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.     

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.     

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.     

Use of the LM-OSL technique for the detection of partial bleaching in quartz

We present a study of the sensitivity to light (ease-of-bleaching) of the trapped charge in sedimentary quartz grains using an optically stimulated luminescence (OSL) technique in which the intensity of the stimulation light is linearly increased during the measurement period. The technique is known as linear modulation OSL (LM-OSL). In controlled laboratory conditions, this technique has been employed to study the ease-of-bleaching of the trapped charge in quartz by comparing the OSL curves of quartz aliquots which have been either: (1) fully bleached, followed by a laboratory dose of β-irradiation, or (2) partially bleached, followed by the laboratory β-dose. The ratio of the OSL signals due to the β-dose from the partly and fully bleached aliquots is illustrated to be a potential indicator of the degree of optical resetting of the OSL signal in dating material. The key parameter governing the ease-of-bleaching is the photoionization cross-section of the trap involved. The concept is also demonstrated in a model study from which very good agreement with the experimental observations has been found. Potential applications of the technique to dating are discussed.     

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.     

Improving the TT-OSL SAR protocol through source trap characterisation

Thermally-transferred optically stimulated luminescence (TT-OSL) extends the age range of OSL dating using quartz. A set of experiments have been undertaken to determine the kinetic parameters of the TT-OSL source traps, and this information has been used to propose an improved TT-OSL single aliquot regenerative (SAR) dose protocol. By combining together OSL and thermoluminescence (TL) measurements on fine-grained quartz, a correspondence between TL peaks and the TT-OSL signal is found. The thermal stability of the main TT-OSL trap was estimated by applying Hoogenstraaten’s method and allowing for thermal quenching; this predicts a lifetime of 4.5 Ma at 10 °C. A set of experiments were undertaken to refine the treatment needed at the end of each SAR cycle to erase the previously acquired TT-OSL signal. An improved TT-OSL SAR protocol using this treatment is proposed, and it is tested on quartz from a young Holocene sample. These tests yielded excellent recycling ratios and excellent dose recovery.     

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.     

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.     

Dating bricks of the last two millennia from Newcastle upon Tyne: a preliminary study

Bricks from a group of four independently dated late medieval buildings in Newcastle upon Tyne have been tested to establish their suitability for luminescence dating as part of a wider study of the dating of post-Roman and medieval brick buildings. The luminescence characteristics of quartz extracted from the bricks were determined using TL (210°C peak) and OSL measurement procedures. Both TL and OSL measurement procedures based on SAR and SARA protocols were applied to determine the palaeodose. The luminescence dates for three sampled locations are in good agreement with the assigned architectural dates; the fourth appears to be older raising doubts concerning the architectural assessment of the sampled phase.     

Non-monotonic dose dependence of the Ge- and Ti-centres in quartz

The dose response of the Ge- and Ti-centres in quartz is studied over a large dose range. After an initial signal increase in the low dose range, both defects show a pronounced decrease in signal intensities for high doses. The model by Euler and Kahan [1987. Radiation effects and anelastic loss in germanium-doped quartz. Phys. Rev. B 35 (9), 4351–4359], in which the signal drop is explained by an enhanced trapping of holes at the electron trapping site, is critically discussed. A generalization of the model is then developed, following similar considerations by Lawless et al. [2005. A model for non-monotonic dose dependence of thermoluminescence (TL). J. Phys. Condens. Matter 17, 737–753], who explained a signal drop in TL by an enhanced recombination rate with electrons at the recombination centre. Finally, an alternative model for the signal decay is given, based on the competition between single and double electron capture at the electron trapping site. From the critical discussion of the different models it is concluded that the double electron capture mechanism is the most probable effect for the dose response.     

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.     

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.     

OSL and IRSL characteristics of quartz and feldspar from southern California,USA

Southern California comprises of a wide range of diverse landscapes and environments, from high mountains with glacial and periglacial sediments to deserts with large sand dunes, extensive alluvial fans and ephemeral playas. Highly active tectonic processes has exposed ancient (c. 2 Ga) plutonic and metamorphic basement from deep within the crust, while similar Palaeozoic, Mesozoic and Cenozoic rocks are also common. A rich array of volcanic lithologies extending into the late Quaternary complement many thick sedimentary sequences that formed in equally diverse ancient environments typical of an accreting active continental margin. In some locations, notably in the Coachella Valley close to Palm Springs and the Salton Sea, low OSL sensitivity and poor characteristics restrict the application of the quartz SAR protocol to date late Pleistocene and Holocene fluvial sediments. In other locations such as the Malibu coastline, high sensitivity of the quartz OSL signal is observed, despite local source rocks being dominated by volcanic lithologies. Problems of poor quartz characteristics, along with uncertainty in predicting quartz OSL behavior for future dating campaigns poses a significant problem for projects, in particular for neotectonic contexts. While K-feldspar has been used extensively to date eolian and fluvial sediments in southern California, little information regarding signal stability is available. We explore the characteristics of both quartz and feldspar sub-samples from eolian, fluvial, lacustrine environments, in order to help develop mineral selection criteria for optical dating applications and clarify these issues. The importance of radiation quenching in quartz grains recently eroded from bedrock and the role of fires in enhancing OSL sensitivity are considered. The relative bleachability of quartz and feldspar fractions, along with thermal stability considerations is discussed. A simple test for quartz OSL signal contamination based on thermal quenching and assistance, and the susceptibility of the OSL signal to IR bleaching is introduced.     

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.     

