The amplitude of a diffracted X-ray beam from any set of planes is dependent upon the following atomic properties of the crystal: 1 position of each atom in the unit cell; 2 the respective atomic scattering factors; and 3 the individual thermal motions. Other factors that directly influence the intensities of the diffracted beam are: 1 the intensity and wavelength of the incident radiation; 2 the volume of crystalline specimen; 3 the absorption of the X-radiation by the specimen; and 4 the experimental arrangement utilized to record the intensity data.
Thus, the experimental conditions are especially important for measurement of diffraction intensities. Only a limited number of Braggs planes are in a position to diffract when monochromatized X-rays pass through a single crystal. Techniques of recording the intensities of all of the possible diffracting hkl planes involve motion of the single crystal and the recording media.
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Recording of these data is accomplished by photographic techniques film or with radiation detectors. A beam passing through a very large number of small, randomly oriented crystals produces continuous cones of diffracted rays from each set of lattice planes. Each cone corresponds to the diffraction from various planes having a similar interplanar spacing. The intensities of these Braggs reflections are recorded by either film or radiation detectors. The Braggs angle can be measured easily from a film, but the advent of radiation detectors has made possible the construction of diffractometers that read this angle directly.
The intensities and d spacings are more conveniently determined with powder diffractometers employing radiation detectors than by film methods. Microphotometers are frequently used for precise intensity measurements of films. An example of the type of powder patterns obtained for four different solid phases of ampicillin are shown in the accompanying figure. For each of these radiations there is an element that will filter off the K radiation and permit the K radiation to pass nickel is used, in the case of copper radiation.
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In this manner the radiation is practically monochromatized. The choice of radiation to be used depends upon the absorption characteristics of the material and possible fluorescence by atoms present in the specimen. Those not familiar with the use of X-ray equipment should seek expert advice. Improper use can result in harmful effects to the operator. Grinding pressure has been known to induce phase transformations; therefore, it is advisable to check the diffraction pattern of the unground sample. In general, the shapes of many crystalline particles tend to give a specimen that exhibits some degree of preferred orientation in the specimen holder.
Crystal Patterns Made Plane and Simple
This is especially evident for needle-like or plate-like crystals where size reduction yields finer needles or platelets. Preferred orientation in the specimen influences the relative intensities of various reflections. Several specialized handling techniques may be employed to minimize preferred orientation, but further reduction of particle size is often the best approach.
Where very accurate measurement of the Braggs angles is necessary, a small amount of an internal standard can be mixed into the specimen. This enables the film or recorder tracing to be calibrated. SlideShare Explore Search You. Submit Search.
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X-ray sources and effects
Tesla also alerted the scientific community about the safe operation of X-ray equipment and the biological hazards associated with exposure. Thomas Edison led the advances in radiology and medical X-ray examinations by developing a fluoroscope in Since the initial pioneering works of the late 19th century, many important discoveries have been made using X-rays which are now used in various applications.
This includes the establishment of X-ray crystallography, an experimental technique which allows the dimensional arrangement of atoms within crystal structures to be determined by the interpretation of diffraction patterns. X-ray crystallography has since made significant contributions to chemistry and material science. Today, X-rays are most notably used for medical imaging, microscopy and astronomy. Crystals were first scientifically investigated in the 17th century for their regularity and symmetry, most notably by Danish geologist Nicolas Steno, who successfully confirmed this symmetry by showing that the angles between the faces are the same for a given type of crystal.
This was followed by the introduction of a notation system for planes in crystal lattices called the Miller indices by William Miller in , which is still used today to identify crystals. In addition, the experiment aimed to examine how the Bragg scattering of X-rays can be used to determine the crystal structures and their lattice constants. There was also a consideration of energy loss by observing the varying intensities of characteristic wavelengths. Theory X-rays are a form of ionising electromagnetic radiation found in the short wavelength and high energy end of the spectrum with wavelengths ranging from 0.
This range of wavelengths of X-rays is of the order of distances between molecules and crystal lattices, making them ideal for spectroscopic techniques for characterisation of the elemental composition of materials. The corresponding energies of this range are eV to keV respectively.
There are two commonly used methods to produce X-rays, by an X-ray tube which is a vacuum tube that linearly accelerates charged particles, or by a synchrotron which is a cyclic particle accelerator that applies electric and magnetic fields. Most natural sources of X-rays are extra-terrestrial such as the Sun and black holes but they are also emitted by the decay of unstable nuclei on Earth. In an X-ray tube a metal filament is the cathode and is heated by a low voltage current in order to emit electrons in a process called thermionic emission.
As a stream of electrons are released into the vacuum, a large electric potential is applied between the cathode and the anode; a metal target.
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This accelerates the electrons towards the anode due to electrostatic attraction. There are two principle mechanisms by which X-rays are produced, by bremsstrahlung or Kshell emission.
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In the first, the emission of X-rays takes place when high-energy electrons or charged particles are decelerated, in speed or direction, by bombarding targets. In this process transitions of electrons between atomic orbit shells take place as the bombardment of electrons can excite and eject inner electrons from the target atoms, provided the incident electron has sufficient energy. This leaves a vacant space in the inner shell and is filled by an electron at the higher level outer shell.
In the course of this transition, the higher level electron is losing kinetic energy as it fills the vacant inner shell. Figure 1: The emission spectra of a heavy metal xray source. These X-rays are called characteristic X-rays with wavelengths that are distinctive for each particular element and transition. The innermost shell from which the incident electron has dislodged an atomic electron is called the K shell. Interference occurs when two waves meet and superimpose either constructively or destructively. Constructive interference takes place when two identical phases meet and superimpose into a wave with combined amplitude.
Related Further Experiments upon the Reflection by a Crystal of Its Characteristic X-Radiation
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