著者

つち山 明 上杉 健太朗 中野 司

出版者

Tokyo Geographical Society

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.109, no.6, pp.845-858, 2000-12-01

被引用文献数

6 6

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X-ray computerized tomography (CT) is a non-destructive method by which crosssectional images of rocks and minerals are obtained using X-ray attenuation. Threedimensional structures of samples can also be obtained by constructing a number of successive images. This review discusses high-resolution X-ray CT machines including an industrial machine in commercial use and that developed by our group at SPring-8, which is the largest synchrotron radiation facility in Japan. Spatial resolution of CT images, which is determined by X-ray detectors and X-ray beam size, is practically limited by sample size due to the limited numbers of X-ray detectors. A resolution of about 1 μm was realized at SPring-8. This resolution is the lower limit for imaging with simple optics. The contrast of an X-ray CT image is expressed as a two-dimensional distribution of CT values, which related to the X-ray linear attenuation coefficient (LAC), , μ. CT values of standard minerals were measured to compare with their values of μ. As μ is a function of X-ray photon energies, beamhardening occurs when we use polychromatic beams. Thus, we cannot compare CT and μ values directly with the industrial scanner, which aplies a polychromatic X-ray beam. If the CT and μ values are normalized by a standard mineral having similar size as samples for the photon energy or the accelerating voltage of an X-ray tube, both values agree well as long asμis less than about 2.5 × μ of Fo<SUB>92</SUB> olivine. We can compare CT and μ values directly in the SPring-8 machine, where monochromatic X-ray beams are available. In this case, normalized CT and μ values agree well in various materials havinga large μ at least including metallic iron. However, absolute CT values are slightly smaller than μ by about 10%, which is probably due to scattered X-ray beams, although the exact reason is not known at present. The high-resolution X-ray method was applied to three-dimensional structures of chondrules, which are characteristic constituents of primitive meteorites, named chondrites. It is known from external shapes and internal textures, which are related to distributions of voids and platy olivine crystals, that chondrules spin at high revolutions of about 50-500 rps during their formation in the primordial solar nebula. This greatly constrains the formation mechanism of chondrules.