Radiation-induced point defects

Shkrob lA, Tadjikov BM, Trifunac AD. (2000) Magnetic resonance studies on radiation-induced point defects in mixed oxide glasses. 1. Spin centers in BjOj and alkali borate glasses. / Non-Cryst Solids 262 6-34. 

In alloys and RPV steels with > 0.07wt%Cu, and irradiation temperatures > 200°C, Cu-enriched solute clusters form. At irradiation temperatures > 325 °C, these can grow to >4nm diameter, and probably transform to the equilibrium fee -Cu phase, but at the temperatures and fluence of interest most CECs in irradiated steels will be bcc." Radiation-induced point defects enhance the substitutional solute diffusion rate and enhance the rate of precipitation. In addition, nucleation of CECs appears to be easier in the presence of matrix defects. The nature of the matrix defects on which CECs nucleate is not clearThe relative importance of homogeneous and heterogeneous nucleation of CECs under irradiation is not agreed, although homogeneous nucleation will, naturally, become more likely as the Cu supersaturation increases. ... 

Radiation-induced point defects are usually preferred over thermal point defects (obtained by quench-in from some higher temperature and freeze-in these point defects) for studying their effects on the physical and mechanical behavior of ceramics. Radiation affects mechanical properties by way of changes in strength. 

In order to elucidate whether such a precipitate can trap positrons, the positron affinities A+ for the host material and the precipitate were calculated [154], The A+ values were found to be relatively high and the positron lifetimes very short for perfect MC carbides. This fact confirms that perfect MC (M s Cr, V, Ti, Mn, Fe, Zr, Nb) carbides are very dense materials that cannot trap positrons when embedded in the Fe matrix. In general, from a PAS point of view, radiation damage can be interpreted as a combination of radiation-induced point defects, dislocations and small vacancy clusters [129,130] that occur mainly in the region of the precipitate-matrix interface. 

The current majority opinion is that both types of point defects are important. Thermal equilibrium concentrations of point defects at the melting point are orders of magnitude lower in Si than in metals. Therefore, a direct determination of their nature by Simmons-Balluffi-type experiments (26) has not been possible. The accuracy of calculated enthalpies of formation and migration is within 1 eV, and the calculations do not help in distinguishing between the dominance of vacancies or interstitials in diffusion. The interpretation of low-temperature experiments on the migration of irradiation-induced point defects is complicated by the occurrence of radiation-induced migration of self-interstitials (27, 28). 

Because boron carbide can be used as the control rod material in a nuclear reactor, in order to interpret its performance it is necessary to establish nature of grown-in and neutron-radiation-induced lattice defects in boron carbide. It was found that the dose received by the irradiated specimen corresponds to transmutation of about eight B nuclei per unit cell in equal number of both Li and " He nuclei (Ashbee 1971). It is believed that the formation of the partial dislocation loops resulting from the agglomeration of point defects are introduced during neutron irradiation. 

It is known that hydrogen incorporated into Si subsequently exposed to ionizing radiation inhibits the formation of induced secondary point defect (Pearton and Tavendale, 1982a). For example, in both Si and Ge a number of electron or y irradiation induced defect states appear to be vacancy-related, and exposure of the Si or Ge to a hydrogen plasma (or implantation of hydrogen into the sample) prior to irradiation induces a degree of... 

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. 

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 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... 

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). 

These results indicate that the radiation induced defects such as some point defects, dislocations and lattice distortions have no influence on the protonic conduction. However, the electronic conduction is modified by sub-band annihilation in gap between valence and conduction bands after neutron irradiation [2, 6, 7],... 

This subsection is devoted to the study of the influence of the various radiation-induced imperfections, on the properties of nonmetallic solids, taking into account the lifetime of these imperfections. From this particular point of view, and in a general way, one may distinguish between structural and electronic imperfections. To the first group, belong all the imperfections which modify the lattice periodicity they comprise lattice defects (Section III,C,1) and dislocations, which are only mentioned as a reminder in this paper. Electronic imperfections are described in Section III,C,2. 

Electronic point defects, displaced electrons, almost always exist in connection with atomic point defects. A purely electronic defect, the so-called self-trapped electron trapped by induced polarization in a solid, has been suggested by Landau 29) but never found. If an incoming quantum imparts enough energy to an electron of one of the atoms of a solid, the electron will be freed from the atom and can wander through the solid. If it is not to be recaptured by the radiation-produced positive ion, it must be trapped at some other point in the solid, one with an effective positive charge. This will almost always be an atomic defect, specifically a negative ion vacancy or an impurity of suitable electron affinity relative to that of the host solid. When an electron is thus removed from an atom, the vacancy in the electronic structure is termed a positive hole. Such a hole has mobility like that of an electron... 

There are three circumstances which make a geometrical reason for an altered catalytic activity probable. If the substrate is a metal with a clean surface, any change upon irradiation must be attributed to atomic point defects or dislocations since electronic defects are excluded by the conductivity of metals. Since dislocations are produced or destroyed by radiation only under special circumstances, the normal explanation for a metal is vacancies, subsurface interstitials, or multiple defects. If, with any nonmetallic type of solid, a catalytic activity is introduced only or especially by heavy-particle bombardment and if the induced activity is little changed by annealing at low temperature, then the arrangement of the atoms rather than the presence or absence of electrons must be important. Finally, if the induced catalytic effect depends... 

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