Fluorescence radiation, light

In Total Reflection X-Ray Fluorescence Analysis (TXRF), the sutface of a solid specimen is exposed to an X-ray beam in grazing geometry. The angle of incidence is kept below the critical angle for total reflection, which is determined by the electron density in the specimen surface layer, and is on the order of mrad. With total reflection, only a few nm of the surface layer are penetrated by the X rays, and the surface is excited to emit characteristic X-ray fluorescence radiation. The energy spectrum recorded by the detector contains quantitative information about the elemental composition and, especially, the trace impurity content of the surface, e.g., semiconductor wafers. TXRF requires a specular surface of the specimen with regard to the primary X-ray light. 

Since only relatively few substances are capable of emitting fluorescent radiation, they can be particularly selectively detected. This means that the selectivity of the chromatographic separation, which is always aimed at, is meaningfully extended by the selectivity of detection. Accompanying substances that absorb radiation but do not emit light do not interfere when the analysis is made by the selective determination of fluorescence ... 

Radiation from a xenon or deuterium source is focussed on the flow cell. An interchangeable filter allows different excitation wavelengths to be used. The fluorescent radiation is emitted by the sample in all directions, but is usually measured at 90° to the incident beam. In some types, to increase sensitivity, the fluorescent radiation is reflected and focussed by a parabolic mirror. The second filter isolates a suitable wavelength from the fluorescence spectrum and prevents any scattered light from the source from reaching the photomultiplier detector. The 90° optics allow monitoring of the incident beam as well, so that dual uv absorption and fluorescence... 

Radiation from a Xenon-radiation or a Deuterium-source is focussed on the flow cell through a filter. The fluorescent radiation emitted by the sample is usually measured at 90° to the incident beam. The second filter picks up a suitable wavelength and avoids all scattered light to reach ultimately the photomultiplier detector. 

Invariably, the energy of the light emitted is less than that originally taken on. By studying the amount and energy of the fluorescence radiation decay rates, depolarization effects, excimer stability, and structure can be determined. 

Figure 12.4—Fluorescence intensity. Depending on the point from which fluorescence is emitted in solution, a variable light intensity will reach the detector. By specific positioning of the excitation and emission windows, it is possible to estimate the re-absorption of fluorescence radiation (by comparison between sectors a and c in the figure) and the absorption of the incident radiation (by comparison between sectors a and b). In practice, fluorescence emitted from the central region of the cell is collected.

In fluorescence, the light that is emitted is of lower energy than the light that is absorbed and the fluorescence stops as soon as the exciting radiation is stopped. In phosphorescence, the phosphorescing molecules remain excited for a period of time after the stimulus has stopped. 

Scintillation counter The sensor, the so-called scintillator, contains a transparent crystal that fluoresces when hit by ionizing radiation, thus a scintillation counter measures ionizing radiation. Light emitted from the crystal is measured by a sensitive photomultiplier tube which is attached to an electronic amplifier in order to count the amplitude of signals produced by the photomultiplier. Liquid scintillation counters are a very efficient and practical way to measure and quantify p radiation (see Figure 10.5b). 

Figure 12 shows results of a thermal-aging experiment. Sheets of Whatman No. 42 filter paper were first exposed to near-UV radiation (BLB fluorescent black lights), after which they were subjected to oven... 

The effects of exposure of FD C Red No. 3 solutions to fluorescent, longwave, and shortwave UV sources were studied by Asker and Jackson (94). They found that exposure to fluorescent lighting (from cool white fluorescent tubes) was more detrimental to the stability of the dye solution than either of the other two UV sources studied. Similarly, the same kind of fluorescent radiation caused a higher degree of degradation of adriamycin solutions than either short wave or long wave UV radiation (47). The results of this study are presented in Table 3. 

Figure 4 (bottom) shows the configuration of the optics to irradiate the seawater and to detect the subsequent signal through the window. A set of lenses, and L2, focuses the fiber image on the water channel. (The diameter of the laser beam at the measuring position was determined to be about 0.6 mm.) The fluorescence emitted and the light scattered in the focused volume are collected by the lens L3 on the surfaces of the fibers F1 and F2 with a two-to-one reduction factor. A dichroic filter transmits the fluorescence radiation to the fiber F j and the scattered radiation to the fiber F2. 

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