The interpretation of quartz optically stimulated luminescence equivalent dose versus time plots

Numerical modelling has shown that the form of the quartz OSL shine plateau (hereafter ‘D_e(t)-plot’) is influenced by the effects of phototransferred TL in the ∼110°C region. It is suggested also that the presence of multiple OSL components (as described by Bailey, Smith and Rhodes, 1997. Partial bleaching and the decay form characteristics of quartz OSL. Radiat. Meas., 27, 123–136; Bailey, 1998. The form of the optically stimulated luminescence signal of quartz: implications of dating. Unpublished PhD thesis, University of London) affects the form of the D_e(t)-plot. Laboratory measurements of a fully reset and artificially dosed sample yielded non-flat D_e(t)-plots, the deviation being greater for the larger of the two simulated palaeodoses, in accordance with theoretical predictions. It is suggested that the so-called ‘shine plateau’ test is of limited use in assessing the bleaching history of quartz sediments.     

Quartz OSL: Effects of thermal treatment and their relevance to laboratory dating procedures

In dating procedures, whether single or multiple aliquot, OSL signals are obtained after samples have been heated in the laboratory with preheat procedures as short as a few seconds or as long as several hours. The OSL signals may be obtained whilst stimulating at a variety of temperatures. These thermal treatments will affect both the trapped electron population(s) in the material and the recombination centres (or trapped hole populations). Several effects may occur simultaneously. Experimental evidence is presented which is used to select the most appropriate thermal treatments in laboratory dating procedures.     

OSL signal of lithium fluoride and its relationship with TL glow-curves

The widely known LiF TL detectors: LiF:Mg,Ti (MTS-N) and LiF:Mg,Cu,P (MCP-N), were investigated with respect to their OSL properties. It was found that both materials exhibit quite substantial OSL sensitivity. In particular, in the case of LiF:Mg,Cu,P this sensitivity was very high, significantly exceeding that of BeO, the standard OSL dosimetric material. LiF:Mg,Cu,P could be a very promising candidate for application in dosimetry, if not for the fading, which was found to be quite high, reaching nearly 80% loss of the signal within 60 h. The OSL signal intensity shows a correlation with the peak 2 of the TL glow curves indicating that the same trapping sites are responsible for both processes. Peak 2 of LiF:Mg,Ti shows a peculiar property, that blue light stimulation removes only about half of its initial intensity, disregarding the duration of stimulation. This suggests, that this peak may have a composite structure and originates from both light-sensitive and light-insensitive trapping centres.     

Intrinsic superlinear dose dependence of thermoluminescence and optically stimulated luminescence at high excitation dose rates

Superlinear dose dependence of thermoluminescence (TL) and optically stimulated luminescence (OSL) has been reported for many materials. The theoretical explanation has been ascribed to competition of either traps or recombination centers, during the excitation stage or during the read-out phase. There has been an account in the literature on superlinearity of OSL associated with merely one trapping state and one kind of recombination center. This had to do with the process taking place during the read-out stage, namely the optical stimulation. In the present work, we report on a model of one trapping state and one kind of recombination center which results in a superlinear filling of the center. Thus, one can expect a superlinear dose dependence of the area under the resulting TL glow peak as well as the OSL signal. We follow this situation by writing the simultaneous nonlinear rate equations for the one-trap-one-recombination-center (OTOR) model and study the expected results by numerical simulation consisting of solving the equations with sets of the trapping parameters. We also present analytical results based on simplifying assumptions, and compare the analytical and numerical results. The effect is significant at relatively high dose rates. The main implication is that when one tries to evaluate by TL dosimetry a dose applied at a high rate, calibration of the TL dosimeter using much smaller dose rates may result in inaccurate results.     

Observations of thermal transfer OSL signals in glacigenic quartz

The conclusions of Rhodes and Pownall (Rhodes, E.J., Pownall, L., 1994. Zeroing of the OSL signal in quartz from young glaciofluvial sediments. Radiation Measurements 23, 329–333) were somewhat discouraging for the prospects of using quartz OSL for dating glacigenic sediments, while the more detailed study of Rhodes and Bailey (Rhodes, E.J., Bailey, R.M., 1997. The effect of thermal transfer on the zeroing of the luminescence of quartz from recent glaciofluvial sediments. Quaternary Science Reviews (Quaternary Geochronology) 16, 291–298) provided more encouragement. Specifically, the latter authors were able to account for the relatively high D_e values observed for recent glacigenic (dominantly glaciofluvial) sediments in terms of an anomalously high thermal transfer effect, rather than simply insufficient bleaching prior to deposition. In other locations, the OSL of quartz from glacigenic material appears to provide reliable age estimates, and does not suffer from these effects (Owen, L.A., Richards, B., Rhodes, E.J., Cunningham, W.D., Windley, B.F., Badamgarav, J., Dorjnamjaa, D., 1998. Relic permofrost structures in the Gobi of Mongolia: age and significance. Journal of Quaternary Science 13 (16), 539–548; Richards, B.W., Owen, L.A., Rhodes, E.J., 2000. Timing of Late Quaternary glaciations in the Himalayas of northern Pakistan. Journal of Quaternary Science 15, 283–297). In this paper, laboratory bleached samples from both the above studies are remeasured, using an experimental design to correct for OSL sensitivity changes, which has some similarities to the single aliquot regenerative dose (SAR) protocol of Murray and Wintle (Murray A.S., Wintle A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements 32, 57–73). Clear evidence of thermal transfer OSL signals, generated during the preheating procedure, is presented. Further measurements, using the full SAR protocol, demonstrate that the source of this charge is related to natural dosing. For one sample, the apparent effect is subtracted effectively by the SAR protocol. Another sample studied in detail shows a striking relationship between the thermally transferred OSL signal and the total TL observed during the ramping of the preceding preheat treatment.     

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.