X-RAY_F11

X-RAY_F11 - GLY 4200 X-Rays and X-ray Mineralogy...

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1 GLY 4200 X-Rays and X-ray Mineralogy X-radiation is a type of electromagnetic radiation, like visible light, UV, IR, etc. The range in wavelength from approximately 10 -6 to 10 -1 nm. They were first discovered by Wilhelm Conrad Roentgen in 1895. In 1912, Friedrich, Knipping, and von Laue first demonstrated that X-rays were diffracted by passage through a crystal. Since diffraction is most effective when the wavelength of the radiation is about the same as the spacing in the diffraction grating (here, a crystal) the wavelength of x-rays tells us something about the spacing of atoms in a crystal. Before examining the behavior of X-rays in crystals, we should review some properties common to all electromagnetic radiation. The Einstein equation is one such property: E = h L = hc/ 8 where E = energy, h = Planck's constant, L = frequency, c = speed of light, and 8 = wavelength. X-rays and heat are generated when electrons hit a target. If all the kinetic energy of an electron is converted to X-ray quanta, we can rewrite the equation as: eV = hc/ 8 where e = charge on the electron, and V = voltage. Replacing constants with their numerical values and expressing voltage in kilovolts, we get: 8 (nm) = 1.24/kV, where kV = kilovolts. This is the minimum wavelength of radiation that may be generated by a given difference in voltage between a cathode and an anode. In practice, most electrons will be involved in several collisions on the way from the filament to the target. They lose some energy in each collision, so the actual output of an X-ray tube will be a continuous spectrum of radiation, with the
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2 Figure 1 - Effect of excitation potential on minimum wavelength shortest wavelength (highest energy) given by the above equation. Figure 1 shows a diagram of the intensity of radiation versus wavelength for excitation potentials of 20, 30, and 40 kilovolts. This type of radiation is known as white radiation, because it is a continuous spectrum of radiation, like white light. (Fig. 136a, p141, Battay) X-rays with shorter wavelengths are more energetic, and more penetrating. They art less absorbed, and do less damage in biologic systems, and are called "hard." Hard X-rays are used for medical and dental work. Long wavelength X- rays are called "soft" radiation, are more easily absorbed in biologic systems, and are more damaging. Soft X-rays are used in crystallography. However, all X-ray can and does cause damage to tissue exposed to it. When working with X-rays, it is very important that all necessary steps be taken to insure that no one is exposed to an X-ray beam, directly or indirectly. Figure 2 shows a schematic diagram of a typical X-ray tube. A current applied to the filament allows electrons to "boil" off. They are focused by the focusing shields, which have an applied negative potential. They are accelerated toward the target by the excitation potential, an applied voltage of 20-100 kilovolts. The entire tube is evacuated, like an incandescent light bulb. The electrons accelerate in flight, and hit the target with a great deal of energy. The
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3 Figure 2 - X-ray tube schematic diagram collision with target atoms produces X-rays. The X-rays are emitted through
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X-RAY_F11 - GLY 4200 X-Rays and X-ray Mineralogy...

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