Three Dimensional X-Ray Computed Tomography in Materials Science
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Three-Dimensional X-Ray Computed Tomography in Materials Science
J.H. Kinney, Q.C. Johnson, U. Bonse, M.C. Nichols, R.A. Saroyan, R. Nusshardt, R. Pahl, and J. M. Brase Introduction Imaging is the cornerstone of materials characterization. Until the middle of the present century, visible light imaging provided much of the information about materials. Though visible light imaging still plays an extremely important role in characterization, relatively low spatial resolution and lack of chemical sensitivity and specificity limit its usefulness. The discovery of x-rays and electrons led to a major advance in imaging technology. X-ray diffraction and electron microscopy allowed us to characterize the atomic structure of materials. Many materials vital to our high technology economy and defense owe their existence to the understanding of materials structure brought about with these high-resolution methods. Electron microscopy is an essential tool for materials characterization. Unfortunately, electron imaging is always destructive due to the sample preparation that must be done prior to imaging. F u r t h e r m o r e , electron microscopy only provides information about the surface of a sample. Threedimensional information, of great interest in characterizing many new materials, can be obtained only by time consuming sectioning of an object. The development of intense synchrotron light sources in addition to the improvements in solid state imaging technology is revolutionizing materials characterization. High resolution x-ray imaging is a potentially valuable tool for materials characterization. The large depth of x-ray penetration, as well as MRS BULLETIN/JANUARY 1988
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Three-Dimensional X-Ray Comput d Tomography in Materials Science dimensional array. An x-ray or photon produces a charge pair in the silicon active layer. These charges are accumulated during an exposure and then read out in digital format by transferring the charges from pixel to pixel onto a readout register. Operating in a charge integrating mode, a CCD can provide high resolution, high contrast digital images of excellent quality. 2 The particular CCD used in our x-ray camera is a virtual phase, frame transfer device. It has a 390 x 580 pixel format, and each pixel is 22 x 22 /im. The full well capacity of the CCD (the number of charges that can be accumulated in a pixel before it saturates) has been measured to exceed 2 x 105 electrons. The rms noise is less t h a n 10 electrons, which means that the dynamic range of the chip is better than 10". This is fully two orders of magnitude better than the best film!3 Figure 1 s h o w s the e x p e r i m e n t a l apparatus with the camera. The CCD array is thermoelectrically cooled to -40°C to reduce the thermally generated dark current to negligible values. X-rays originating from a synchrotron storage ring pass through the sample and are converted to visible light on a high resolution p h o s p h o r screen. The image formed on the phosphor screen is projected onto the CCD with a bi-convex lens that
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