Radiation defects

In contrast to metals and semiconductors, the valence electrons in polymers are localized in covalent bonds.The small current that flows through polymers upon the application of an electric field arises mainly from structural defects and impurities. Additives, such as fillers, antioxidants, plasticizers, and processing aids of flame retardants, cause an increase of charge carriers, which results in a decrease of their volume resistivity. In radiation cross-linking electrons may produce radiation defects in the material the higher the absorbed dose, the greater the number of defects. As a result, the resistivity of a radiation cross-linked polymer may decrease. Volume resistivities and dielectric constants of some polymers used as insulations are in Table 8.3. It can be seen that the values of dielectric constants of cross-linked polymers are slightly lower than those of polymers not cross-linked. 

Particle irradiation effects in halides and especially in alkali halides have been intensively studied. One reason is that salt mines can be used to store radioactive waste. Alkali halides in thermal equilibrium are Schottky-type disordered materials. Defects in NaCl which form under electron bombardment at low temperature are neutral anion vacancies (Vx) and a corresponding number of anion interstitials (Xf). Even at liquid nitrogen temperature, these primary radiation defects are still somewhat mobile. Thus, they can either recombine (Xf+Vx = Xx) or form clusters. First, clusters will form according to /i-Xf = X j. Also, Xf and Xf j may be trapped at impurities. Later, vacancies will cluster as well. If X is trapped by a vacancy pair [VA Vx] (which is, in other words, an empty site of a lattice molecule, i.e., the smallest possible pore ) we have the smallest possible halogen molecule bubble . Further clustering of these defects may lead to dislocation loops. In contrast, aggregates of only anion vacancies are equivalent to small metal colloid particles. 

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... 

Equation (2.2.24) means homogeneous generation of particles A and B with the rate p (per unit time and volume), whereas (2.2.25) comes from the statistical independence of sources of a different-kind particles. Physical analog of this model is accumulation of the complementary Frenkel radiation defects in solids. Note that depending on the irradiation type and chemical nature of solids (metal or insulator), dissimilar Frenkel defects could be either spatially correlated in the so-called geminate pairs (see Chapter 3) or distributed at random. We will focus our attention on the latter case. 

Fig. 3.1. Models of basic radiation defects in ionic solids.

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]. 

There are three reasons why the temperature change can affect the tunnelling luminescence of radiation defects in wide-gap insulators characterized by a strong electron-phonon interaction ... 

If radiation defects are charged, their aggregation is accompanied by the internal electric fields (which are the greater, the less particle mobility and larger irradiation intensity p). Quantitative analysis of this problem has been done in [37]. (Probably, this effects has been observed experimentally in [38].)... 

The kinetics of defect accumulation requires further theoretical studies at temperatures at which they become mobile. The creation under the action of radiation of an ordered structure from the accumulation of radiation defects is an effect akin to those discussed in the theory of self-organization of structures - synergetics [14]. 

Yu.H. Kalnin, V.L. Vinetskii, O.M. Kirshon and Yu.Yu. Krikis, Abstracts of the All-Union Seminar on Simulation of Radiation Defects (Ioffe Institute of the Acad. Sci. USSR, Leningrad, 1986) p. 46. 

In our opinion, this book demonstrates clearly that the formalism of many-point particle densities based on the Kirkwood superposition approximation for decoupling the three-particle correlation functions is able to treat adequately all possible cases and reaction regimes studied in the book (including immobile/mobile reactants, correlated/random initial particle distributions, concentration decay/accumulation under permanent source, etc.). Results of most of analytical theories are checked by extensive computer simulations. (It should be reminded that many-particle effects under study were observed for the first time namely in computer simulations [22, 23].) Only few experimental evidences exist now for many-particle effects in bimolecular reactions, the two reliable examples are accumulation kinetics of immobile radiation defects at low temperatures in ionic solids (see [24] for experiments and [25] for their theoretical interpretation) and pseudo-first order reversible diffusion-controlled recombination of protons with excited dye molecules [26]. This is one of main reasons why we did not consider in detail some of very refined theories for the kinetics asymptotics as well as peculiarities of reactions on fractal structures ([27-29] and references therein). 

A wide range of condensed matter properties including viscosity, ionic conductivity and mass transport belong to the class of thermally activated processes and are treated in terms of diffusion. Its theory seems to be quite well developed now [1-5] and was applied successfully to the study of radiation defects [6-8], dilute alloys and processes in highly defective solids [9-11]. Mobile particles or defects in solids inavoidably interact and thus participate in a series of diffusion-controlled reactions [12-18]. Three basic bimolecular reactions in solids and liquids are dissimilar particle (defect) recombination (annihilation), A + B —> 0 energy transfer from donors A to unsaturable sinks B, A + B —> B and exciton annihilation, A + A —> 0. 

In ref. 59, the effect of temperature jumps on the intensity of tunneling recombination luminescence from y-irradiated vitreous solutions of biphenyl, naphthalene, anthracene, and phenanthrene in toluene was studied. A comparison of the ITL studies with the EPR and absorption spectroscopy data on the kinetics of the decay of the radiation defects in these y-irradiated... 

