D.I. Vainshtein

EPR study of radiation-induced defects in the thermoluminescence dating medium zircon (ZrSiO4)

Mineral zircon has been considered as a possible medium for luminescence dating. The development of a suitable material for luminescence dating requires detailed knowledge of the processes taking place during, for example, exposure to ionizing radiation, long-term storage, annealing at moderate temperatures, excitation with (visible-UV) light. In this paper we have described our efforts to obtain relevant dating information by investigating the electron paramagnetic resonance (EPR) spectra of a variety of paramagnetic defects in mineral zircon (ZrSiO4) crystals as a function of the irradiation dose, annealing time and the temperature. The rare-earth ions Dy3+ and Tb3+, which play a crucial role as hole traps and recombination centres, have been investigated in detail and the behaviour of the intensity of the EPR spectra associated with these impurities can be understood in terms of a theoretical model describing the luminescence related processes in zircon. In addition, a number of defects, which can be characterized by SiOmn- have been identified and investigated. Also the behaviour of some of these centres has been analysed in the framework of the theoretical model.

Thermoluminescence of ZrSiO4 (zircon): A new dating method?

Zircon appears to be a suitable medium for thermoluminescence (TL) dating of sediments from the Quaternary. TL of zircon results predominantly from internal irradiation, due to the relatively high internal concentrations of α-emitting U and Th. The internal dose predominates over the external one that is caused by the surrounding geological layers and cosmic rays. Measurement of the TL buildup forms the basis for the development of a geochronometer, to measure the time elapsed since burial of the sediment by more recent layers. The separation and selection procedures, which are used to concentrate the high quality, transparent and colorless part of the zircon fraction of the sediments are an important part of the zircon TL measurements methodology. By improving the procedures, the colored (i.e. light absorbing) grains are excluded from the measurements. For all sand samples, the 3D TL spectra show Dy3+ peaks at low temperatures and Tb3+ bands at high temperatures. The Dy3+ peaks fade rapidly but we have found that after storage for 16 weeks in the dark, the peaks associated with Tb3+ are stable at room temperature for at least two years. Zircons were formed many millions to several billions years ago and therefore we suspected that the problems with zircons are related with ‘‘old’’ radiation damage. In this paper we will focus on two major problems of zircon dating: fading and zoning. We will show that if suitable procedures are used during the preparation stage and the dating experiments, these problems can be solved to a large extent.

Theory of the late stage of radiolysis of alkali halides

Abstract Recent results on heavily irradiated natural and synthetic NaCl crystals give evidence for the formation of large vacancy voids, which were not addressed by the conventional Jain–Lidiard model of radiation damage in alkali halides. This model was constructed to describe metal colloids and dislocation loops formed in alkali halides during earlier stages of irradiation. We present a theory based on a new mechanism of dislocation climb, which involves the production of VF centers (self-trapped hole neighboring a cation vacancy) as a result of the absorption of excess H centers. Voids are shown to arise due to the reaction between F and VF centers at the surface of halogen bubbles. Critical parameters associated with the bubble-to-void transition are evaluated. Voids can grow to sizes exceeding the mean distance between colloids and bubbles, eventually absorbing them, and, hence, igniting a back reaction between the halogen gas and metal. The amount of radiation damage in alkali halides should be evaluated with account of void formation, which strongly affects the radiation stability of material.

A new mechanism for radiation damage processes in alkali halides

We present a theory of radiation damage formation in alkali halides based on a new mechanism of dislocation climb, which involves the production of VF centers (self-trapped hole neighboring a cation vacancy) as a result of the absorption of H centers of dislocation lines. We consider the evolution of all experimentally observed extended defects: metal colloids, gas bubbles, and vacancy voids. Voids are shown to arise and grow large due to the reaction between F and VF centers at the surface of halogen bubbles. Voids can ignite a back reaction between the radiolytic products resulting in decomposition of the irradiated material.

A kinetic model of zircon thermoluminescence

Abstract A kinetic model of zircon thermoluminescence (TL) has been constructed to simulate the processes and stages relevant to thermoluminescent dating such as: filling of electron and hole traps during the excitation stage both for natural and laboratory irradiation; the time dependence of fading after laboratory irradiation; TL experiments both after laboratory and natural irradiation. The goal is to inspect qualitative behavior of the system and to unravel the processes and determine the parameters controlling TL phenomena of zircon. The input parameters of the model, such as types and concentrations of the TL centers and energy distributions of the hole and electron traps, were obtained by analyzing the experimental data on fading of the TL-emission spectra of samples from different locations. EPR data were used to establish the nature of the TL centers. Glow curves and 3D TL emission spectra are simulated and compared with the experimental data on time-dependent TL fading. Theoretical dating curves for combined natural plus laboratory irradiation have been calculated for as-irradiated, faded and preheated samples.

