Luminescence dating: laboratory procedures and protocols

In the last 30 years, from 1967 to 1997, the use of luminescence signals from naturally occurring minerals has gone though a major metamorphosis, from thermoluminescence (TL) dating of pottery to optically stimulated luminescence (OSL) dating of sediments. Laboratory procedures for dating sediments have been adapted from those for pottery and new procedures have been developed as the need arises.

The majority of sediment dating applications are carried out on quartz and potassium-rich feldspars and the general characteristics of the TL and OSL signals from these minerals are reviewed. For sediments some new problems were encountered, with some grains perhaps not being completely bleached at deposition. For OSL signals there is no simple procedure for the selection of a thermally stable signal, as there had been in the case of pottery.

Many different laboratory protocols have been developed as our understanding of the fundamental behaviour of luminescence signals from quartz and feldspar has improved. These protocols are explained and discussed, giving the advantages and disadvantages of each procedure as applied to different types of sediment.

This review is presented as a guide to the selection of the most appropriate procedure for a particular dating application.     

Study of the 110°C TL peak sensitivity in optical dating of quartz

As the 110°C TL emission in quartz uses the same luminescence centers as the OSL emission, the 110°C TL signal from a test dose may be used to monitor the OSL sensitivity change. It is thus important to study the relationship between the 110°C TL peak and the OSL sensitivity in studies related to optical dating from quartz. We have conducted a series of experiments using sedimentary quartz, where the annealing temperatures were varied between 260 and 1000°C before the measurement of OSL and 110°C TL sensitivities. Another series of experiments on two sedimentary quartz samples investigated the 110°C TL peak and OSL dose-dependent sensitivity change after different annealing temperatures. In these experiments, the 110°C TL and OSL signals from the test dose are shown to have similar sensitization characteristics: the 110°C TL sensitivity change is proportional to the OSL sensitivity change if the annealing temperature is lower than 500°C. It is concluded that the 110°C TL signal can be used to correct the OSL sensitivity change in the single-aliquot additive-dose protocol.     

Optically stimulated luminescence (OSL) study of synthetic stishovite

Optically stimulated luminescence (OSL) of synthetic stishovite was investigated for a future dating technique of meteor impact craters. Luminescence around 330 nm was measured on the γ-ray irradiated stishovite under two stimulating light sources of infrared laser (830 nm) and blue light emitting diode set (470 nm). Thermoluminescence (TL) studies before and after the OSL measurements showed the intensities around 100–200°C and 220–350°C to increase and those around 350–450°C to decrease. This indicates that a part of deep-trapped charges excited during the OSL measurements were retrapped by shallower traps. The infrared stimulated luminescence (IRSL) after the TL measurement up to 450°C could not be detected, while the blue light stimulated luminescence (BLSL) after TL had about one-tenth of the intensity before TL. This indicates that a part of the charges in shallower traps were detrapped thermally and returned to the deeper traps which were related to BLSL. The result implies that some of the BLSL-related traps are quite stable at room temperature and could be used for geological dating. In addition, two paramagnetic centers produced by sudden release of high pressure in synthesis process were found in the unirradiated stishovite by electron spin resonance (ESR). Their g-factors are g=2.00181 and g=2.00062 for an axial signal and g=2.00305 for the other isotropic signal. These signals could be used for an evidence of impacts if those signals could be stored in geological time.     

Luminescence spectra of alkalifeldspars and plagioclases

Feldspars are widely used as dosimeters in dating archaeological and geological materials by thermally or optically stimulated luminescence (TL, OSL). The TL, (IR)OSL and phosphorescence (following gamma irradiation) spectra of 26 samples covering the orthoclase-albite-anorthite ternary system, which were characterized by microscopy and X-ray diffraction analysis, were measured using the Freiberg high sensitive luminescence spectrometer. The range of observation for TL (up to 350°C) and phosphorescence was (200–800) nm. IR (880 nm) stimulated luminescence light has been detected between 330 nm and 660 nm. Representative examples of our systematic investigations on the luminescence behaviour of alkalifeldspars and plagioclases are presented.     

