Radiation-induced defects

Of special interest in the recent years was the kinetics of defect radiation-induced aggregation in a form of colloids-, in alkali halides MeX irradiated at high temperatures and high doses bubbles filled with X2 gas and metal particles with several nanometers in size were observed [58] more than once. Several theoretical formalisms were developed for describing this phenomenon, which could be classified as three general categories (i) macroscopic theory [59-62], which is based on the rate equations for macroscopic defect concentrations (ii) mesoscopic theory [63-65] operating with space-dependent local concentrations of point defects, and lastly (iii) discussed in Section 7.1 microscopic theory based on the hierarchy of equations for many-particle densities (in principle, it is infinite and contains complete information about all kinds of spatial correlation within different clusters of defects). 

Solution polymerization of VDE in fluorinated and fluorochlorinated hydrocarbons such as CEC-113 and initiated with organic peroxides (99), especially bis(perfluoropropionyl) peroxide (100), has been claimed. Radiation-induced polymerization of VDE has also been investigated (101,102). Alkylboron compounds activated by oxygen initiate VDE polymerization in water or organic solvents (103,104). Microwave-stimulated, low pressure plasma polymerization of VDE gives polymer film that is <10 pm thick (105). Highly regular PVDE polymer with minimized defect stmcture was synthesized and claimed (106). Perdeuterated PVDE has also been prepared and described (107). 

In a more general application, thermoluminescence is used to study mechanisms of defect annealing in crystals. Electron holes and traps, crystal defects, and color-centers are generated in crystals by isotope or X-ray irradiation at low temperatures. Thermoluminescent emission during the warmup can be interpreted in terms of the microenvironments around the various radiation induced defects and the dynamics of the annealing process (117-118). ... 

A series of calculations on defect centers induced by radiation damage in alpha-quartz is reported. Ab initio SCF-MO calculations were carried out on a 21 atom cluster, Si50i6 % surrounded by 956 point-ions, designed to simulate alpha-quartz. This two-region approach made it possible to represent the long-range electrostatic effects, present in the crystal, in the SCF-MO cluster. 

It was discovered by Al shits et al. (1987) that static magnetic fields of order 0.5T affect the motion of dislocations in NaCl crystals. This is not an intrinsic effect but is associated with impurities and/or radiation induced localized defects. Also, magnetic field effects have been observed in semiconductor crystals such as Si (Ossipyan et al., 2004). 

Santana, P, Pena, L.A, Haimovitz-Friedman, A, Martin, S., Green, D., McLougWin, M., Cordon-Cardo, C., Schuchman, E.H., Fuks, Z. and Kolesnick, R., 1996, Acid sphingomyehnase-deficient human lymphoblasts and mice are defective in radiation-induced apoptosis. Cell 86 189-199... 

Hu, B., Pan, Y., Botis, S., Rogers, B., Kotzer, T., Yeo, G. 2008. Radiation-induced defects in drusy quartz, Athabasca Basin, Canada A new aid to exploration of uranium deposits. Economic Geology, 103, 1571-1580. 

Before discussing the types of radicals actually observed in DNA, it is important to first summarize the work on model systems. These include studies of irradiated nucleosides and nucleotides from which one can usually determine the structures of the free radical products. The emphasis here will be to summarize the results on EPR/ENDOR studies of irradiated DNA bases at low temperatures in efforts to study the primary radiation-induced defects. 

In single crystals of deoxyadenosine [45], the site of oxidation seems to be the deoxyribose moiety. This brings up an interesting point. In studies of the radiation-induced defects in nucleosides and nucleotides, one often sees evidence of damage to the ribose or deoxyribose moiety. These radicals have not been discussed here because much less is known about sugar-centered radicals in irradiated DNA. 

Another approach by Hiittermann et al. [50] has been to use pulsed EPR techniques to study the radicals present in DNA fibers equilibrated in D2O and then irradiated and observed at 77 K. This work supports the conclusions that the primary radiation-induced defects are Cyt and Gua. Also reported are contributions from Thy and an allyl radical found on thymine [Thy(Me—H) ]. Also discussed are three components tentatively assigned as adenine and guanine anions and a species whose dominant hyperfine interaction involves the N1 of cytosine. 

