x-ray diffraction technique

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X-Ray Diffraction :







INTRODUCTION X-Rays : X-rays are short wave length electromagnetic radiations produced by the deceleration of high energy electrons or by electronic transitions of electrons in the inner orbital of atoms. X-ray region 0.1to100 A ˚ Analytical purpose 0.7 to 2 A ˚ More energetic (i.e. can penetrate deeper into a material ). 4


PRODUCTION OF X-RAYS The process of producing X-rays may be visualized in terms of Bohr’s theory of atomic structure. The atom is composed of nucleus and numerous electrons. The electrons are arranged in shells.(K,L,M) When a high energy beam of electrons strike the target metal it knockout an electron completely from one of the inner shell(K,L,M) of that atom. 5

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One of the outer electrons will fall into the vacated orbital, with the simultaneous emission of energy in the form of x-rays. The energy of x-ray is equal to the difference in energy between two levels involved. If a K-shell looses its electron and it is replaced by the electron from the L-shell as a resulting X-ray is termed as K-X-ray . The energy of K- Xray is termed as E k . 6


EQUATION: EK=EL-EK EL is the energy of L-shell. EK is the energy of K-shell. K-rays are divided into two types K-alpha, K-beta. If the electron falling into the K-shell comes from the closest shell,(L) it is K-alpha X-rays. If the electron falling into the K-shell comes from the nearest shell ,(M)it is K-beta X-rays. 7

Theory of x-ray at atomic level :

Theory of x-ray at atomic level 8 Inner orbit Outer orbit 8

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d   A   C B dSin  The path difference between ray 1 and ray 2 = 2d Sin For constructive interference: n = 2d Sin Ray 1 Ray 2  Deviation = 2  Constructive interference of the reflected beams emerging from two different planes will take place if the difference in path lengths of two rays is equal to whole number of wavelengths. BRAGG’s EQUATION 9


INSTRUMENTATION X-ray source:1.Crooke’s tube 2.Coolidge tube Collimator Monochromator-1.Filter type 2.Crystal type Detectors: a)Photographic methods b)Counter methods: 1.Geiger-muller counter 2.Proportional counter 3.Scintillation counter 4.Solid-state semi-conductor detector 5.Semi conductor detectors 10



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PRODUCTION OF X-RAYS: X-rays are generated when high velocity electrons impinge on a metal target. Approximately 1% of the total energy of the electron beam is converted into x-radiation. The remainder being dissipated as heat. Many types of x-ray tubes are available which are used for producing x-rays. 12 source

Generation of x-rays:

Generation of x-rays 13

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Coolidge tube Called as hot cathode tube. Works with a very good quality vacuum (about 10 -4 Pa, The electrons are produced by thermionic effect from a tungsten filament heated by an electric current. There are two designs: 1.End-window tubes : Have thin "transmission target" to allow X-rays to pass through the target 2.side-window tubes: An Electrostatic Lens to focus the beam onto a very small spot on the anode. A window designed for escape of the generated X-ray photons. Power 0.1 to 18 kW . 14

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COLLIMATOR Inserted in the diffracted-beam to get a narrow x-ray beam. It consists two sets of closely packed metal plates seperated by a gap. The left end of the collimator shown is mounted on the X-ray tube. The yellow-colored region at the left end determines the the size of the beam. The green region at the right end removes parasitic radiation. 16

Materials used:Nacl,LiF,quartz etc,. :

Materials used: Nacl,LiF,quartz etc,. Filter type A window that absorbs undesirable radiation and allows required wavelength to pass. Eg:Zr absorbs x-rays emitted by Mo. Crystal type Positioned in the x-ray beam so that the angle of the reflecting planes satisfied the Bragg’s equation for the required wave length. Characteristics of a crystal: Mechanically strong and stable The mosaicity and resolution of the crystal, should be small. MONOCHROMATOR 17



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Counter methods 1. Geiger-muller counter: Filled with an inert gas like argon. Measures ionizing radiation . Detect the emission of nuclear radiation: alpha particles , beta particles or gamma rays Advantages: a)Trouble free b)Inexpensive Disadvantages: a)Cannot be used to measure energy of ionizing radiation. b)Used for low counting rates c)Efficiency falls off below 1A 19

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2. Proportional counter: Filled with heavier gas like xenon or krypton as it is easily ionized. Output pulse is dependent on intensity of X-rays falling on counter. Count the particles of ionizing radiation and measures their energy. Advantages: a)Count high rates with out significant error. Disadvantages: a)Associated electronic circuit is complex. b)Expensive. 20

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3.Scintillation counter: Measures X-rays of shorter wavelengths. The sensor , called a scintillator , consists of a transparent crystal , usually phosphor, plastic (usually containing anthracene ), or organic liquid that fluoresces when struck by ionizing radiation . The PMT is attached to an electronic amplifier to count and possibly quantify the amplitude of the signals. Advantages: a)Count high rates. 21

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4.Solid state semi-conductor detector: The electrons produced by X-ray beam are promoted into conduction bands and the current which flows is directly proportional to the incident X-ray energy. Disadvantage: Maintainted at very low Temp to minimise the noise and prevent deterioration of the detector. 5.Semi-conductor detectors: Silicon-lithium drifted detector. The principle is similar to gas ionization detector. Voltage of pulse=Q/C Application : In neutron activation analysis Semi-conductor detector 22


REFERENCES 1)Instrumental methods of chemical analysis ,B.K.sharma,17 th edition 1997-1998,GOEL publishing house.page no:329-359 2)Principles of instrumental analysis,5 th edition ,by Dougles a.skoog,f.James holles,Timothy A.Niemen.page no:277-298 3)Instrumental methods of chemical analysis ,Gurudeep R.chatwal,sham k.anand,Himalaya publications page no:2.303-2.332 4) Instrumental Methods Of Chemical Analysis – H. Kaur pg.no:727-729,737 5) http://www.scienceiscool.org/solids/intro.html 6) http://en.wikipedia.org/wiki/X-ray_crystallography 23

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