Vacancy decay

Fig. 12. Kinetics of the supersaturation vacancy decay (S) for ceCu-Al alloys quenched from 873 K. The reacted fraction during stage 1 (xi) is also shown for each alloy. In Fig. lid, the supersaturation decay is plotted for Cu-19 at% Al, showing two different dislocation densities representative of deformed (( i) and annealed ( 2) materials (/> = 0.33Ks ...

An accurate knowledge of the individual rates of vacancy decay is interesting in several fields. Firstly, transition rates present a sensitive tool to investigate details of atomic structure since they probe static properties (atomic wave functions) as well as dynamic properties (electron correlation and relaxation). Secondly, an accurate knowledge of relative decay rates is important in practical applications In experimental studies of ion-atom collisions either fluorescence or electron emission is detected and the ionization cross sections are derived. In the L-shell case uncertainties of fluorescence and Coster-Kronig yields are a limiting factor upon deriving ionization cross sections. ... 

The experimental vacancy decay rates may be compared to theoretical predictions. A comparison of relative rates normalized to unity for the total decay rate (i.e., fluorescence and Coster-Kronig yields) may be misleading For example, assume that one decay channel is strongly dominant and that the transition rate only of this channel is predicted incorrectly by theory. In such a case the dominant channel has a yield close to unity both experimentally and theoretically and thus only little deviation will be observed for this channel. However, experimental and theoretical yield of a weak decay channel depend strongly on the rate of the strong channel due to the normalization, and thus for the weak channel a significant deviation arises. For this reason, a conclusive comparison between experiment and theory has to be made for absolute rates. In order to convert the experimentally determined relative rates to absolute values, a normalization... 

It is seen, that the static rearrangement makes the cross section too small, while the Is-vacancy decay into and ... 

From other work on Pb(CH3)4 it is known that an Auger cascade connected with an L or M vacancy in the lead atom leads to the development of a charge of up to -t-17. This results in the total destruction of the molecule through a Coulomb explosion. On the basis of the 4/5 rule and the 14% internal conversion, one can estimate that for Pb(CH3)4 the molecule should remain intact in at least 69% of the decays, corresponding to the transformation ... 

So far, we have discussed only the detection of y-rays transmitted through the Mossbauer absorber. However, the Mossbauer effect can also be established by recording scattered radiation that is emitted by the absorber nuclei upon de-excitation after resonant y-absorption. The decay of the excited nuclear state proceeds for Fe predominantly by internal conversion and emission of a conversion electron from the K-shell ( 90%). This event is followed by the emission of an additional (mostly Ka) X-ray or an Auger electron when the vacancy in the K shell is filled again. Alternatively, the direct transition of the resonantly excited nucleus causes re-emission of a y-photon (14.4 keV). 

Reactions of eh with H and OH were once considered diffusion-controlled see, however, Elliot et al. (1990). The rate constants, 2.5—3.0 x 1010 M-1s 1 (see Table 6.6), are high. In both cases, a vacancy exists in the partially filled orbitals of the reactants into which the electron can jump. Thus, hydrogen formation by the reaction eh + H may be visualized in two steps (Hart and Anbar, 1970) eh + H—H, followed by H + H20— OH" This reaction has no isotope effect, which is consistent with the proposed mechanism. The rate of reaction with OH is obtained from the eh decay curve at pH 10.5 in the absence of dissolved hydrogen or oxygen, where computer analysis is required to take into account some residual reactions. At higher pH (>13), OH exists as O- and the rate of eh + O—"02 has been measured as 2.2 x 1010 M-1s-1. 

The significance of this length parameter A can be understood by examining the predicted steady-state vacancy concentration profile in the porous electrode as shown in Figure 26a. At steady state, the model predicts that the mixed conductor will be reduced by an amount that decays exponentially with distance... 

Silver Activation. Doping zinc sulfide with silver leads to the appearance of an intense emission band in the blue region of the spectrum at 440 nm, which has a short decay time. Weak luminescence in the green (520 nm) and red regions can also occur. The blue band is assigned to recombination at substitutionally incorporated silver ions [5.314], [5.315]. The red band is caused by luminescence processes in associates of silver ions occupying zinc positions with neighboring sulfur vacancies... 

When fired in a reducing atmosphere, Y3A15Oi2 exhibits a pronounced afterglow due to traps formed by oxygen vacancies [5.356]. Subsequent annealing in air diminishes this effect and leads to decay times of 200 300 ns therefore, Y3Al5Ol2 Ce3+ is used in flying-spot scanner tubes. The emission maximum is at 550 nm. This phosphor is classified under P46 (TEPAC) and KG (WTDS) (see Section 5.5.4.3). 

As it is known, I centres are the most mobile radiation-induced radiation defects in alkali halides and therefore they play an essential role in low-temperature defect annealing. It is known, in particular, from thermally-stimulated conductivity and thermally-stimulated luminescence measurements, that these centres recombine with the F and F electron centres which results in an electron release from anion vacancy. This electron participates in a number of secondary reactions, e.g., in recombination with hole (H, Vk) centres. Results of the calculations of the correlated annealing of the close pairs of I, F centres are presented in Fig. 3.11. The conclusion could be drawn that even simultaneous annealing of three kinds of pairs (Inn, 2nn and 3nn in equal concentrations) results in the step-structure of concentration decay in complete agreement with the experimental data [82]. 

As it was discussed in Chapter 3, neutral point defects in all solids interact with each other by the elastic forces caused by overlap of deformation fields surrounding a pair of defects. These forces are effectively attractive for both similar and dissimilar defects (interstitial-interstitial, vacancy-vacancy and interstitial-vacancy, respectively) and decay with the distance between defects as... 

Figure 7.8 shows the joint correlation function for vacancies for a fixed dose rate of p = 1017 cm 3s 1 but varying the temperature from 0°C to 150°C. The two low-temperature curves - at 0°C and 50°C - decay monotonously with r, as it is predicted by equation (6.3.4). However, as the temperature increases further, curves 3 and 4 reveal a new type of behaviour... 

Ideas about the tunneling mechanism of the recombination of donor acceptor pairs in crystals seem to be first used in ref. 51 to explain the low-temperature of photo-bleaching (i.e. decay on illumination) of F-centres in single crystals of KBr. F-centres are electrons located in anion vacancies and are generated simultaneously with hole centres (centres of the Br3 type which are called H-centres) via radiolysis of alkali halide crystals. 

Figure 1.2 Schematic diagram to show X-ray emission to fill vacancy caused by nuclear decay. An L-shell electron (A) is shown filling a K-shell vacancy (B). In doing so, it emits a characteristic K X-ray.

Electron capture decay produces a vacancy in the atomic electron shells and secondary processes that lead to filling that vacancy by the emission of X-rays and Auger electrons occur. These X-rays permit the detection of EC decays. 

These features of lines of various spectra (X-ray, emission, photoelectron, Auger) are determined by the same reason, therefore they are discussed together. Let us briefly consider various factors of line broadening, as well as the dependence of natural line width and fluorescence yield, characterizing the relative role of radiative and Auger decay of a state with vacancy, on nuclear charge, and on one- and many-electron quantum numbers. 

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