with current detector technology and are exploring the goal of making phase-based CAT scans of the interiors of objects.(K.A. Nugent et al., Physical Review Letters, 30 September 1996.)
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