X-ray Fluorescence - Essay Example

X-Ray Fluorescence Introduction X-rays were discovered by Wilhelm K. Rontgen (1845 – 1923), a German physicist (Shackley 7). 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. 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 radioisotope’s half life. X-ray tube, basically made up of tungsten filaments for production of X-rays serves as an alternative source. The principle of its operation bases on the acceleration of electrons in an electrical field and their subsequent deceleration in an anode made up of a suitable material. This electron beam region would be evacuated so as to deter any collision with gas molecules. The X-ray tube would thus emit energy characteristic of the anode material together with Bremsstrahlung radiation, a spectrum resulting from the use of an X-ray tube whose anode material in Rh. With the peak intensity of Bremsstrahlung radiation being about half the maximum energy, target selection would be pegged on selecting a target whose excitation would optimise that of the target element. Alternatively, line energy in the important element’s region that has no significant effect on the background could be selected. Even though Rh has been commonly used, other elements such as Ti, Cr, Cu, Y, Mo, Sc and Fe have also been highly used (Sharma 526). This principle of operation applies to all X-ray tubes with the difference being in the cathode and anode polarity and exit window arrangement. Figure 3: Typical X-ray tube from Sharma (526) Detection In the case of energy dispersive analysis, both dispersion and detection would be combined into a single operation. Solid state detectors such as Ge(Li), Si(Li), PIN diode and Silicon Drift Detector, SDD together with proportional counters have been employed for this function, all working under the same principle. The oncoming X-ray photon would ionise the detector atoms producing charges proportional to its energy. This charge would then be collected with the process recurring for subsequent photons. Thereafter, a spectrum would be developed through the division of the spectrum of the energy into discrete bins and making a count record of the pulses in each bin. For wavelength dispersive analysis, radiations of single wavelengths would be produced by a monochromator and passed onto a detector resembling a Geiger counter, referred to as a photomultiplier, responsible for counting the passing photons (Bounakhla and Tahri 28). The counter encompasses a chamber full of X-ray photon ionised gas. The central electrode normally charged at +1,700 V against the conducting chamber walls and the photons would trigger a current cascade in form of a pulse across the resultant field. This signal would then be amplified and transmitted into digital counts that produce the analytical data. Quantitative XRF There exists a direct relationship between the characteristic X-ray lines intensities and the element concentration in the sample. Nonetheless, Sharma (528) acknowledges the complexity in this relationship noting that the intensity of an emission line would be dependent on energy spectrum, intensity of the X-rays used to excite, the presence of other elements in the sample, geometry of the sample, geometry of the source and the efficiency of the X-ray detector. The scholar gives an example of a brass sample where a portion of Zn K? X-rays would interact with Cu atoms leading to the enhancement of Cu lines and simultaneously reducing Zn lines, referred to as matrix effects. To address such complexities, quantitative analysis software like Amptek’s XRS-FP have been employed. Analysis of the spectrum involves first the determination of each peak’s intensity referred to as the net area. This would require that the peaks be separated from the background and loss processes such as correction for escape peaks and overlapping peaks be separated. This would be followed by correcting for the detector and window sensitivity, excitation spectrum and geometric effects. Finally, the matrix effects would be corrected for. With the increase in complexity of measurement conditions so would the analysis complexity also increase. Conclusion XRF entails the measurement of an element’s X-rays intensity as a function of wavelength or intensity. The collision of high energy X-rays from radioisotopes or X-ray tube with the target element produces photons whose characteristics identify the element. Detectors serve to identify the elements based on the intensity of the pulse of the resultant counts. Works Cited Bounakhla, M. and Mounia Tahri. “X-Ray Fluorescence Analytical Techniques.” Web. 27 March 2013. http://www.cnstn.rnrt.tn/afra-ict/NAT/xrf/XRF%20V1.pdf Shackley, M. S., ed. “An Introduction to X-Ray Fluorescence (XRF).” X-Ray Florescence Spectrometry (XRF) in Geoarchaeology. New York, NY: Springer, 2011. Sharma, S. Atomic and Nuclear Physics. Delhi: Dorling Kindersley, 2008. Read More