M.R. Krbetschek

Spectral information from minerals relevant for luminescence dating

The paper reviews basic spectral features of luminescence from minerals used in dating and allied research. Luminescence production is a result of multiple interactions within the imperfect crystal lattice and spectral information is not limited to the emission of light. Results of spectral investigations of luminescence emission during thermal stimulation (TL) or optical stimulation (OSL) form the main part of the paper. However, information on luminescence excitation and light absorption spectroscopy is also presented and possible links between luminescence production in minerals and particular lattice defects are considered. Quartz and feldspars, the most commonly used minerals, receive special attention, but the review includes other materials such as polymineral fine-grained fractions from sediments, zircon, calcite and other salts (halite, sulfate), meteorites, flint, volcanic materials (obsidian, tephra), ceramics and metallurgical slags. Although a wide range of different luminescence emission wavebands occur, it can be shown that certain emissions dominate in particular materials. Basic dosimetric properties are often known just for single emission wavebands of a particular mineral, and are listed in this case. The paper also aims to provide a starting point and inspiration for the study of other TL and OSL emissions, with particular regard to their potential and suitability for dating and related dosimetry tasks. These investigations, involving palaeodose determination based on an emission waveband with known characteristics, need careful separation of the particular emission peak, which may be influenced by its behaviour during the dating procedure (sample preparation, irradiation, preheat treatments, luminescence measurements, etc.). Spectral information available in this context and some technical remarks on the experimental conditions will be given to pave the way for conventional TL or OSL measurements in luminescence dating and dosimetry using natural or semi-natural materials.

Radioluminescence dating of sediments: methodological aspects

Abstract Radioluminescence (RL) at 865 nm (1.42 eV) — a characteristic of potassium feldspar — has been applied successfully in the dating of Quaternary sediments. The signal can be interpreted as the infrared (IR)-emitting electron transition into a light sensitive trap. Direct measurement of the electron density is thus possible, and this is the basic advantage of this new method. Further investigations have been carried out to develop a precise and reliable IR-RL dating procedure. Besides the already applied additive dose total bleach technique, that of total bleach regeneration and a combination of both (slide technique) were tested in the dating of different aeolian and waterlaid sediments. New bleaching experiments show that about 10 min of light exposure (about 300–700 nm) is sufficient to reach the saturation level of the IR-RL signal. Palaeodose determinations of recently deposited sediments result, in most cases, in an age equivalent to

The basic principle of radioluminescence dating and a localized transition model

The IR signal of the radioluminescence of potassium feldspars is caused by the luminescent transition of electrons into optically active traps. This allows the direct determination of the density of trapped electrons and therefore a method of sediment dating with higher precision and accuracy than conventional luminescence dating. The principle behind it and its advantages are presented, in particular the fact that it is a real single aliquot dating technique. The explanation of both radioluminescence and IR-optically stimulated luminescence (IR-OSL) in terms of a band model is possible after the introduction of a localized transition. In contrast to previous models, the process of dose accumulation in the sediment was simulated using a dose rate as low as in real sediments. Preheat experiments indicate that the recombination centres are the unstable part of the luminescence process. The parameters of these centres are equal to those previously assigned to thermally unstable electron traps. Furthermore, the sources of systematic errors in conventional IR-OSL dating are discussed.

Investigations of feldspar radioluminescence: potential for a new dating technique

The radioluminescence (RL) properties of different types of feldspar were investigated by means of spectral measurements. An increase of the radioluminescence signal can be related to the accumulated dose and seems to be proportional to the number of filled traps. Therefore the increase of the RL signal can be used for dosimetry purposes. Radioluminescence growth curves were measured for two samples to calculate the dose rate of the excitation source. Measurements, computer simulations and the principle of a new dating technique are presented.

CCD-camera based high sensitivity TL/OSL-spectrometer

Abstract A high sensitivity TL/OSL-spectrometer has recently been built at the Freiberg Luminescence Dating Laboratory. It detects luminescence over a range of 200–800 nm simultaneously with a grating spectrograph and an attached liquid-nitrogen cooled CCD-array. The completely computer-controlled system allows free user defined measurement cycles in a temperature range of 20–700°C and optical stimulation from UV to IR with monochromatic light from a 200 W mercury lamp. The general construction of the apparatus and examples of spectra are presented.

Feldspar IRSL emission spectra at high and low temperatures

Abstract The physical basis of feldspar dating using the Infrared Stimulated Luminescence (IRSL) method is still unknown to a large extent. This study shows the IRSL behaviour of a sediment-extracted microline in a temperature range from −130 to +200°C. The low temperature region was used to calculate the activation energy to 0.15 eV. The behaviour above room temperature, as well as additional experiments on museum specimens, gives cause for the introduction of a long lived state (some tens of milliseconds) to the IRSL model, which is situated in the recombination path of the 560 nm emission. Some consequences on dating are discussed.

A study of thermoluminescence emission spectra and optical stimulation spectra of quartz from different provenances

The present study examines (1) variability in the thermoluminescence (TL) emission spectrum and optical stimulation (OSL) spectrum of quartz from different provenances and, (2) possible correlations between spectral features and the nature of the complex growth curves (ranging from saturating exponential to those described by a cubic polynomial function), so as to determine the validity of the currently used experimental protocols. The results suggest that commonly used UV emission for dating constitutes only a minor component of the total quartz emission and in view of this a dose-dependent contribution from blue/blue-green emission peak to the UV detection is likely. The OSL stimulation spectrum shows a definite change in stimulation response between 500-520 nm, hence implying that stimulation in this window may contribute as an additional source of scatter in multi-grain samples.

New methods for a better internal 40K dose rate determination

Abstract Feldspar samples, which were extracted by means of liquid density separation from sediments, often show a considerable amount of quartz. This quartz amount can lead to an underestimation of the internal 40 K dose rate, if not taken into account when determining the potassium concentration of the sample. This paper outlines how a more exact estimation of the 40 K dose rate is possible, by using a chemical analysis of the potassium, sodium and calcium concentration of the feldspar separate and a calculation of the potassium concentration of the pure feldspar fraction. In the second part a newly developed method for determining the internal 40 K dose rate is presented. By detailed investigations of the afterglow behaviour of numerous feldspar samples after an irradiation with a defined β- or γ-dose (in the text designated as radio-phosphorescence), we observed that the peak emission of the red (700–750 nm) radio-phosphorescence depends on the potassium concentration of the feldspars. Pure potassium feldspars show a red luminescence maximum at about 700 nm, however it shifts with decreasing K-concentration to 750 nm for albites and plagioclases. This effect makes it possible to determine the potassium concentration of the pure feldspar fraction by means of the radio-phosphorescence signal.

A systematic study of the radioluminescence properties of single feldspar grains

Spectral radioluminescence properties of a large number of single grains were measured. The main emission in K-feldspar grains occurs in the infrared range with the same dose characteristic as shown by multi-grain samples. For plagioclases, large differences in spectral radioluminescence between single-grains and multi-grain aliquots were detected. Differences in the saturation of the dose characteristics of the individual emissions have been observed. Consequences for dating application are discussed.