Numerous data about the processes of the tunneling recombination of radiation defects have been obtained in studies on tunneling recombination luminescence. The recombination luminescence of y-irradiated alkali halide crystals was discovered in the mid-1960s [58, 59] in studying the transfer of electrons from Ag and T1 atoms (electron donors) to Cl2 particles (electron acceptor). The Ag and T1 atoms are formed as a result of the action of irradiation on alkali halide crystals which contain Ag+ or Tl+ additives in amounts of about 10 3M. The electrons generated by the irradiation reduce the Ag4 or TU ions to Ag° or Tl° while the hole centres are stabilized in the form of the Cl2 ion occupying two anion positions in the lattice. The hole centres of this kind, whose structure is depicted schematically in Fig. 16, are referred to as Vk-centres. 

For the last few years numerous electron tunneling processes on the surface of highly dispersed (specific surface area of about 100-200m2g x) oxides have been discovered and investigated [67-70]. The processes studied represent reactions of recombination of radiation defects. In highly dispersed oxides a considerable portion of these defects is known to be stabilized on the surface. 

Similar results on the kinetics and temperature dependence of the recombination of electron and hole centres have been obtained [68-71] in studying highly dispersed samples of magnesium oxide MgO. As distinct from CaO, however, in MgO the hole centres are mainly stabilized on the surface (so-called Vs -centres) while the electron centres are stabilized both on the surface (Fs+ -centres) and in the volume (F1 -centres). After irradiation is over a slow recombination of radiation defects is observed... 

Yu. V. Trushin, Computer-Aided Modeling of Radiation Defects in Crystals, Akad. Nauk SSSR, Fiz.-Tekh. Inst., Leningrad, 1983. 

Abstract. The spectral dependence of photoluminescence and optical conductivity for the solid C6o and Cd-Q)0 films (with the admixture of C70 fullerenes) are studied under irradiation by argon ions with different doses. The fragmentation of the C6o molecules and the formation of the radiation defects, which are accompanied by appearance and increase in the intensity of the new component of the excitons emission, by decrease in the high-frequency optical conductivity spectral dependence to an analogous characteristic for the amorphous carbon films are observed with an increase in the radiation dose. This testifies that with the destruction of the molecules structure by ions the growth of the number of electrons, which are in the sp2 -hybridized state takes place. Furthermore, with the appearance of radiation defects the formation of the traps of the free charge carriers, which lead to a total decrease in the optical conductivity occurs. 

Keywords radiation defects optical transitions electron structure fragmentation of... 

The formation of radiation defects under irradiation of the fullerene films by the bombarding particles leads to the essential modification of electronic subsystem, which determines their optical and electrophysical properties. However, the mechanisms of radiation defect formation with the use of different types of irradiation and dose load, and also the nature of a change in the electronic properties in this case are studied insufficiently. It is necessary to note that in the case of the condensed state of fullerenes not only the radiation damages of the molecular polyhedrons, which by themselves influence the redistribution of... 

Thus, the ion irradiation leads to the modification of the structure of electronic states in the energy gap of HOMO-LUMO. In this case the electron spectra of all traps both dimeric, and impurity, are affected, that indicates about the changes in the atomic structure of fullerenes as a result of their radiation destruction and about the accumulation of radiation defects in a crystal lattice. The effects of radiation exposure indicated affect also distribution of electron energy in the valence and vacant bands, that specifies the transformation of optical interband transitions. 

It is possible to assume that in the case of the C60 films irradiated by Ar+ ions the fragmentation of molecules, which leads to the growth of the concentration of carbon atoms, whose electronic state strives to the sp2 -hybridization, i.e., to the more graphite-like phase occurs. With the formation of radiation defects, in... 

Baur et al [3] first observed a defect-related PL exhibiting an NP line at 0.931 eV in their unintentionally doped samples. They attributed this PL to the 3T2 — 3A2 transition of V3+. Kaufmann et al [15] performed intentional doping of GaN with V by ion implantation. They found a PL at 0.82 eV after post-growth annealing, which they attribute to a V-related radiation defect. 

The aim of the present review is (1) to outline the experimental techniques used to explore the primary radiation induced defects in nucleic acid constituents in the solid-state, (2) to provided an updated review of what is currently known about these primary induced radiation defects in DNA, (3) to consider the transformations the primary radicals undergo in order to look at biologically relevant lesion such as strand breaks, (4) to see how theoretical calculations are currently being used to assist in making free radical assignments, and finally, (5) to look at unsolved problems and make suggestions for future work. 

Abstract. The spectral dependence of photoluminescence and optical conductivity for the solid C60 and Cd-C6o films (with the admixture of C70 fullerenes) are studied under irradiation by argon ions with different doses. The fragmentation of the C60 molecules and the formation of the radiation defects, which are accompanied by appearance and increase in the intensity of the new component of the excitons emission, by decrease in the high-frequency optical conductivity o e) and... 

Furthermore, with the appearance of radiation defects the formation of the traps of the free charge carriers, which lead to a total decrease in the optical conductivity... 

---