Radiation, absorption sources

A non exhaustive description of the history of X-ray Absorption Spectroscopy (XAS) can be found in Ref. 1. The modem EXAFS (Extended X-ray Absorption Fine Structure) technique began in the early seventies of the last century. It corresponds to the concomitance of both theoretical and experimental developments. Between 1969 and 1975, Stem, Sayers and Lytle succeeded in interpreting theoretically the X-ray Absorption Structures observed above an absorption edge [2], while during the same period, the advent of synchrotron radiation (SR) sources reduced drastically the acquisition time of a spectrum if compared to data obtained with conventional X-ray tubes. XAS provides essential information about the local atomic geometry and the electronic and chemical state of a specific atom, for almost any element of the periodic table (Z>5). This prime tool for... 

One of the environmental sources of nitrite is represented by the irradiation of nitrate itself. The fact that nitrite, too, is a photoactive compound implies that the photochemical reactivity of nitrate and nitrite cannot often be dissociated, although the relative contributions to hydroxyl generation can be derived from the concentration values, photolysis quantum yields and radiation absorption calculations [6,8,12,14]. 

The Planck-Kirchhoff law allows a good approximation of the spectral radiance of any thermal radiator, the sources as well as the samples and detectors. Thermal radiators are characterized by a definite temperature as well as by their absorption coefficients f(i>) or a(i>), which describe the characteristic spectrum of the radiator ... 

The set of components that enable us to record a radiation absorption spectrum consists of radiation source, monochromator, cuvette, and detector with the data treatment system (Fig. 2.1). 

Nuclear radiation absorption methods have many technical applications. These methods are not to be confused with radioisotope tracer methods, although radioisotopes may be used as radiation sources. In the tracer method the chemical properties of the radionuclide are important while in the applications discussed in this section only the type and energy of radiation emitted are irqx>rtant. 

The radiation absorption and transmission characteristics of polymers can be used to identify polymer types. The basic principle of spectrophotometric methods of analysis is to provide an energy source (such as infrared radiation, IR) to which a sample is exposed, and to convert to some measure a response that is characteristic of the moiecuies. This response is, most often, absorption of a fraction of the incident tight. 

The parallelism between time and space is pursued in applying the same concept of reversibility to both. A space distribution of energy can be a source of irreversibility as demonstrated in case study H8 Attenuated Propagation in Chapter 11 dealing with attenuated propagation due to radiation absorption. This is a property of space which is made clearer than in the classical theories. 

In addition to the artificial radiation hght sources above described, sun-hght can be used to illuminate the photocatalysts. The earth receives about 1.7 X lO kW of solar radiation (1.5 x 10 kWh per year). Extraterrestrial radiation has an intensity of 1367 W m in a wavelength range between 200 and 5000 nm, which is reduced to 280-4000 nm when it reaches the ground, due to absorption phenomena by atmospheric compounds such as ozone, oxygen, carbon dioxide, aerosols, water vapor, clouds, etc. 

Spectral interferences are due to incomplete isolation of the radiation absorbed by the analyte from other radiation or radiation absorption detected by the instrument. In AAS spectral interferences by thermal emission of concomitants transmitted by the monochromator or received by the detector as stray light are eliminated by modulating the primary radiation and tuning the amplifier to the same modulation frequency (see Sec. 1.1). Spectral interferences can therefore only be caused by absorption of radiation by overlapping atomic or molecular lines, or by scattering of source radiation by nonvolatilized particles formed by the concomitants. Spectral interferences are best corrected for by an efficient background corrector (see Sec. 1.4). It must be stressed that the method of additions (see Sec. l.S) by definition cannot be used to correct for any spectral interferences. 

Various types of radiation are used a-rays, p-rays, y-rays. X-rays, and infrared radiation. A continuous stream of radiation is emitted from a constant radiation source (X-ray tube or radioisotope), passes through the material whose thickness is being measured, and strikes the radiation sensor. As radiation passes through the extrudate, some of the radiation is absorbed and, as a result, the radiation reaching the sensor is less intense. The amount of absorption depends on the material s density and thickness. If the density is constant, the amount of radiation absorption is a direct measure of thickness. 

The experimental layout is presented in Fig. 14.5. A Co( Fe) isotope I with activity of lOmCi in a chromium matrix was used as a source of Mossbauer radiation. This source has a spectrum in the form of single line of natural width. The source is fixed in the Plexiglas disc and put in the center (/ = 2.5 cm) or near the edge (/ = I cm) of the resonant absorber 2, having a form of cylinder with diameter D = 2 cm and length L = 5 cm, made of Fe isotope (200 mg) in stainless steel (100 mg). The thickness of absorber 7 mg cm provides the requirement of total absorption of resonant radiation (for co - coeg < T/2) and almost full transparency for nonresonant radiation (for co - coeg > T/2). 

Tanner and Tolbert (1975) also used a radiation procedure to measure eggshell thickness. However, rather than using beta-backscatter as a means of measurement, they used a ganuna-radiation densitometer that had been used to measure density of small sections of wood cores. The radiation source was Fe. They concluded that.. radiation absorption methods are only slightly superior to optical methods in measuring this effect. The optical methods are much faster and cheaper. It should be noted that their procedure, as contrasted with that of Fox ef d/. (1975), required destruction of the egg and use of laboratory equipment. 

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