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chapter3 - 1 CHAPTER 3 X-RAY DIFFRACTION IN CRYSTAL I....

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Unformatted text preview: 1 CHAPTER 3 X-RAY DIFFRACTION IN CRYSTAL I. X-Ray II. Diffraction III. Diffraction of Waves by Crystals IV. X-Ray Diffraction V. Bragg Equation VI. X-Ray Methods VII. Neutron & Electron Diffraction Bertha Rntgens Hand 8 Nov, 1895 2 X-RAY X-rays were discovered in 1895 by the German physicist Wilhelm Conrad Rntgen and were so named because their nature was unknown at the time. He was awarded the Nobel prize for physics in 1901. Wilhelm Conrad Rntgen (1845-1923) 3 X-RAY PROPERTIES X ray, invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light. The wavelength range for X rays is from about 10-8 m to about 10-11 m, the corresponding frequency range is from about 3 10 16 Hz to about 3 10 19 Hz. 4 x-ray 10-10 1A E ~ 10 4 ev X-RAY ENERGY Electromagnetic radiation described as having packets of energy, or photons . The energy of the photon is related to its frequency by the following formula: h E = =Wavelength , = Frequency , c = Velocity of light c = hc E = h c E = 5 PRODUCTION OF X-RAYS Visible light photons and X-ray photons are both produced by the movement of electrons in atoms. Electrons occupy different energy levels, or orbitals, around an atom's nucleus. When an electron drops to a lower orbital, it needs to release some energy; it releases the extra energy in the form of a photon. The energy level of the photon depends on how far the electron dropped between orbitals. 6 7 Evacuated glass bulb Anode Cathode X rays can be produced in a highly evacuated glass bulb, called an X-ray tube, that contains essentially two electrodesan anode made of platinum, tungsten, or another heavy metal of high melting point, and a cathode. When a high voltage is applied between the electrodes, streams of electrons (cathode rays) are accelerated from the cathode to the anode and produce X rays as they strike the anode. X-RAY TUBE 8 Monochromatic and Broad Spectrum of X-rays X-rays can be created by bombarding a metal target with high energy (> ) electrons. Some of these electrons excite electrons from core states in the metal, which then recombine, producing highly monochromatic X-rays. These are referred to as characteristic X-ray lines. Other electrons, which are decelerated by the periodic potential of the metal, produce a broad spectrum of X-ray frequencies. Depending on the diffraction experiment, either or both of these X-ray spectra can be used. 4 10 4 10 9 ABSORPTION OF X-RAYS The atoms that make up your body tissue absorb visible light photons very well. The energy level of the photon fits with various energy differences between electron positions....
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chapter3 - 1 CHAPTER 3 X-RAY DIFFRACTION IN CRYSTAL I....

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