X - RAY DIFFRACTION TECHNIQUE

T.B.EKNATH BABU (T.B.E.K.B)

STUDENT AT ARULMIGU KALASALINGAM COLLEGE OF PHARMACY

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X – RAY DIFFRACTION TECHNIQUEBASIC PRINCIPLE: In 1901 roentgen received a noble prize for the discovery of x- ray these raysexist in the region of 0.01-10nm but 0.08-0.2nm is most useful region for analytical purpose. In anatom the electrons are arranged in layers or shell’s

· K-shell · L- shell · M-shell · N-shell

The electrons migrate from the vacant outer orbital to inside vacant orbit to fill up the vacant slot.The kind of energy generated leads to origin of the x- rays, time scale is approximately 10-12 -10-4sec The x-ray are generated by – Bombardment of metal target with beam of high energy electrons.Exposure of matter to primary x-rays beam to generate secondary x-ray showing fluorescence. Useradioactive element which on disintegration leads to x-ray formation. From synchronization ofradiation source but the last is most expensive process

X-RAY SOURCE: In x-ray instruments, sources are- Tube, Radioisotopes, Secondary fluarescencent sources. The most common source is a highly evacuated tube. The anode is heavy, hollow, water cooled blockof copper with a metal target plated. The metal having high melting point, good thermal conductivityand large atomic number (N). Such metal are silver, iron, copper, chromium, tungsten, rhodium,cobalt, molybdenum.

INSTRUMENTATION o X-RAY SOURCE o COLLIMATOR o MONOCHROMATOR o DETECTOR



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X-ray source: X-ray source A. Coolidge tube B. Synchroton radiation C. Radioisotopes D. Secondary fluorescent sources

COOLIDGE TUBE:

The Crookes tube was improved by William Coolidge in 1913. The Coolidge tube, also called hotcathode tube, is the most widely used. It works with a very good quality vacuum (about 10−4 Pa, or10−6 Torr).In the Coolidge tube, the electrons are produced by thermionic effect from a tungsten filament heatedby an electric current. The filament is the cathode of the tube. The high voltage potential is betweenthe cathode and the anode, the electrons are thus accelerated, and then hit the anode.There are two designs: end-window tubes and side-window tubes.End window tubes usually have "transmission target" which is thin enough to allow X-rays to passthrough the target (X-rays are emitted in the same direction as the electrons are moving.) In onecommon type of end-window tube, the filament is around the anode ("annular" or ring-shaped), theelectrons have a curved path.(half of a toroid)What is special about side-window tubes is:· An Electrostatic Lens to focus the beam onto a very small spot on the anode· The anode is specially designed to dissipate the heat and wear resulting from this intense focused

barrage of electrons. Some anodes are:· Mechanically spun to increase the area heated by the beam.(Medical "rotating anode")· Cooled by circulating coolant. (indirectly on most rotating anodes)



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· The anode is precisely angled at 1-20 degrees off perpendicular to the electron current so as toallow escape of some of the X-ray photons which are emitted essentially perpendicular to thedirection of the electron current.

· The anode is usually made out of tungsten or molybdenum.· The tube has a window designed for escape of the generated X-ray photons.The power of a Coolidge tube usually ranges from 0.1 to 18 kW.

COLLIMATOR: Collimator: A series of closely spaced, parallel metal plates or by a bundle of tubes, 0.5mm orsmaller in diameter. collimator, device for changing the diverging light or other radiation froma point source into a parallel beam. This collimation of the light is required to make specializedmeasurements inspectroscopy and in geometric and physical optics. The collimator may bea telescope with an aperture at the principal focal length of the lens. Light from the luminous sourceis focused on this slit by another lens of similar focal length, and the slit then serves as the luminousobject of the optical system.

