Primary radiation sources

Gamma-ray sources can also be used without the need for an elaborate power supply, allowing use in small portable instruments. The sources include a radioactive isotope which supply gamma rays with characteristic energy and can supply an X-ray or gamma-ray beam with the flux of 10 photons s sr . The main advantages of radioisotope excitation over X-ray tube excitation are the monoenergetic character of radioisotope-emitted X-rays, and that it is an inexpensive technique that is easily commercially available. 

Synchrotron radiation (SR) is the electromagnetic radiation emitted by high-energy electrons (several GeV), circulating at highly relativistic speed in storage rings of a synchrotron accelerator. The SR spectrum is continuous and its 

It is also possible to create characteristic secondary X-ray emission using other incident radiation to excite the sample. When ions of suffident energy (usually MeV protons) produced by an ion accelerator are employed as an excitation source, the technique is named particle-induced X-ray emission (PIXE), The sensitivity of the PIXE method is very high for Fe and neighboring elements with a similar Z number, i.e. approximately 20 essential metallic elements. The lower detection limit for a PIXE beam is given by the ability of the X-rays to pass through the window between the chamber and the X-ray detector, which sharply decreases with the decrease of atomic number. The upper limit is dependent on the ionization cross-section, the probability of the K electron shell ionization, which decreases with the increase of atomic number. 

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The intensity units for ( ) are related to a primary radiation source, the candela, Cd. The definition of a candela is ... 

This means that HR-CS AAS, due to its special features, does not need any modulation of the source or any selective amplifier. This also means that a potential source of noise has been eliminated, as both AC operation of hollow cathode lamps and the mechanical choppers are contributing to noise in LS AAS. In addition, other problems that are associated with strong emission of the atomizer source in LS AAS - such as the emission noise caused by the nitrous oxide -acetylene flame in the determination of Ba and Ca due to the CN band emission [3] - are equally absent in HR-CS AAS for the same reasons, that is, the higher intensity of the primary radiation source, and the high resolution. 

Again, in HR-CS AAS these problems are essentially nonexistent for the same reasons as given above. Firstly, because of the relatively constant, very intense emission of the primary radiation source, there are no more weak lines that is, the same high SNR will be obtained on all analytical lines, regardless of their spectral origin. The only factors that will have an influence will be the absorption coefficient and the population of the low excitation level in case nonresonance lines are used. Secondly, because of the high resolution of the monochromator, and the visibility of the entire spectral environment of the analytical line in HR-CS AAS, potential spectral interferences can easily be detected, and in addition cannot influence the actual measurement, except in the rare case of direct line overlap. However, even in this case, HR-CS AAS provides an appropriate solution, as discussed in the previous section. 

In most instruments, the radiant flux is modulated periodically. This can be achieved by modulating the current of the primary source or with the aid of a rotating sector (g) in the radiation beam. Accordingly, it is easy to differentiate between the radiant density emitted by the primary source and that emitted by the flame. Both single beam and dual-beam instruments (see also Fig. 77) are used. In the latter the first part of the radiation of the primary source is led directly into the monochromator, whereas the second part initially passes through the flame. In this way fluctuations and drift can be compensated for insofar as they originate from the primary radiation source or the measurement electronics. Furthermore, the spectrometer can be provided with equipment for a quasi-simultaneous measurement of the line and background absorption [253]. 

The primary radiation sources used in AAS have to fullfil several conditions ... 

Furthermore, the radiant density of the D2 lamp in a large part of the spectrum is fairly low. Hence, the procedure limits the number of analytical lines which can be used and the number of elements that can be determined. As the spectral radiance of the D2 lamp is generally low as compared with that of a hollow cathode lamp, the latter must be operated at a low radiant output (low current), which means that detector noise limitations and poor detection limits are soon encoun-terd. Finally, as work is carried out with two primary radiation sources, which are difficult to align as they have to pass through the same zone of the atom reservoir, this may lead to further systematic errors. 

Although monochromatic excitation can improve the minimum detection limits over a restricted range of elements, it compromises performance in other areas. Most obvious is the need for higher intensity in the primary radiation source. 

Spectral interferences in AAS due to direct overlapping of the emission lines from the primary radiation source and the adsorption line of another element in the atom cell are very rare. To give rise to a spectral interference, the lines must not merely be within the band pass of the monochromator, but must actually overlap with each other s spectral profile (i.e., be within 0.01 nm). Spectral interferences resulting from the overlap of molecular bands and lines are more of a problem in AAS. Examples of this type of interference are the nonspecific absorption at 217.0 nm which affects lead (e.g., sodium chloride gives a strong molecular absorption at this wavelength), and the calcium hydroxide absorption band on barium at 553.55 nm. Spectral interferences may usually be eliminated by the use of background correction. 

The ability of AAS to differentiate between two elements is only dependent on the widths of the lines emitted from the primary radiation source ( 0.001-0.002 nm) and of the absorption lines ( 0.001-0.005 nm). The monochromator in an AAS instrument has the sole task of separating the analytical line from other emission lines of the primary source. Experience has shown that this can be achieved for practically all elements with a bandpass of 0.2 nm. For a number of elements... 

Similar to Zeeman AAS, the Smith-Hieftje technique can be used for lines over the entire spectral range, and uses only one primary radiation source, so that alignment and stability are optimum. The technique is simple and much cheaper than Zeeman AAS. It does not suffer from limitations due to Zeeman splitting of molecular bands. However, as self-reversal is not complete, the technique can be used only for rather low background absorbance, sensitivity is decreased as the... 

Principle of atomic absorption spectrometry. 1, primary radiation source 2, atomizer 3, sample 4, combustion gases 5, optical dispersive system 6, detector 7, data acquisition and processing and 8, data editing. (From Ebdon, L. and Evans, E.H., An Introduction to Analytical Atomic Syectrometry, John Wiley Sons, West Sussex, 1998,206. With permission.)... 

An XRF facility usually consists of a primary radiation source, a detector, an electronics system, and sometimes an optics system. Various XRF... 

In mobile EDXRF systems, where small dimensions are essential, sealed radionuclides are often used as primary radiation sources instead of X-ray tubes. The radionuclides have to be selected with respect to... 

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