Unified model for all stages of the F-coloring of NaCl

A unified model for the F-coloring of NaCl is here proposed which accounts for some of the important features of the coloring curve as well as its dependence on a number of factors. The model assumes the creation of anionic Frenkel pairs as the primary mechanism of damage. Then, the rate of F-coloring is determined by the competition of secondary thermally-activated processes which include trapping of the free interstitial by impurities (or lattice defects) and aggregation to other free interstitials or already formed interstitial clusters. This model is, obviously, as oversimplified one and cannot account for all features found in the irradiation coloring of alkali halides. However, it provides a simple but useful scheme which satisfactorily explains the behavior of NaCl, and probably some other alkali halides (particularly KCl) in a rather wide range of experimental conditions.Special attention has been paid to the shape of the curve and its modification by impurities, radiation intensity and temperature. Some of the important characteristics of these effects have now been qualitatively explained.The use of a computer to solve the kinetic equations of the model yields curves which are quite similar to the experimental ones and permits the simulation of a number of experiments. In some cases semiquantitative agreement with experiment is obtained.     

Thermoluminescence of NaCl:Tl at 340°K

Thermoluminescence and optical absorption have been studied in X-irradiated NaCl:Tl crystals. It is suggested that the filled electron traps and the filled hole traps responsible for 340°K glow peak are the F-centres and the TI⁺⁺ ions respectively. The emission at 340°K results from the thermal release of F-electrons and their recombination with the holes at TI⁺⁺ centres.     

The magnetic properties of Eu nanoclusters formed in NaCl crystals during plastic deformation and aggregation

Plastic deformation of aged crystals and diffusion-controlled aggregation of Eu²⁺ ions in freshly quenched NaCl: Eu crystals causes the formation of complexes bound by exchange interaction. The solution of large-sized precipitates by quenching followed by impurity aggregation results in the nucleation of magnetically sensitive clusters comprising few atoms. A constant magnetic field with induction 5 T increases the probability of the transition of magnetically sensitive clusters from the high-to low-spin metastable state and induces changes in their atomic structure.     

Effect of X-irradiation on impurity-vacancy dipoles in lead and calcium-doped NaCl

In the present paper the effects of the room temperature X-irradiation in calcium and lead-doped NaCl have been reinvestigated in order to get a deeper insight into the nature of the radiation induced halogen interstitial traps. Special attention has been paid to determine whether a correlation exists between the change in the concentration of Ca²⁺ (Pb²⁺-cation vacancy dipoles and the color center formation. Our results indicate that for both systems, the dipoles are the fundamental traps for the interstitial defects in agreement with the theoretical models recently developed for F-center formation in alkali halides doped with divalent impurities. This conclusion is, however, in disagreement with that reported by Ikeya et al in a previous work on calcium-doped NaCl in which it was concluded that the isolated positive ion vacancies were the dominant traps for the halogen interstitial defects. Some comments about this discrepancy are given.     

Colloid production and precursor damage in heavily irradiated NaCl

Results for electron irradiated NaCl at 150°C indicate the existence of a highly stable defect structure which is a precursor for colloid production and appears to act as a centre for colloid nucleation. Evidence from ionic conductivity, stored energy and positron capture suggests that divacancies rather than dislocations are involved.     

Lyoluminescence mechanism of gamma and additively coloured alkali halides in pure water

The light emitted on dissolution of gamma and additively coloured crystals of NaCl, NaBr, KCl, KBr, CsCl and CsBr in pure water is studied. Experiments on gamma irradiated crystals have proved that the light emission originates from the recombination of released F-centres with trapped holes (V₂-centres) at the water-solid interface. Exceptionally pure additively coloured crystals having only F-centres produced no light when dissolved in pure water. However, relatively impure crystals emit light on dissolution due to the recombination of F-centres with traces of impurity ion centres in these crystals.     

