H.W. den Hartog

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.

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.

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!).

Kinetics of nucleation and coarsening of colloids and voids in crystals under irradiation

The kinetics of nucleation and coarsening of vacancy clusters in irradiated crystals are considered with account of their elastic interaction with point defects resulting in the biased absorption of vacancies and interstitial atoms. It is shown that in the technologically important range of high dose rate (or low temperature) irradiation, the nucleation rate and the final number density of clusters are determined by the bias parameters rather than by irradiation conditions. The model is applied to the evolution of sodium colloids and chlorine bubbles in NaCl resulting in the formation of voids followed by a sudden fracture of the material, which presents a potential problem in rock salt nuclear waste repositories. The number densities and mean sizes of colloids, bubbles and voids are evaluated and compared with experimental data.

Protocol for TL dating with zircon: Computer simulation of temperature and dose rate effects

Natural zircon is irradiated internally by U and Th impurities. After exposure to ionizing irradiation zircon exhibits thermoluminescence (TL), which can be used to calculate the irradiation dose and the sample age. A kinetic model for TL of zircon developed earlier is used to model the processes relevant for dating. The response of zircon to irradiation at different dose rates is simulated for different temperatures. Several scenarios for the dating procedure are considered, including laboratory added irradiation, fading and preheat. It is shown that by irradiating the sample at elevated temperatures one can imitate natural irradiation, i.e. it is possible to reproduce the structural state of the trap system (distribution functions of filled electron and hole traps), which is responsible for the TL behavior. This implies that the dose dependence of the TL signal from samples, which had been irradiated under natural conditions, can be produced by irradiation at an elevated temperature.

New scenario for the accumulation and release of radiation damage in rock salt and related materials

Rock salt might be a promising geological medium for a radioactive waste repository. However, we have observed that even a basically stable compound such as NaCl may become unstable after heavy irradiation. As a result of the irradiation, dislocations, Na and Cl2 precipitates and large voids are produced followed ultimately by sudden explosion-driven fracture of the material. We present a new concept of the radiation-induced micro-structural evolution, which explains the phenomena observed in heavily irradiated NaCl samples. This concept can be a prototype of a more general assessment of radiation effects in crystalline radwaste and repository materials, which is necessary for the evaluation of critical effects and maximization of the safety of repositories.

Metal–insulator and magnetic transitions in heavily irradiated NaCl–KBF4

We have performed electron spin resonance (ESR), nuclear magnetic resonance (NMR) and static magnetic susceptibility measurements on heavily irradiated NaCl–KBF4 single crystals in the temperature range 4.2≤T ≤350 K. In these samples, up to about 10% of the NaCl molecules are transformed into extremely small metallic Na particles and Cl2 precipitates. At high temperatures a one-line ESR signal, i.e. common mode due to strong exchange interaction between conduction electrons and F-aggregate centres, is observed. We propose that the smooth decrease of the ESR spin susceptibility with decreasing temperature, which can be as large as 50%, is due to a metal–insulator transition, taking place at about 40 K. In the same temperature range, the linewidth increases by 18 ± 2 G with decreasing temperature. This anomalous broadening is explained by a reduction of the exchange narrowing at low temperatures. NMR spin–lattice relaxation on 23Na shows a Korringa-type behaviour down to 10 K, which suggests that the conducting phase in heavily irradiated NaCl–KBF4 behaves as a three-dimensional metal. SQUID experiments have revealed anti-ferromagnetic ordering at 40 K and a ferromagnetic phase below 20 K. The nature of the observed effects is discussed.

Raman scattering and quantum confinement in heavily electron-irradiated alkali halides

In this paper we will study the properties of several unusual Raman scattering peaks in heavily irradiated NaCl with vast amounts of colloidal sodium and chlorine precipitates. It appears that the laser excitation light interacts with both the electronic and vibration systems of the Na colloids, which gives rise to new Raman scattering peaks, which can be associated with electronic and vibrational excitations confined in extremely thin (about 6 nm) quantum wires.

A model for void-induced back reaction between radiolytic products in NaCl

Abstract A kinetic model is formulated for the chemical reaction between radiolytic sodium colloids and gas bubbles, which are brought into contact with each other during the exposure to ionising radiation by the growing voids. The reaction starts with the evaporation of Na atoms into the void due to the localized heat release caused by reactions between chlorine molecules colliding with the colloid surface. It is shown that this exothermic and autocatalytic reaction leads to a sudden temperature increase inside the void, which gives rise to thermoelastic stresses in the surrounding matrix. Tangential stresses might exceed the threshold stress required for localized cleavage of the matrix resulting in crack formation and mechanical instability of NaCl under high dose irradiation.