MONOCHROMATORS: Filter: When the wavelength of two spectral lines is nearly the same there is an element may be usedas a filter to reduce the intensity of the line with the shorter wavelength.MONOCHROMATORS FILTERS Absorbs undesirable radiation Eg ; zirconium filter used formolybdenum radiation CRYSTAL MONOCHROMATOR Suitable analyzing crystal graphite,NaCl, LiF, quartz Flat crystal monochromator Curved crystal monochromator

DETECTORS: DETECTORS Two methods- Photographic method Counter methods Detection is based on theability of X-rays to ionize matter

PHOTOGRAPHIC METHOD: PHOTOGRAPHIC METHOD Records position and intensity of x-ray beam D-density unitsIo-intensity of incident ray I - intensity of transmitted ray Densitometer is used to measure D D=logIo/I



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DISADVANTAGES Time consuming method Exposures of several hours are required Rarely usedfor quantitative measurements

COUNTER METHODS: COUNTER METHODS Geiger – Muller tube counter Proportional counter Scintillation counter

GEIGER MULLER COUNTER: GEIGER MULLER COUNTER

Filled with argon xenon or krypton gas Each photon of x-ray interacts with an atom of argon causeslose of electron- photoelectron Has large KE. This K.E is lost to ionize several other atoms of thegas. ADVANTAGES Inexpensive Relatively trouble free DISADVANTAGES Counts low ratesonly Efficiency falls of at wavelength below 1Amstrong Cannot be used to measure the energy ofx-ray

SCINTILLATION DETECTOR: SCINTILLATION DETECTOR

PROPORTIONAL COUNTER: PROPORTIONAL COUNTER Construction is similar to GMC Filled with xenon or crypton Outputpulse is proportional to intensity of the x-rays ADVANTAGE Counts high rates DISADVANTAGECircuit is complex and expensive



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Diffraction pattern: · Laue photographic method · Bragg x-ray spectrometer method · Rotating crystal method· Powder method

Laue photographic method: Transmission method: In this method the crystal is held stationary in abeam of x-rays , after passing through the crystal is diffracted and is recorded on a photographicplate. Laue pattern can be used to orient crystals for solid state experiments. Back reflection method:This method provides similar information as the transmission method.Bragg’s x-ray spectrometer method: Using the Laue’s photograph, Bragg analyzed the structuresof crystals of sodium chloride, KCl and ZnS. Bragg devised a spectrometer to measure the intensityof x-ray beam. The spectra obtained in this way can be employed for crystallographic analyses. Thisis based on the Bragg’s equation: nλ =2d sin θ This equation gives the condition which must besatisfied for the reflection of x-rays from a set of atomic planes.Rotating crystal method: In this method monochromatic x-radiation is incident on a single crystalthat is rotated about one of its axes. The reflected beams lie as spots on the surface of cones that arecoaxial with the rotation axis. The diffracted beam directions are determined by intersection of thereciprocal lattice points with the sphere of reflection. By remounting the crystal successively aboutdifferent axes, one can determine the complete distribution of reciprocal lattice points. One mountingis sufficient if the crystal is cubic but two or more may be needed if the crystal has lower symmetry.Powder method: In these method the crystal is replaced by a large collection of very small crystals,randomly oriented, and a continuous cone of diffracted rays is produced. There are some importantdifferences, with respect to rotating crystal method. The cone obtained with a single crystal are notcontinuous because the diffracted beams occur only at certain points along the cone, whereas the



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cones with the powder method are continuous. The cone produced in the powder method isdetermined by the spacing of prominent planes and are not uniformly spaced.

· Applications of XRD· electron density The powder xrd pattern may be thought of as� The elucidation of structure of

penicillin by xrd paved the way for the later synthesis of penicillin. �and accordingly, theposition of the atoms in complex structures, such as penicillin may be determined from acomprehensive mathematical study of the x-ray diffraction pattern. finger print Xrd can alsobe used to determine whether the compound is solvated or not�of the single crystal structure,and it may be used conduct qualitative and quantitative analysis.

· Applications of XRD Particle size determination State of anneal in metals� Tooth enameland dentine have been examined by xrd. � It is used to assess the weathering and degradationof natural and synthetic , minerals. � Determination of Cis-Trans isomerism �by applying therelation. v= V. δθ. cos θ / 2n Where v = the volume or size of an individual crystalline V= thetotal volume of the specimen irradiated n = the number of spots in a diffraction ring at aBragg angle θ δθ = the divergence of the X –ray beam

Science

X - RAY DIFFRACTION TECHNIQUE