Effect of radiation-induced emission of schottky defects on the formation of colloids in alkali halides

Formation of vacancy clusters in irradiated crystals is considered taking into account radiation-induced Schottky defect emission (RSDE) from extended defects. RSDE acts in the opposite direction compared with Frenkel pair production, and it results in the radiation-induced recovery processes. In the case of alkali halides, Schottky defects can be produced as a result of the interaction of extended defects with excitons, as has been suggested by Seitz in 1954. We consider a model that takes into account excitonic mechanisms for the creation of both Frenkel and Schottky defects, and which shows that although the contribution of the latter mechanism to the production of primary defects may be small, its role in the radiation-induced evolution of microstructure can be very significant. The model is applied to describe the evolution of sodium colloids and the formation of voids in NaCl, which is followed by a sudden fracture of the material, presenting a potential problem in rock salt-based nuclear waste repositories. The temperature, dose rate and dose dependence of colloid growth in NaCl doped with different types of impurities is analyzed. We have found that colloid growth may become negative below a threshold temperature (or above a threshold dose rate), or below a certain impurity concentration, which is determined by the RSDE, that depends strongly on the type and concentration of the impurities. The results obtained with the model are compared with experimental observations.     

The kinetics of barium precipitation at dislocations in NaCl monocrystals

The kinetics of barium precipitation at dislocations in NaCl monocrystals has been studied in thermally and mechanically treated NaCl + 4 ppm BaCl₂ samples by investigating the isothermic variation of ionic conductivity as a function of time. The course of precipitation which takes place at dislocations located at grain boundaries can be divided into three time regions characterized by diffusion of impurities to dislocation cores at grain boundaries, nucleation, formation of new grain boundaries, etc. At higher number of dislocations an interruption of the precipitation appears due to a local free energy of nucleation minimum at radiusr ₀=6·92×10^–8 cm.     

Ranges and range theories

Irradiation of ionic crystals causes the displacement of lattice ions and the formation of primary defects in the form of vacancies and interstitials. At high temperatures when these defects are mobile secondary defect reactions will produce various types of defect clusters. In some compounds clustering can lead to the formation of small particles of the metal constituent, referred to as colloids. A well-known example of this effect occurs in the alkali halides, where the colloids form as the result of large-scale aggregation of the primary F centres, so that the metallic region in this case derives from primary defects on the anion sublattice. The latent image of the photographic process in silver halides is also an example of the formation of a small metal colloid, and other crystals such as hydrides and azides can also be partially decomposed into metallic particles by irradiation with ionizing radiation. Recently metal colloids have been found as a result of displacement damage in the oxides Li₂ and Al₂O₃. This article reviews some of the background properties of colloids in ionic crystals and describes some examples of colloid formation by irradiation. Colloid growth in NaCl is described in more detail, since recent experimental and theoretical work provides a more complete picture than in other compounds. The Jain-Lidiard theory explains many features of the behaviour observed during high dose irradiation at high temperatures, and some comments are made about ways in which the theory could be developed further.     

The influence of thermal processing on the relative mobility of edge and screw dislocations in NaCl crystals

The effect of thermal processing on the mobility of dislocations is investigated in NaCl crystals doped with impurities of various types—high-solubility impurities (Ca²⁺) and low-solubility impurities (Pb²⁺). The results obtained after aging and thermal processing indicate that the type of impurity and its state (e.g., level of aggregation) in the crystal have a strong effect on the relative mobility of edge and screw dislocations, and also on the parameters of double transverse slip. Fiz. Tverd. Tela (St. Petersburg) 41, 1041–1043 (June 1999)     

Electronic relaxations of radiative defects of the anion sublattice in caesium bromide crystals and exoemission of electrons

The thermostimulated exoelectron emission (TSEE) is applied for investigation of the processes of radiative defects recombination in the nearsurface layer. Results of TSEE studies of radiatively excited CsBr crystals are presented. Dose dependences of the decay kinetics, TSEE spectrum structure and exosums were studied. Concentration of exoemission-active centres (EAC) and TSEE kinetics parameters have been calculated on the base of the bulk thermoactivated anion sublattice defect recombination. The attained result correlate with the electron centres concentration for irradiated crystals. In the framework of the Auger-like anion defects recombinational bulk model of exoemission from irradiated wide-band-gap crystals, the energy spectra of the exoelectrons excited on the F-centres are attained.     