Thermoluminescence of zircon: a kinetic model

The mineral zircon, ZrSiO4, belongs to a class of promising materials for geochronometry by means of thermoluminescence (TL) dating. The development of a reliable and reproducible method for TL dating with zircon requires detailed knowledge of the processes taking place during exposure to ionizing radiation, long-term storage, annealing at moderate temperatures and heating at a constant rate (TL measurements). To understand these processes one needs a kinetic model of TL. This paper is devoted to the construction of such a model. The goal is to study the qualitative behaviour of the system and to determine the parameters and processes controlling TL phenomena of zircon. The model considers the following processes: (i) Filling of electron and hole traps at the excitation stage as a function of the dose rate and the dose for both (low dose rate) natural and (high dose rate) laboratory irradiation. (ii) Time dependence of TL fading in samples irradiated under laboratory conditions. (iii) Short time annealing at a given temperature. (iv) Heating of the irradiated sample to simulate TL experiments both after laboratory and natural irradiation. The input parameters of the model, such as the types and concentrations of the TL centres and the energy distributions of the hole and electron traps, were obtained by analysing the experimental data on fading of the TL-emission spectra of samples from different geological locations. Electron paramagnetic resonance (EPR) data were used to establish the nature of the TL centres. Glow curves and 3D TL emission spectra are simulated and compared with the experimental data on time-dependent TL fading. The saturation and annealing behaviour of filled trap concentrations has been considered in the framework of the proposed kinetic model and compared with the EPR data associated with the rare-earth ions Tb3+ and Dy3+, which play a crucial role as hole traps and recombination centres. In addition, the behaviour of some of the SiOmn- centres M has been compared with simulation results.

New mechanism for radiation defect production and aggregation in crystalline ceramics

In many ceramic solids, the number of primary displaced ions is different for different sublattice components, either because the ion masses and displacement energies differ in simple binary collisions (like in alumina) or because radiolytic displacements occur on a single sublattice (like in halides). However, irradiation produces not only metal colloids or gas precipitates, but stoichiometric dislocation loops, and voids as well. We propose a secondary displacement mechanism of vacancy production at a dislocation as a result of its interaction with a primary interstitial ion in another sublattice which explains the observed phenomena.

Mineral zircon: A novel thermoluminescence geochronometer

Mineral zircon contains trace amounts (typically 10-1000 v ppm) of the f -emitters uranium and thorium, which irradiate this mineral internally. This outstanding feature of zircon turns out to be extremely useful when this mineral is applied as a thermoluminescence (TL) dating medium, because the build-up of the age-dependent luminescence is dominated by the presence of well-defined internal radioactive sources and the contributions to the dose from external radiation sources are two orders of magnitude smaller. The results presented in this paper have led us to the conclusion that for zircon dating it is necessary to carefully select the best and homogeneous zircon grains of the highest optical quality. For successful dating experiments on very young and historically well-defined coastal dune sands, selection of the most stable luminescence component by means of narrow band interference filters is needed. Our results suggest that ultimately optical zircon dating will allow us to determine the age of extremely young samples ( e.g. 12 months!).

Explosive decomposition of heavily irradiated NaCl

Abstract In heavily irradiated NaCl explosions can be initiated during irradiation or later, after the irradiation, when the samples are heated to temperatures in the range 100–250°. As a result of the irradiation Na and Cl2 precipitates, dislocations and voids are produced, along with stored energy (the maximum value observed until now ∼ 76kJ/mol, which is about 18.5% of the enthalpy of formation of NaCl, 411.2kJ/mol). This implies that heavily irradiated NaCl is a highly energetic material. We have observed that the samples, which revealed large radiation-induced voids, explode rather easily. In these samples the instability connected with large voids (hot spots) probably initiates the explosive release of stored energy, which is in many cases accompanied by characteristic (explosive) sounds. In this paper we will discuss the nature of the explosions and show that a basically stable insulating compound, such as NaCl, may become unstable after heavy irradiation.

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.