![]() These are the x-rays that we measure in diffraction experiments, as the scattered x-rays carry information about the electron distribution in materials. If the wavelength of these scattered x-rays did not change (meaning that x-ray photons did not lose any energy), the process is called elastic scattering (Thompson Scattering) in that only momentum has been transferred in the scattering process. When x-ray photons collide with electrons, some photons from the incident beam will be deflected away from the direction where they originally travel, much like billiard balls bouncing off one anther. X-rays primarily interact with electrons in atoms. These powerful sources, which are thousands to millions of times more intense than laboratory x-ray tubes, have become indispensable tools for a wide range of structural investigations and brought advances in numerous fields of science and technology. Synchrotron radiation is emitted by electrons or positrons travelling at near light speed in a circular storage ring. In recent years synchrotron facilities have become widely used as preferred sources for x-ray diffraction measurements. (The energy E of a x-ray photon and its wavelength is related by the equation E = hc/ l, where h is Planck's constant and c the speed of light) (check out this neat animated lecture on x-ray production) ![]() Common targets used in x-ray tubes include Cu and Mo, which emit 8 keV and 14 keV x-rays with corresponding wavelengths of 1.54 Å and 0.8 Å, respectively. When a free electron fills the shell, a x-ray photon with energy characteristic of the target material is emitted. The high energy electrons also eject inner shell electrons in atoms through the ionization process. As electrons collide with atoms in the target and slow down, a continuous spectrum of x-rays are emitted, which are termed Bremsstrahlung radiation. In a x-ray tube, which is the primary x-ray source used in laboratory x-ray instruments, x-rays are generated when a focused electron beam accelerated across a high voltage field bombards a stationary or rotating solid target. X-rays are produced generally by either x-ray tubes or synchrotron radiation. The energetic x-rays can penetrate deep into the materials and provide information about the bulk structure. Because the wavelength of x-rays is comparable to the size of atoms, they are ideally suited for probing the structural arrangement of atoms and molecules in a wide range of materials. For diffraction applications, only short wavelength x-rays (hard x-rays) in the range of a few angstroms to 0.1 angstrom (1 keV - 120 keV) are used. X-rays are electromagnetic radiation with typical photon energies in the range of 100 eV - 100 keV.
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