Nucleation of damage centres during ion implantation of silicon

Nucleation of damage centres in single crystal silicon held at room temperature and bombarded in a “random” direction with light ions is not at all times heterogeneous, as has sometimes been assumed. Homogeneous nucleation of interstitial clusters occurs at low ion fluences and this is characterized, for an extremely short period, by a linear dependence of the state of disorder on ion fluence, followed by a longer period during which disorder a (fluence)^1/2. During these periods of ‘nucleation’ and ‘primary growth’ small interstitial clusters behave as unsaturable traps. For higher fluences there is a smooth transfer to a form of damage increase which is not of displacement cascade origin. In this period of ‘secondary growth’ the state of disorder varies linearly with ion fluence, and interstitial clusters behave as nucleation traps.

It is shown how marked effects on the state of disorder due to changes in the mass of the bombarding ion, in the flux of the impinging beam, or in the temperature and impurity content of the bombarded crystal, can be simply traced to an early assistance for or a resistance to the onset of hetero- over homogeneous nucleation.     

Crystalline properties and colour centres in alkali halide thin films

Abstract

Thin films of KCl, KBr, RbCl have been obtained by thermal evaporation on amorphous substrates with different deposition parameters. The crystalline structure and orientation have been determined, and the films resulted to be policrystals with high uniformity of orientation. Production of colour centres, achieved by irradiation with low-energy electrons, leads to F center concentrations barely observed in large crystals. The colouration kinetics is similar to that in the bulk, and shows after a maximum an exponential decay at high doses because of centre aggregation coupled to thermal effects. The films exhibit a bleaching process of the colour centres at room temperature, whose kinetics depends on the irradiation damage.     

Development of NaCl:K(6T) phosphor for thermoluminescence dosimetry

The present paper discusses the development of the NaCl:K (10 ^–1 molar fraction of potassium) phosphor as dosimeter material for beta-radiation thermoluminescent dosimetry (TLD). The TLD grade NaCl:K(6T) material was prepared by annealing the NaCl:K powder obtained from aqueous solution by the method of recrystalization and annealed at 600°C for two hours in open air. It is observed that thermal glow curves of NaCl:K(6T) specimen exhibit well defined glow peak around 170°C alongwith a weaker peak at lower temperature. The high-temperature peak is found to grow supralinearily with increase in beta dose in the range 10–2×10³ rad. Since the nature of glow curves under the influence of different doses remains more or less identical, it is believed that the phosphor does not undergo radiation damage and give high intrinsic TL around peak III. Examination of the system for fundamental dosimetry requirement shows that the NaCl:K(6T) material can be used as a suitable TLD material in beta radiation.     

Aggregation and precipitation stages in NaCl:Pb2+ studied by UV absorption

UV absorption measurements during isochronal annealing of homogenised and as-grown single crystals show two distinct stages in NaCl:Pb²⁺ where number density of free impurities reduce. With the help of our earlier light scattering results the low temperature stage has been attributed to aggregation of impurity-vacancy dipoles whereas the stage appearing at the higher temperature is assigned to the precipitation process.     

Radiation hardening in niobium-dependence of the yield stress on neutron dose

Abstract

Niobium samples were neutron-irradiated at reactor ambient temperatures (approximately 50 °C for polycrystals and 90°C for single crystals) to doses from 4 × 10¹⁵ to 8 × 10¹⁸ neutrons/cm² (E> 1 MeV). The density and size distribution of radiation-produced defect clusters, observed by transmission electron microscopy, were measured in polycrystalline niobium over a range of doses from 2 × 10¹⁷ to 4.4 × 10^l8 neutrons/cm². The TEM results were correlated with yield stress measurements as a function of dose. The radiation hardening was analyzed on the basis of a planar dispersed barrier model. It was found that the observed increases in yield stress at low doses were consistent with the measured density and size distribution of the defect clusters, with a cluster strength of (0.5–0.8) Gb² (G, shear modulus; b, Burgers vector). This corresponds to strong barrier hardening. At doses above about 10¹⁸ neutrons/cm², the hardening rate decreased sharply; this apparent saturation is discussed in terms of the coarsening of defect clusters, dislocation channeling, and the effect of interstitial impurities.     

