SOFTWARE
4. X-ray spectrometry
Since the discovery of X-rays by Roentgen in 1896, the electromagnetic spectrum between 0.1 and 200
A
has been a source of significant contributions to our fundamental knowledge of atomic structure. Bombarding matter with high-energy particles such as electrons or protons generates X-rays. When an atom is bombarded in this manner, an electron is ejected from one of the inner shells of the atom. This vacancy is immediately filled by an electron from, , \
a higher-energy shell, creating a vacancy in that shell that is, in turn, filled by an electron from yet a higher shell. Thus, by a series of transitions, L ~K, M~L, N~M, each new vacancy is filled until the excited atom returns to its ground state. This electronic transition results in the emission of a characteristic X-ray spectral line whose energy, hv, is equal to the difference between the binding energies of the two electrons involved in the transition. Only certain electronic transitions are permitted by quantum-mechani.~al selection rules. The energies or wavelengths of the X-ray spectral lines are the basis for qualitative analysis.
4.1 Energy dispersive X-ray spectrometry (EDX)5
As the electron beam of the scanning electron microscope (SEM) is scanned across the sample surface, it generates X-ray fluorescence from the atoms in its path. The energy of each X-ray photon is characteristic of the element that produced it.
A solid-state detector composed of Si and Li, is responsive to the energies of the characteristic X-ray radiation. Instruments utilizing these detectors are called energy-dispersive X-ray spectrometers. The energy of a characteristic X-ray produces an electronic pulse- amplitude distribution in the detector. The EDX microanalysis system collects the X-rays, sorts and plots them by energy and automatically identifies and labels the elements responsible for the peaks in this energy distribution.
Hence, an entire X-ray spectrum from several elements in a specimen can be stored at one time.
The EDX data are typically compared to either known or computer-generated standards to produce a full quantitative analysis showing the sample composition. The EDX instrument is also capable of giving maps of distributions of elements over areas of interest.
The detector must operate at liquid-nitrogen temperatures, and a reservoir attached to the detector must be filled periodically with liquid nitrogen. The preamplifier, pulse processor and pileup-rejection circuitry constitute a sophisticated electronic package to maintain the quantitative properties of the signals produced in the detector. These are necessary to ensure high performance with respect to energy resolution, data-acquisition times and signal-to-noise ratio (i.e. detection limit).
5http://www2.arnes.si/-sgszmeral/eds/eds.html
4.2 Scanning electron microscopy (SEM)6
In the scanning electron microscope (SEM), a very fine "probe" of electrons with energies up to 40 keV is focused at the surface of the specimen in the microscope and scanned across it in a pattern of parallel lines. A number of phenomena occur at the surface under electron impact:
most important for scanning microscopy is the emission of secondary electrons with energies of a few tens eV and re-emission or reflection of the high-energy backscattered electrons from the primary beam. The intensity of emission of both secondary and backscattered electrons is very sensitive to the angle at which the electron beam strikes the surface, i.e. to topological features on the specimen. The emitted electron current is collected and amplified; variations in the resulting signal strength as the electron probe scans across the specimen are used to vary the brightness of the trace of a cathode ray tube being scanned in synchronism with the probe.
There is thus a direct positional correspondence between the electron beam scanning across the specimen and the fluorescent image on the cathode ray tube.
The magnification produced by the scanning microscope is the ratio between the dimensions of the final image display and the field scanned on the specimen. Usually, the magnification range of SEM is between 10 and 200 000 X and the resolution is between 4 and 10 nm (40 - 100 angstroms).
6http://www2.arnes.sil-sgszmeral/sem/sem/html
4.3 X-ray diffraction (XRD)
In wavelength-dispersive spectrometers, a schematic of which appears in Fig. 5, wavelengths are separated by Bragg diffraction from a single crystal. The X-ray tube is usually of high intensity (approx. 3 kW) with a stabilized high-voltage supply. This is necessary because large losses of characteristic radiation occur due to the relatively low reflectivity of the dispersive crystals. The detector is mounted on a goniometer, which allows the detector to accept one wavelength at a time at the 28 diffraction angle, and covers a broad range from a few degrees to 150 degrees. Either a proportional or scintillation counter detector is used, or both in
tandem arrangement. The associated electronics include a DC power supply, linear amplifier and recorder.
Detector
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X-ray tube Sample
Fig. 5. A schematic representation ofan XRD spectrometer
Bombarding a suitable target with electrons produces the X-rays. When the electrons hit the target, they "move" electrons around the orbitals, which results in a series of emission wavelengths as the atom returns to an unexcited state. The resulting electron excitation in the source produces a broad band of energies that are high energy X-rays. The sample is irradiated with the high energy X-rays to produce a secondary beam of fluorescent X-rays. These X-rays are passed through a collimator and thus directed to a single analyzer crystal that separates the wavelengths. The wavelengths of the X-rays produced by the powdered sample and diffracted by the analyzer crystal obey the Bragg equation:
nJ.
=
2dsinOwhere A
=
wavelength of X-rays, d=
spacings of atoms in powdered sample and n=
integer.The Bragg equation links the d-spacings on the powdered sample to the angle of turn of the analyzer crystal. The data obtained shows a series of lines of varying intensities at different 28 values, obtained as the analyser crystal turns. A qualitative analysis of the sample is thus carried out (Brady, J.E.et al. (1993)).
References
Brady, J.E.; Holum, J.R.; Chemistry: The Study of Matter and its Changes; John Wiley and ons; p 386 Chapter 9 (1993)
Chittur,K.; http://www.eb.uah.edu/-kchitturlbmreview/node3.html
Miller, R.G.J.; Laboratory Methods in Infrared Spectroscopy;Heydon and Son Ltd,London, N.W.; p 137 (1988)
APPENDIX 4
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