First steps toward spatially resolved OSL dating with electron multiplying charge-coupled devices (EMCCDs): System design and image analysis

Spatially resolved OSL (SR-OSL) measurements are made with a scanning or imaging system that records luminescence emissions from known sources within a sample. This work is a first step toward the development of an SR-OSL methodology based on the imaging of ultraviolet OSL emissions with an EMCCD camera. We present UV OSL images of natural quartz extracts, OSL/TL/IR images of sandstone, image-based OSL measurements from aluminum oxide grains, and dose recovery experiments on aluminum oxide and Risø calibration quartz. Camera sensitivity and quantitative reproducibility indicate that EMCCD-based OSL dating is feasible. Further experimentation is necessary to adequately define the error sources and measurement limitations of EMCCDs in the OSL regime.     

Kinetics of red, blue and UV thermoluminescence and optically-stimulated luminescence from quartz

The duration over which charge is retained at trapping sites is of fundamental importance for trapped electron dating. Here, we report measurements of the kinetic parameters of the prominent thermoluminescence (TL) glow peaks of quartz, and of the optically-stimulated luminescence (OSL) signal from quartz utilised for optical dating. The similarity in trapping lifetimes of the 325°C TL peak and the dominant component of the OSL signal are taken as further support for the hypothesis of their common origin in the same trapped electron population.     

The timing of OSL sensitivity changes in a natural quartz

The timing of 110°C thermoluminescence (TL) and optically stimulated luminescence (OSL) sensitivity changes is explored using a natural (aeolian) quartz sample from Australia that was previously found to exhibit marked dose-dependent sensitivity change. The changes occur asynchronously; the 110°C TL sensitivity changes after dosing plus pre-heating, whereas the OSL sensitivity changes (proportionately with dose) after optical bleaching. Although the magnitude and saturation characteristics of the sensitivity changes are found to be similar, their non-synchronicity negates a direct link between the phenomena. Implications of these results for dating procedures are discussed. In particular, the results indicate that a single aliquot additive dose procedure similar to that developed for coarse-grained potassium feldspar should be possible for some quartz samples.     

Relative response of TL and component-resolved OSL to alpha and beta radiations in annealed sedimentary quartz

Knowledge of the relative luminescence response to alpha and beta radiation is very important in TL and OSL dating. In the present study the relative alpha to beta response is studied in a sedimentary quartz sample, previously fired at 900 °C for 1 h, in the dose region between 1 and 128 Gy, for both thermoluminescence (TL) and linearly modulated optically stimulated luminescence (LM – OSL). The LM – OSL measurements were performed at room temperature and at 125 °C. All OSL signals were deconvolved into their individual components. Comparison of OSL curves after alpha and beta irradiation strongly supports that quartz OSL components follow first order kinetics in both cases. In the case of TL, the relative alpha to beta response is found to be very different for each TL glow-peak, but it does not depend strongly on irradiation dose. In the case of LM – OSL measurements, it is found that the relative behaviour of the alpha to beta response is different for three distinct regions, namely the fast OSL component, the region of medium OSL component originating from the TL glow-peak at 110 °C when stimulation takes place at room temperature and finally the region of slow OSL component. Following stimulation at ambient temperature, the relative alpha to beta response of all components was not observed to depend significantly on dose, with the value of ratio being 0.03 and a tendency to decrease with increasing dose. However, in the case of measurements performed at 125 °C, the relative response of the fast components is much enhanced, and for the remaining components it increases with increasing dose. Special care must be taken to examine the relative alpha to beta response of the fast component at 125 °C which contrasts the relative response of the TL peak at ca. 325 °C. The implications for the dating of annealed quartz are also briefly discussed.     

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.     

Quantification of cross-irradiation and cross-illumination using a Risø TL/OSL DA-15 reader

The automated Risø TL/OSL DA-15 reader is widely used for luminescence dating applications. The nature of the device is such that the irradiation and illumination of a sample during the OSL procedures may affect discs in adjacent positions. It is critical to confirm that these factors do not introduce significant systematic errors. The effects of such cross-talk (cross-irradiation and cross-bleaching) are examined and quantified, both for specific and general cases. Depending upon the protocol used, cross-irradiation and cross-bleaching could accumulate to have an effect on the D_e estimate, and for the latter this could be significant (i.e. 10%).     

