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X-ray Fluorescence - Essay Example

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These rays have a long history in commercial elemental analysis since the discovery of the correlation between a sample’s atomic weight and the X-rays radiating from it, a fact that enabled Henry G. J. Moseley number elements in 1913 through the observation of K-line transitions as observed in X-ray spectrum…
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X-ray Fluorescence
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Download file to see previous pages J. Moseley number elements in 1913 through the observation of K-line transitions as observed in X-ray spectrum. This formed the basis of element identification through X-ray fluorescence spectroscopy by considering the relationship between the atomic number and the frequency. X-Ray fluorescence, XRF refers to the emission of characteristic secondary, also referred to as fluorescent X-rays by bombarding a material with X-rays at high energy or gamma rays so that the material gets excited. The wavelength of X-rays range between 50 and 100 A related to energy in the relationship: E = h? where h is Planck constant, 6.62 * 10-24 and ? is the frequency in Hertz. High energy X-rays would be required for XRF as the soft X-rays get absorbed by the target element, with the absorption edges depending on ionisation energies of the respective electrons, unique to each element. While the energy dispersive XRF, EDXRF methodology detects all elements from Na through to U, the wavelength dispersive XRF, WDXRF detects down to Be (Shackley 34). How XRF Works When the atoms of the target material absorb the high energy photons from the X-rays or gamma-rays, the electrons at the inner shell would be ejected from the atom transforming them to photoelectrons. As a result, the atom would be left at an excited state having a vacancy in its inner shell. The outer shell electrons would then fall into this resultant vacancy in the process emitting photons whose energy equals the difference in energy between the two states. It would be appreciated that each element has its unique energy level set, implying that each element would emit characteristic pattern of X-rays unique to itself which Sharma (527) refers to as characteristic X-rays. With increase in the concentration of the corresponding element, there would also be an increase in the X-ray intensity. This phenomenon also applies in the quantitative analysis of elements through the production of optical emission spectra. With characteristic X-rays resulting from transition between the energy levels in an atom, the electrons that transition from energy level Ei to Ej would emit X-rays with energy Ex = Ei – Ej. With each element having unique atomic energy level set, a unique X-rays set would be emitted characteristic of the element (Sharma 526). Considering Bohr’s atomic model (see fig. 1), with atomic levels designated as K, L, M and so forth, each with additional sub-shells, a transition between these shells would result in the emission of characteristic X-rays. Fig. 1. Bohr’s atomic model from Sharma (527) As such, M X-ray would result from transition to M shell, so would K X-ray be a result of transition to K shell. K?1 X-ray would result from an electron dropping from M3 shell to fill in a vacancy in the K shell (see fig. 2). The emitted X-ray would have energy EX-ray = EK – EM3. Figure 2: X-ray line labelling from Bounakhla and Tahri (12) Sources According to Bounakhla and Tahri (21), radioisotopes provide the simplest source for configuration since one selects a source that emits X-rays slightly above the target element’s absorption edge energy. They have found wide application due to their stability and smallness in size in the context where monochromatic and continuous sources would be required. It serves well with regard to ruggedness, reliability, simplicity and in the consideration of cost of equipment. For safety, emissions would be limited to approximately 107 photons. The activity would be described in terms of disintegration rates of the radioisotopes where this activity would decrease from initial activity, A0 to final activity At for a duration of time, t. At = A0e(-0.693t/T?) where T? is the ...Download file to see next pagesRead More
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