Aggregation pathways and Suzuki phase formation in doped alkali halides

A model about Suzuki phase formation in Alkali Halides doped with divalent cationic impurities is presented. It considers rearrangement of small aggregates as dimers and trimers. Experimental data of Suzuki phase nucleation at room temperature in NaCl:Cd²⁺ corroborate this model.     

Deformation-induced excitation of the luminescence centres in coloured alkali halide crystals

The present paper reports the deformation-induced excitation of the luminescence centres in coloured alkali halide crystals. The peaks of the mechanoluminescence (ML) in γ-irradiated KCl, KBr, KI, NaCl and LiF crystals lie at 455, 463, 472, 450 and 485 nm, i.e. at 2.71, 2.67, 2.62, 2.75 and 2.56 eV, respectively. From the similarity between the ML spectra and the thermoluminescence (TL) and afterglow spectra, the ML of KCl, KBr, KI, NaCl and LiF crystals can be assigned to the deformation-induced excitation of the halide ions in V₂-centres or any other hole centres. For the deformation-induced excitation of the halide ions in V₂-centres, or in other centres, the following four models may be considered: (i) free electron generation model, (ii) electron–hole recombination model, (iii) dislocation exciton radiative decay model and (iv) dislocation exciton energy transfer model. The dislocation exciton energy transfer model is found to be suitable for the coloured alkali halide crystals. According to the dislocation exciton energy transfer model, during the deformation of solids the moving dislocations capture electrons from the F-centres and then they capture holes from the hole centres and consequently the formation of dislocation excitons takes place. Subsequently, the energy released during the decay of dislocation excitons excites the halide ions of the V₂-centres or any other hole centres and the light emission occurs during the de-excitation of the excited halide ions, which is the characteristic of halide ions. The mechanism of ML in irradiated alkali halide crystals is different from that of the TL in which the electrons released form F-centres due to the thermal vibrations of lattices reach the conduction band and the energy released during the electron–hole recombination excites the halide ions in V₂-centres or in any other hole centres. It is shown that the phenomenon of ML may give important information about the dislocation bands in coloured alkali halide crystals.     

Recent studies on radiation induced F-center and colloid particle formation in synthetic NaCI and natural rock salt

Radiation induced F-center and Na metal colloid formation have been studied, at temperatures between 100 and 350°C, in melt growth synthetic NaCl crystals and natural rock salt samples, using equipment for making optical measurements during 1–3 MeV electron irradiation. In both types of NaCl the damage formation kinetics are qualitatively similar. However, there are large quantitative differences which depend on irradiation temperature, dose rate, total dose, inherent strain, strain applied prior to irradiation and other factors. Natural rock salt samples from different localities exhibit markedly different radiation induced colloid formation rates. An extrapolation of the current data to the doses expected in radioactive waste depositories indicate that natural salt immediately adjacent to the planned waste canisters could develop between 0.1 and 50 percent colloidal sodium in 50 to 400 years.     

Generation modes of Eu<Superscript>2+</Superscript> nonequilibrium dimers and influence of a magnetic field on their reconstruction in NaCl: Eu crystals

Three generation modes have been experimentally revealed for impurity-vacancy Eu²⁺ dimers with the atomic structure determined by the spin-selective transitions to equilibrium configurations in a constant magnetic field of 5–15 T in NaCl: Eu crystals. It has been shown that, under conditions of linearly increasing mechanical stresses, the plastic strain of the NaCl: Eu crystals accelerates the aggregation of impurities and generates magnetically sensitive dimers several tens of times more rapidly than in the case of diffusion-controlled aggregation. Under conditions of linearly increasing strains, magnetically sensitive dimers arise as a result of the destruction of impurity precipitates by moving dislocations. The identification of dimers and the determination of their average spin in the process of spin-dependent reorganization of the atomic structure of the dimers have been performed using optical spectroscopy and SQUID magnetometry.