X-RAY DIFFRACTION : X-RAY DIFFRACTION Slide 2: INTRODUCTION:
X-rays are discovered by Roentgen who called them x-rays because the nature at first was unknown so, x-rays are also called Roentgen rays. x-ray diffraction was discovered by max.The wavelength range is 10-7 to about 10-15 metre.
The penetrating power of x-rays depends on energy also, there are two types of x-rays.
i) Hard x-rays: which have high frequency and have more energy. ii) soft x-rays: which have less penetratig and have low energy PRINCIPLE: : PRINCIPLE: X-ray diffraction is based on constructive interference of monochromatic x-rays and a crystalline sample. These x-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation ,collimated to concentrate and directed towards the sample. The interaction of incident rays with the sample produces constructive interference when conditions satisfy Bragg’s law. INSTRUMENTATION : INSTRUMENTATION The instrumentation consists of the following parts, they are
Production of x-rays
Detector Slide 8: 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. COOLIDGE TUBE: : COOLIDGE TUBE: Slide 10: COLLIMATOR:
In order to get a narrow beam of x-rays, the x-rays generated by the target material are allowed to pass through a collimator which consists of two sets of closely packed metal plates separated by a small gap.
The collimator absorbs all the x-rays except the narrow beam that passes between the gap. Collimator: : Collimator: MONOCHROMATORS: : MONOCHROMATORS: In order to do monochromatisation of x-rays , two methods are available.These are
Monochromators DETECTORS : DETECTORS The x-ray intensities can be measured and recorded either by photographic or counter methods.
Both these types of methods depends upon ability of x-rays to ionize matter and differ only in the subsequent fate of electrons produced by the ionizing process. A) photographic method: : A) photographic method: In order to record position and intensity of x-ray beam a plane cylindrical film is used.
The film after exposing to x-rays is developed,the blackening of the developed field is expressed in terms of density units D given by
D is related to the total x-ray energy that causes the blackening of the photographic film and measured by densitometer.
. B) Counter methods: : B) Counter methods: These are of many types, like
Geiger-muller tube counter
Solid-state semiconductor detector
Semiconductor detector a)Geiger-muller tube counter: : a)Geiger-muller tube counter: b) Proportional counter: : b) Proportional counter: Construction is similar to Geiger-tube counter only but proportional counter is filled with a heavier gas like xenon or krypton.
Heavier gas is preferred because it is easily ionised. c) Scintillation counter: : c) Scintillation counter: Slide 19: In this detector, there is a large sodium iodide crystal activated with a small amount of thallium.
When x-rays incident upon crystal, the pulses of visible light are emitted which can be detected by photomultiplier tube. d) Semi-conductor detectors: : d) Semi-conductor detectors: Slide 21: When x-ray falls on a semiconductor or a silicon lithium-drifted detector,it generates an electron(-e) and a hole(+e) in a fashion analogous to the formation of a primary ion pair in a proportional counter.
The principle is similar to that of gas ionization detector as used in a proportional counter, except that the materials used are in a solid state. X-RAY DIFFRACTION METHODS : X-RAY DIFFRACTION METHODS These are generally used for investigating the internal structures and crystal structures of various solid compounds.
Bragg’s x-ray spectrometer method
Rotating crystal method
Powder method b)Back-reflection method: : b)Back-reflection method: 2)Bragg’s x-ray spectrometer method: : 2)Bragg’s x-ray spectrometer method: This method is based on Bragg’s law, bragg analysed the structures of Nacl,KaI and ZnS. Slide 26: APPLICATIONS: : APPLICATIONS: structure of crystals.
particle size determination.
soil classification based on crystallinity.
analysis of industrial dusts can be effected and their relationship to industrial disease ascertained.
corrosion products can be studied. REFERENCES: : REFERENCES: Instrumental methods of chemical analysis
Gurudeep R. chatwal
Instrumentation by Skoog. Slide 30: THANK YOU