X-Ray Combined.ppt PDF

X-RAY X-rays were discovered in 1895 by the German physicist Wilhelm Conrad Röntgen and were so named because their nature was unknown at the time. He was awarded the Nobel Wilhelm Conrad Röntgen prize for physics in 1901. (1845-1923) ...

X-RAY X-rays were discovered in 1895 by the German physicist Wilhelm Conrad Röntgen and were so named because their nature was unknown at the time. He was awarded the Nobel Wilhelm Conrad Röntgen prize for physics in 1901. (1845-1923)  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 × 1016 Hz to about 3 × 1019 Hz.  Properties of X-rays X-rays travel in straight lines. X-rays cannot be deflected by electric field or magnetic field. X-rays have a high penetrating power. Photographic film is blackened by X-rays. Fluorescent materials glow when X-rays are directed at them. Photoelectric emission can be produced by X- rays. Ionization of a gas results when an X-ray beam is passed through it. Make shadows of absorbing material on photosensitive paper  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: E h c E hc     =Wavelength , ‫ = ע‬Frequency , c = Velocity of light hc E    x-ray ≈ 10-10 ≈ 1A° E ~ 104 ev  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.  Production of X-rays (1) X-rays are produced when rapidly moving electrons that have been accelerated through a potential difference of order 1 kV to 1 MV strikes a metal target. Evacuated glass tube Target Filament  Production of X-rays (2) Electrons from a hot element are accelerated onto a target anode. When the electrons are suddenly decelerated on impact, some of the kinetic energy is converted into EM energy, as X-rays. Less than 1 % of the energy supplied is converted into X-radiation during this process. The rest is converted into the internal energy of the target.  Generation of X-rays (K- Shell Knockout) An electron in a higher orbital immediately falls to the lower energy level, releasing its extra energy in the form of a photon. It's a big drop, so the photon has a high energy level; it is an X- ray photon. The free electron collides with the tungsten atom, knocking an electron out of a lower orbital. A higher orbital electron fills the empty position, releasing its excess energy as a photon. LIGHT INTERFERENCE  Constructive & Destructive Waves Constructive interference Destructive İnterference. is the result of results when two out-of-phase synchronized light waves light waves cancel each other that add together to out, resulting in darkness. increase the light intensity. Light Interference  Diffraction from a particle and solid Single particle To understand diffraction we also have to consider what happens when a wave interacts with a single particle. The particle scatters the incident beam uniformly in all directions Solid material What happens if the beam is incident on solid material? If we consider a crystalline material, the scattered beams may add together in a few directions and reinforce each other to give diffracted beams  X-Ray Diffraction & Bragg Equation English physicists Sir W.H. Bragg and his son Sir W.L. Bragg developed a relationship in 1913 to explain why the cleavage faces of crystals appear to reflect X-ray beams at certain angles of incidence (theta, θ).This observation is an example of X-ray Sir William Henry Bragg (1862-1942), William Lawrence Bragg (1890-1971) wave interference. o 1915, the father and son were awarded the Nobel prize for physics "for their services in the analysis of crystal structure by means of Xrays".  n λ = 2d sinΘ (1) derived by the English physicists Sir W.H. Bragg and his son Sir W.L. Bragg in 1913 to explain why the cleavage faces of crystals appear to reflect X-ray beams at certain angles of incidence (theta, ). The variable d is the distance between atomic layers in a crystal, and the variable lambda  is the wavelength of the incident X-ray beam, n is an integer  Deriving Bragg's Law Bragg's Law can easily be derived by considering the conditions necessary to make the phases of the beams coincide when the incident angle equals and reflecting angle. The rays of the incident beam are always in phase and parallel up to the point at which the top beam strikes the top layer at atom z (Fig. 1). The second beam continues to the next layer where it is scattered by atom B. The second beam must travel the extra distance AB + BC if the two beams are to continue traveling adjacent and parallel. This extra distance must be an integral (n) multiple of the wavelength () for the phases of the two beams to be the same: n = AB +BC (2). Recognizing d as the hypotenuse of the right triangle Abz, we can use trigonometry to relate d and  to the distance (AB + BC). The distance AB is opposite  so, AB = d sin(3). Because AB = BC eq. (2) becomes, n = 2AB (4) Substituting eq. (3) in eq. (4) we have, n = 2 d sin(1) and Bragg's Law has been derived.  