Radiation self-absorption

Self-Absorption—Absorption of radiation (emitted by radioactive atoms) by the material in which the atoms are located in particular, the absorption of radiation within a sample being assayed. 

Self-absorption is a phenomenon whereby emitted radiation is reabsorbed as it passes outwards from the central region of the flame (cf. arc/spark spectrometry). It occurs because of interaction with ground state atoms of the analyte in the cooler outer fringes of the flame and results in attenuation of the intensity of emission. It is particularly noticeable for lines originating from the lowest excited level and increases with the concentration of the analyte solution (Figure 8.24). 

To avoid this handicap, Boersch and coworkers 2) used coupled resonators. The first active laser cavitiy generates the radiation whose absorption is to be measured. The probe is placed in a second cavity, which is coupled to the first one and which is undamped by an active medium just below the threshold for self-oscillation. This arrangement enables changes in the refractive index as small as An 10 ° or absorption coefficients down to a 10 to be detected. 

Due to absence of a stable ground state, no self-absorption of the excimer radiation can occur. 

Monochromatic UV radiation is emitted by excimer lamps, in which microwave discharge5 or a radio-frequency-driven silent discharge6 generates excimer-excited states of noble gas halide molecules, which decay by the emission of monochromatic UV radiation. In the ground state, the excimer molecules decay into atoms. Therefore, no self-absorption of the UV radiation can occur. All photons are coupled out of the discharge.4... 

In X-ray fluorescence, self-absorption due to optical quenching of the sample is always observed. For solid samples, radiation can be observed only at a depth of a few micrometres. The intensity P of radiation, after travelling a distance dx in a material which a lineic absorption coefficient p (cm ), will decrease by dP for a penetration angle of 90°. The integrated form of the expression dP = —pP dx is comparable to that described in colorimetry ... 

Reversed Radiation.—A pressure broadened resonance emission line with radiation in the middle of the line virtually absent because of self-absorption near the walls, (e.g., the 2537 A. line obtained from medium pressure mercury lamps). 

A further factor which affects the shape of a resonance line is the transport of resonance radiation through the. parent gas. Milne s early theory of self-absorption by the imprisonment of resonance radiation has been revised by Holstein (3) and Bichermau (4), taking into account the incoherent scattering of resonance photons. Furthermore, Walsh (" ) has extended the imprisonment lifetime calculation for cases where Doppler and collision broadening of the resonance line are simultaneously present, and in addition this author has examined the complications caused by the, hfs. The line shape, linewidth, and other properties of self-absorbed lines hare been discussed recently bv Tako (6), who summarizes the various effects of self-absorption as follows ... 

The photon hvx emitted in a radiating transition is reabsorbed with a certain probability by another molecule X or Y. By absorbing the photon the molecule makes a transition into the excited states X and Y, respectively. Thus the Y molecules are excited only by the radiating and nonradiating transfer of the excitation energy. The photons hvY emitted in a radiating transition which do not undergo self-absorption leave the scintillator. 

The focus of the experiments is on the work in the radioanalytical chemistry laboratory - initial sample processing, radioanalyte purification, and preparation of the sample for counting. Certain aspects of radiation detection, such as counting efficiency and self-absorption for the various radiations, are addressed in these experiments. We expect that the student learns about the principles and applications of radiation detection instruments in a separate radiation detection course, which can be presented before, after, or in parallel with this course. 

All these limitations do not exist in HR-CS AAS. Firstly, the radiation intensity of the source is always high enough to provide a significantly better SNR than the LS in conventional AAS. In a first approximation the radiation intensity, and hence the SNR for all lines, is of the same magnitude, although it degrades somewhat in the far UV. Secondly, the resolution of the monochromator is such that only the center of the line is detected by the analytical pixel. As the source emits a continuum, and the analytical pixel is adjusted in such a way that it is always in the line center, none of the phenomena associated with LS exist, for example, line shift, self absorption, or the presence of other lines emitted by the lamp [3]. 

Radiation of the solution background is governed and minimized by self-absorption of the radiation, i.e., by the attenuation of the radiation intensity by the radioactive medium itself (thin-foil method). 

The spatial distribution of emitting species produced in the electron-hole recombination process is one of important reasons for a difference between the PL and EL spectra, and a characteristic determining the EL quantum efficiency. The self-absorption of the short-wavelength part of the fluorescence can be utilized for determining the spatial distribution of EL. The principle of the method, as discussed in Sec. 3.1 and used for photoexcited states in Sec. 3.2, has been adapted to the recombination radiation as follows [41] the unknown spatial distribution of the EL light intensity, if/ x) from a plate-shaped emitting sample, is related to the experimentally observed EL signal, El(Zo), by the expression... 

It should be noted that the spectral emission is influenced by the. self-absorption of the emitted radiation by the sample. If the temperature distribution is homogeneous, this effect is already included in the determination of the absorptivity. In inhomogenous samples, the self-absorption may be neglected if the absorptivity is below 5%. In this case, the overall emission can be treated as the sum of the emission of all infinitely thin layers into which the sample can be divided (Pepperhoff and Grasz, 1955). Otherwise, the emission of all inner layers must be corrected by transmission factors before summation. For practical calculations, the sample volume can be divided into different layers, each of which is assumed to be in thermal equilibrium. 

If the vibrational temperature is determined by using the intensities of different bands, a distinct value is obtained for each band. These values do not represent the arithmetic mean of all temperatures. Due to the nonlinear increase of the spectral radiance by the black body radiator, the hot zones appear more pronounced than the cold ones. On the other hand, the influence of the more distant zones with respect to the observer is reduced by stronger self-absorption. The vibrational temperatures deduced from bands with high absorption coefficient are therefore lower than those derived from bands with smaller absorption coefficient. Nevertheless, all thus obtained temperature values are between the lowest and the highest temperature of the sample. The method of fitting calculated spectral profiles to the observed ones has been successfully applied in these cases, too. 

A number of problems arise in connection with the use of emission IR spectroscopy (IRES). One of them arises from the existence of temperature gradients, which can cause self-absorption of the emitted radiation by the colder outer parts of the sample itself another is concerned with the selective reflection that occurs in the vicinity of strong absorption bands. This reduces the absorptance and hence the emittance. Moreover, perturbations can be created by reflections and emission by the cell elements. These problems, however, can in part be overcome so that IR emission spectra can be successfully recorded and are widely used, for example, in the fields of polymer and corrosion science and mineralogy. Some uses of IRES... 

The overall counting efficiency rj in eq. (7.1) depends on the frequency H of the decay mode measured in relation to the activity, tlie self-absorption S of the radiation in the radioactive sample, the contribution B of backscattered radiation, the geometrical arrangement G of the sample with respect to the counter, the absorption... 

Figure 7.5. Self-absorption S of the radiation of Ca in CaCOs as a function of the thickness of the sample.

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