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Distinguish between continuous & characteristic x-ray spectra?(physics 2nd sem)
LiVeRpOoL FoOtBaLL cLuB
on 2010-06-03 09:30:00check any of the wbut phy books!!!!
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on 2010-06-03 09:30:00Continuous and Characteristic X-ray Spectra When the target material of the X-ray tube is bombarded with electrons accelerated from the cathode filament, two types of X-ray spectra are produced. The first is called the continuous spectra. The continuous spectra consists of a range of wavelengths of X-rays with minimum wavelength and intensity (measured in counts per second) dependent on the target material and the voltage across the X-ray tube. The minimum wavelength decreases and the intensity increases as voltage increases. The second type of spectra, called the characteristic spectra, is produced at high voltage as a result of specific electronic transitions that take place within individual atoms of the target material. This is easiest to see using the simple Bohr model of the atom. In such a model, the nucleus of the atom containing the protons and neutrons is surrounded by shells of electrons. The innermost shell, called the K- shell, is surrounded by the L- and M - shells. When the energy of the electrons accelerated toward the target becomes high enough to dislodge K- shell electrons, electrons from the L - and M - shells move in to take the place of those dislodged. Each of these electronic transitions produces an X-ray with a wavelength that depends on the exact structure of the atom being bombarded. A transition from the L - shell to the K- shell produces a Ka X-ray, while the transition from an M - shell to the K- shell produces a Kb X-ray. These characteristic X-rays have a much higher intensity than those produced by the continuous sprectra, with Ka X-rays having higher intensity than Kb X-rays. The important point here is that the wavelength of these characteristic x-rays is different for each atom in the periodic table (of course only those elements with higher atomic number have L- and M - shell electrons that can undergo transitions to produce X-rays). A filter is generally used to filter out the lower intensity Kb X-rays. For commonly used target materials in X-ray tubes, the X-rays have the following well-known experimentally determined wavelengths: Element Ka Wavelength (l) Å Mo 0.7107 Cu 1.5418 Co 1.7902 Fe 1.9373 Cr 2.2909 X-ray Diffraction and Bragg\'s Law Since a beam of X-rays consists of a bundle of separate waves, the waves can interact with one another. Such interaction is termed interference. If all the waves in the bundle are in phase, that is their crests and troughs occur at exactly the same position (the same as being an integer number of wavelengths out of phase, nl, n = 1, 2, 3, 4, etc.), the waves will interfere with one another and their amplitudes will add together to produce a resultant wave that is has a higher amplitude (the sum of all the waves that are in phase. If the waves are out of phase, being off by a non-integer number of wavelengths, then destructive interference will occur and the amplitude of the waves will be reduced. In an extreme case, if the waves are out of phase by an odd multiple of 1/2l [(2n+1)/2l ], the resultant wave will have no amplitude and thus be completely destroyed. The atoms in crystals interact with X-ray waves in such a way as to produce interference. The interaction can be thought of as if the atoms in a crystal structure reflect the waves. But, because a crystal structure consists of an orderly arrangement of atoms, the reflections occur from what appears to be planes of atoms. Let\'s imagine a beam of X-rays entering a crystal with one of these planes of atoms oriented at an angle of q to the incoming beam of monochromatic X-rays (monochromatic means one color, or in this case 1 discreet wavelength as produced by the characteristic spectra of the X-ray tube). Two such X-rays are shown here, where the spacing between the atomic planes occurs over the distance, d. Ray 1 reflects off of the upper atomic plane at an angle q equal to its angle of incidence. Similarly, Ray 2 reflects off the lower atomic plane at the same angle q. While Ray 2 is in the crystal, however, it travels a distance of 2a farther than Ray 1. If this distance 2a is equal to an integral number of wavelengths (nl), then Rays 1 and 2 will be in phase on their exit from the crystal and constructive interference will occur. If the distance 2a is not an integral number of wavelengths, then destructive interference will occur and the waves will not be as strong as when they entered the crystal. Thus, the condition for constructive interference to occur is nl = 2a but, from trigonometry, we can figure out what the distance 2a is in terms of the spacing, d, between the atomic planes. a = d sin q or 2a = 2 d sin q thus, nl = 2d sin q This is known as Bragg\'s Law for X-ray diffraction. What it says is that if we know the wavelength ,l , of the X-rays going in to the crystal, and we can measure the angle q of the diffracted X-ray
Ishwarya
on 2010-06-19 09:30:00@Administrator- You really work hard..don\'t you? O.O @Sahu- Open Amol Chakraborty.....X-ray chappie...!!! You\'ll find everything!!