Infrared (IR) stimulated luminescence from modern bricks in retrospective dosimetry applications

It has frequently been observed that certain roof tiles and bricks, especially from relatively modern European buildings, do not contain enough quartz grains in a suitable grain size range to permit dose reconstruction using thermoluminescence (TL) or optically stimulated luminescence (OSL) methods. In this paper the feasibility of using infrared-stimulated luminescence (IRSL) on the feldspar fraction of such bricks and tiles has been investigated. Appropriate preheating treatments were employed in order to select the most stable signals, and procedures were developed to enhance the signal to noise ratio. The possible effect of anomalous fading under application of these procedures was tested. In the dose range above 100 mGy, it has been demonstrated that using IRSL on the feldspar fraction of such material provides a feasible alternative to the use of green-light-stimulated luminescence (GLSL) on the quartz fraction, for the purposes of retrospective dosimetry. Furthermore, since the use of IRSL as described in this paper involves the measurement of polymineral fine grain fractions of bricks, a technique for the calibration of the built-in β source against the γ source in Secondary Standard Dosimetry facilities for routine use of the technique is described.     

Lumineszenzdatierung quartärer Sedimente

The purpose of this paper is to discuss briefly the basis, technique and efficiency of luminescence dating. Thermoluminescence and optically-stimulated luminescence are useful for dating of quaternary sediments. Contents: Sediment Dating, Thermoluminescence Dating, Thermoluminescence (TL), Dating Method, Sources of Error, Dosimetry, Dating Techniques, Optical Dating (OSL), Sediments, References.     

Comparison of the properties of various optically stimulated luminescence signals from potassium feldspar

Various optically stimulated luminescence signals from K-feldspar have been used to determine the equivalent doses of sediment samples. Understanding the properties of these optical signals is critical to evaluate their applicability and limitations to optical dating. In this paper, some properties of IRSL, post-IR OSL and post-IR IRSL signals (detected in the UV region using U-340 filters) from a museum sample of K-feldspar were investigated by analyzing the relationships between optical and TL signals, and the effect of optical bleaching and heating on optical signals. The trap parameters of the different optical signals were calculated using the pulse annealing method. The results show that this sample exhibits two regenerated TL peaks at ～140 and ～330 °C. Corresponding to the low temperature TL peak, the OSL and post-IR OSL signals appear to be more associated with lower temperature TL than the IRSL signal measured at 50 °C. Corresponding to the high temperature TL peak, the post-IR IRSL signals mainly originate from the more thermally stable traps associated with the high temperature TL, compared with the IRSL and post-IR OSL signals. However, the post-IR IRSL_225 °C signal is shown to be hard to be bleached by blue light and simulated sunlight, compared with the IRSL_50 °C and low temperature post-IR IRSL signals. The implication for optical dating is that the elevated temperature post-IR IRSL signals can be preferentially applied over other signals from K-feldspar, but it is desirable that the effectiveness of the pre-depositional zeroing of these signals is assessed.     

TL/OSL properties of crystalline inclusions from heavy,barytes loaded,concrete

In the present work, we report the luminescence data obtained from heavy, barytes loaded, concrete containing many crystalline inclusions, extracted from a shielding block located at CERN. The use of both Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) signals, resulting from a specific trap at about 200 °C, is investigated for retrospective dosimetry purposes. By applying thermal cleaning experiments the TL signal of interest was isolated. Basic TL and OSL properties as thermal and optical stability, repeatability and mainly the linearity of the TL and OSL signals as a function of beta dose were investigated. The implications of all these luminescence properties to retrospective dosimetry are also briefly discussed.     