This review has looked at the direct effects of ionizing radiation on nucleic acids. The first step was to review detailed EPR/ENDOR experiments on irradiated model compounds at low temperatures in order to study the primary radiation-induced defects. Next, it was shown how these EPR spectra are used to simulate the EPR spectra of the DNA polymer. 

Lithic Tool Forgery and the ESR Age. Thermal stability of radiation-induced defects were studied in a stone tool excavated from Paleolithic Kami-Takamori site and supplied by archaeologists in our laboratory. The age agreed well with those of tephra in the same sediment studied by several dating methods. The age is related with the heating event which does not necessarily mean heating by ancient man as written clearly and cautiously in the paper.79... 

After thermalization, the defects begin to migrate, recombine, cluster, or precipitate provided the temperature is high enough to activate the motion of point defects. The various possible processes depend on defect concentration and their spatial distribution as well as on defect mobility and their interaction energies. As in non-metallic crystals, internal and external surfaces act as sinks for at least a part of the radiation induced defects in metals. 

The first two terms in the bracket correspond to the customary chemical free energy density and the gradient energy respectively. The third term takes into account the ballistic flux. D is the Darken interdiffusion coefficient (Eqn. (4.78)), but adapted to the radiation induced increased defect concentration of the alloy. 

The authors of this book started working on chemical kinetics more than 10 years ago focusing on investigations of particular radiation - induced processes in solids and liquids. Condensed matter physics, however, treats point (radiation) defects as active particles whose individual characteristics define kinetics of possible processes and radiation properties of materials. A study of an ensemble of such particles (defects), especially if they are created in large concentrations under irradiation for a long time, has lead us to many-particle problems, common in statistical physics. However, the standard theory of diffusion-controlled reactions as developed by Smoluchowski... 

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]. 

V. Kirsanov, A. Suvorov and Yu. Trushin, Processes of Radiation-Induced Defect Production in Metals (Energoatomisdat, Moscow, 1985). 

In the last several decades, both experimental data and theoretical studies [5, 9, 13-15] have revealed the effect of similar defect aggregation in the course of the bimolecular A+B —> 0 reaction under permanent particle source (irradiation) - the phenomenon similar to that discussed in previous Chapters for the diffusion-controlled concentration decay. Radiation-induced aggregation of similar defects being observed experimentally at 4 K after prolonged X-ray irradiation [16] via both anomalously high for random distribution concentration of dimer F2 centres (two nearest F centres) and directly in the electronic microscope [17], permits to accumulate defect concentrations whose saturation value exceed by several times that of the Poisson distribution. 

At low temperatures dissimilar-defect segregation arises under permanent particle source due to local fluctuations of particle (defect) densities. Radiation-induced production by a change of two or more similar defects (say, A) nearby creates a germ of their aggregate which is more stable and has a greater chance to survive rather than two isolated defects A since the probability that two or more defects B will be created statistically in the same... 

As it has been said above, accumulation of radiation-induced (Frenkel) defects takes place in all kinds of solids irrespective which of the two basic recombination mechanisms - annihilation or tunnelling recombination - occurs [9, 11, 13, 17-20, 39, 42, 99-107]. In a good approximation this process could be considered as the A + B —> 0 reaction with the particle input. In many cases strong arguments exist for the clustering of radiation-induced... 

We note in conclusion that taking account of correlation of defects in genetic pairs, formation of pairs of new defects (e.g., owing to the tunnelling mechanism of recombination), and of radiation-induced disclocation loops, etc., substantially complicate the development of rigorous and universal theory of the kinetics of defect accumulation. The temperature dependence of the efficiency of defect accumulation contains substantial information on the correlation within genetic pairs and on the nature of their interaction [119, 124] and is also of great theoretical importance. 

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