Applications of Bragg's Law 1) In x-ray diffraction (XRD) the interplanar spacing (d-spacing) of a crystal is used for identification and characterization purposes. In this case, the wavelength of the incident x-ray is known and measurement is made of the incident angle (Θ) at which constructive interference occurs. Solving Bragg's Equation gives the d-spacing between the crystal lattice planes of atoms that produce the constructive interference. A given unknown crystal is expected to have many rational planes of atoms in its structure; therefore, the collection of "reflections" of all the planes can be used to uniquely identify an unknown crystal. In general, crystals with high symmetry (e.g. isometric system) tend to have relatively few atomic planes, whereas crystals with low symmetry (in the triclinic or monoclinic systems) tend to have a large number of possible atomic planes in their structures. 2) In the case of x-ray fluorescence spectroscopy (XRF), crystals of known d-spacings are used as analyzing crystals in the spectrometer. Because the position of the sample and the detector is fixed in these applications, the angular position of the reflecting crystal is changed in accordance with Bragg's Law so that a particular wavelength of interest can be directed to a detector for quantitative analysis. Every element in the Periodic Table has a discrete energy difference between the orbital "shells" (e.g. K, L, M), such that every element will produce x-rays of a fixed wavelength. Therefore, by using a spectrometer crystal (with fixed d-spacing of the crystal) and positioning the crystal at a unique and fixed angle (Θ), it is possible to detect and quantify elements of interest based on the characteristic x-ray wavelengths produced by each element.  Types of X-ray Diffraction method There are many types of X-ray camera to sort out reflections from different crystal planes. We will study only three types of X- ray photograph that are widely used for the simple structures. 1.Laue photograph 2.Rotating crystal method 3.Powder photograph  LAUE METHOD The Laue method is mainly used to determine the orientation of large single crystals while radiation is reflected from, or transmitted through a fixed crystal. The diffracted beams form arrays of spots, that lie on curves on the film. The Bragg angle is fixed for every set of planes in the crystal. Each set of planes picks out and diffracts the particular wavelength from the white radiation that satisfies the Bragg law for the values of d and θ involved.  Back-reflection Laue Method In the back-reflection method, the film is placed between the x-ray source and the crystal. The beams which are diffracted in a backward direction are recorded. One side of the cone of Laue reflections is defined by the transmitted beam. The film intersects the cone, with the diffraction spots generally Single lying on an hyperbola. Crystal X-Ray Film  Transmission Laue Method In the transmission Laue method, the film is placed behind the crystal to record beams which are transmitted through the crystal. One side of the cone of Laue reflections is defined by the transmitted beam. The film intersects the cone, with the diffraction spots generally lying on an ellipse. Single Film X-Ray Crystal  Crystal structure determination by Laue method The Laue method is mainly used to determine the crystal orientation. Although the Laue method can also be used to determine the crystal structure, several wavelengths can reflect in different orders from the same set of planes, with the different order reflections superimposed on the same spot in the film. This makes crystal structure determination by spot intensity difficult.  ROTATING CRYSTAL METHOD In the rotating crystal method, a single crystal is mounted with an axis normal to a monochromatic x-ray beam. A cylindrical film is placed around it and the crystal is rotated about the chosen axis. As the crystal rotates, sets of lattice planes will at some point make the correct Bragg angle for the monochromatic incident beam, and at that point a diffracted beam will be formed.  ROTATING CRYSTAL METHOD Lattice constant of the crystal can be determined by means of this method; for a given wavelength if the angle  at which a reflection occurs is known, can bed hkl determined. a d  h2  k 2  l 2  Rotating Crystal Method The reflected beams are located on the surface of imaginary cones. By recording the diffraction patterns (both angles and intensities) for various crystal orientations, one can determine the shape and size of unit cell as well as arrangement of atoms inside the cell. Film THE POWDER METHOD If a powdered specimen is used, instead of a single crystal, then there is no need to rotate the specimen, because there will always be some crystals at an orientation for which diffraction is permitted. Here a monochromatic X-ray beam is incident on a powdered or polycrystalline sample. This method is useful for samples that are difficult to obtain in single crystal form.  