Luminescence sensitivity changes in quartz as a result of annealing

Retrospective dosimetry using optically stimulated luminescence (OSL) on quartz extracted from (for example) bricks needs to account for strong OSL sensitivity changes that are known to occur depending on the previous thermal treatment of the sample. Non-heated quartz exhibits OSL orders of magnitude less per unit radiation than that for heated material. The reason these temperature-induced sensitivity changes occur in quartz is presently not well understood. This phenomenon is also seen in the related area of luminescence dating in which sedimentary quartz and quartz from heated archaeological samples show very different OSL sensitivities. In this paper we report on studies of the effects of high temperature annealing on the OSL and phototransferred TL (PTTL) signals from sedimentary and synthetic quartz. A dramatic enhancement of both OSL and PTTL sensitivity was found especially in the temperature range 500–800°C. Computer simulations of the possible effects are shown to produce data that agree in all essential details with the experimental observations. It is further demonstrated that the enhanced OSL sensitivity as a function of annealing temperature is not a pre-dose effect.     

Behavioural studies of stimulated luminescence from feldspars

Since the work of Mejdahl in the mid-1980s feldspars have been used widely for palaeodosimetry. They have proved particularly popular because of the technical ease with which an optically stimulated luminescence signal can be obtained, and the potential for their use in dating over a wider time range than quartz. However, while the use of infrared stimulated lminescence has been a great success, the realisation of the potential for dating over a wide time range has been hampered by a series of behavioural problems, in particular concerned with the stability of the luminescence signal. This paper surveys the current state of knowledge of a range of aspects of the stimulated luminescence behaviour of feldspar. Particular attention is paid to the thermal and optical properties of the thermoluminescence (TL) and optically stimulated luminescence (OSL) signals from feldspars since these are of greatest relevance to their use as palaeodosimeters. Results obtained from well-characterised museum specimens demonstrate the variety of responses that are seen from different types of feldspar, particularly in their TL behaviour. Less variation is seen in their OSL response. In contrast, the response of potassium-rich feldspars separated from Quaternary sediments tends to be very consistent when many grains are analysed simultaneously, suggesting that averaging of the signal simplifies the behaviour of the system.     

Investigating the thermally transferred optically stimulated luminescence source trap in fired geological quartz

The pre-dosed thermoluminescence (TL) emission of quartz has been found to be useful in retrospective dosimetry and archaeometry. Though the pre-dosed optically stimulated luminescence (OSL) and TL emissions have been reported to be similar, the former has been found to be un-reliable for the equivalent dose estimation. As this measurement protocol involves thermal heating at around 400 °C, the work reported in this paper investigated the influence of this heating on the OSL using fired specimens from various regions. The results suggested that the discrepancy in the behaviour of two emissions is caused by the presence of the thermally transferred optically stimulated luminescence (TT-OSL) induced by thermal-activation involved in the pre-dose treatment. This transferred signal was observed to be very significant in the case of samples containing a prominent higher-temperature TL peak at ∼375 °C. The characterization of this signal based on (i) the nature of the glow curves, (ii) thermal-annealing of the OSL trap, (iii) observation of the TT-OSL, (iv) bleaching of the source trap and (v) the correlation between TL and OSL seems to suggest that the trap corresponding to this TL peak is the source trap in the TT-OSL emission mechanism.     

Crushing effects on TL and OSL on quartz: relevance to fault dating

The effect of crushing on the TL and OSL signals in quartz was investigated to examine the feasibility of using OSL and TL for dating of faulted rocks. 275°C TL signals were not reduced by crushing by hand (mortar and pestle), in a pellet die press, or using an automatic mortar and pestle, but the sensitivity of the 100°C TL signal to a test dose decreased in crushed samples. Green light-stimulated OSL signal intensities were not decreased after crushing, but showed strongly increased inter-aliquot variability after crushing. We concluded that OSL and TL in quartz might be useful to characterize the crushing history of quartz in fault rocks, but their usefulness in dating fault events was not evident because we did not find evidence of signal zeroing under the conditions used in this study.     

Absorbed dose evaluations in retrospective dosimetry: methodological developments using quartz

Dose evaluation procedures based on luminescence techniques were applied to 50 quartz samples extracted from bricks that had been obtained from populated or partly populated settlements in Russia and Ukraine downwind of the Chernobyl NPP. Determinations of accrued dose in the range 30–300 mGy were obtained using TL (210°C TL and pre-dose) and OSL (single and multiple aliquot) procedures. Overall, good inter-laboratory concordance of dose evaluations was achieved, with a variance (1σ) of ±10 mGy for the samples examined.