NIST studies extending the scope of electron-beam tomography.Tomography is the science of reconstructing three-dimensional structures from a set of two-dimensional images--for example, a set of absorption photographs of the same object taken from different angles, using penetrating radiation such as radio waves Radio waves Electromagnetic energy of the frequency range corresponding to that used in radio communications, usually 10,000 cycles per second to 300 billion cycles per second. , x rays, or electron beams. Reconstruction of the full three-dimensional structure from such images requires a model of the attenuation Loss of signal power in a transmission. Attenuation The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities. of the radiation beam as it passes through the object. Electron microscopy electron microscopy Technique that allows examination of samples too small to be seen with a light microscope. Electron beams have much smaller wavelengths than visible light and hence higher resolving power. can be used to extend these concepts to the nanoscale. While most studies have emphasized obtaining excellent spatial resolution (Data West Research Agency definition: see GIS glossary.) A measure of the accuracy or detail of a graphic display, expressed as dots per inch, pixels per line, lines per millimeter, etc. It is a measure of how fine an image is, usually expressed in dots per inch (dpi). on small samples, samples larger than 1 [micro]m have received little consideration. For such samples, electrons are sure to undergo multiple scattering, which violates the standard assumption of exponential attenuation used in tomography. New theoretical analysis shows that even if multiple scattering is present, tomography may be performed by generalizing the relationship of the detected signal to the material thickness. Little else in the analysis or the instrument needs to change. A paper published in the June 2, 2003, issue of Applied Physics Letters Applied Physics Letters is a weekly peer-reviewed scientific journal published by the American Institute of Physics devoted to the publication of new experimental and theoretical papers about applications of physics to science, engineering, and modern technology. describes a simulation of tomographic reconstruction using scanning transmission electron micrographs of a model photonic band gap crystal. A simulated tomographic reconstruction of a polymer sample over 8 micrometers on a side was achieved. CONTACT: Zachary Levine, (301) 975-5453; zachary.levine@nist.gov. |
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