THE POWDER METHOD The powder method is used to determine the value of the lattice parameters accurately. Lattice parameters are the magnitudes of the unit vectors a, b and c which define the unit cell for the crystal. For every set of crystal planes, by chance, one or more crystals will be in the correct orientation to give the correct Bragg angle to satisfy Bragg's equation. Every crystal plane is thus capable of diffraction. Each diffraction line is made up of a large number of small spots, each from a separate crystal. Each spot is so small as to give the appearance of a continuous line.  The Powder Method If a the monochromatic A sample sample of some consists hundredsx-ray of crystals some beam istens (i.e. of directed a atrandomly powdered a single sample) crystal, show orientatedthen that crystals, single only the onediffracted the or two beams form continuous diffracted cones. diffracted beams beams are mayseen to result. A circle of film is used to record lie on the surface of several the diffraction pattern as shown. cones. The cones may Each cone intersects the film emerge in all giving diffraction directions, lines. The lines forwards are seen as and backwards. arcs on the film.  Powder diffraction film When the film is removed from the camera, flattened and processed, it shows the diffraction lines and the holes for the incident and transmitted beams.  Application of XRD XRD is a nondestructive technique. Some of the uses of x-ray diffraction are; 1. X-ray powder diffraction is most widely used for the identification of unknown crystalline materials (e.g. minerals, inorganic compounds). Determination of unknown solids is critical to studies in geology, environmental science, material science, engineering and biology. 2. Other applications include:  characterization of crystalline materials  identification of fine-grained minerals such as clays and mixed layer clays that are difficult to determine optically  determination of unit cell dimensions  measurement of sample purity With specialized techniques,  Advantages and disadvantages of X-rays Advantages; X-ray is the cheapest, the most convenient and widely used method. X-rays are not absorbed very much by air, so the specimen need not be in an evacuated chamber. Disadvantage; They do not interact very strongly with lighter elements.  X-RAY FLUORESCENCE (XRF) When a primary x-ray excitation source from an x-ray tube or a radioactive source strikes a sample, the x-ray can either be absorbed by the atom or scattered through the material. The process in which an x-ray is absorbed by the atom by transferring all of its energy to an innermost electron is called the “photoelectric effect.” During this process, if the primary x-ray had sufficient energy, electrons are ejected from the inner shells, creating vacancies. These vacancies present an unstable condition for the atom. As the atom returns to its stable condition, electrons from the outer shells are transferred to the inner shells and in the process giving off a characteristic x-ray whose energy is the difference between the two binding energies of the corresponding shells. The process of detecting and analyzing the emitted x-rays is called “Xray Fluorescence Analysis.” In most cases the innermost K and L shells are involved in XRF detection. A typical x-ray spectrum from an irradiated sample will display multiple peaks of different intensities. The characteristic x-rays are labeled as K, L, M or N to denote the shells they originated from. Another designation alpha (α), beta (β) or gamma () is made to mark the x-rays that originated from the transitions of electrons from higher shells. Hence, a Kαx-ray is produced from a transition of an electron from the L to the K shell, and a Kβx-ray is produced from a transition of an electron from the M to a K shell, etc. Since within the shells there are multiple orbits of higher and lower binding energy electrons, a further designation is made as α1, α2 or β1, β2, etc. to denote transitions of electrons from these orbits into the same lower shell. The XRF method is widely used to measure the elemental composition of materials. Since this method is fast and non-destructive to the sample, it is the method of choice for field applications and industrial production for control of materials. Depending on the application, XRF can be produced by using not only x-rays but also other primary excitation sources like alpha particles, protons or high energy electron beams. Sometimes, as the atom returns to its stable condition, instead of emitting a characteristic x- ray it transfers the excitation energy directly to one of the outer electrons, causing it to be ejected from the atom. The ejected electron is called an “Auger” electron. This process is a competing process to the XRF. The X-Ray Fluorescence Process