Investigations of the green- and infrared-stimulated luminescence using the recuperation effect

We used the recuperation effect (see also “Recuperation of infrared stimulated luminescence of feldspars” in this issue) for investigating the connection between infrared- and greenlight-stimulated luminescence (IRSL and GLSL) of feldspars by performing a “double-bleach recuperation” experiment. A diode system was used for infrared (IR) stimulation, and a filtered-light unit for greenlight (GL) stimulation. Powdered feldspar samples (2 mg each) of known chemical characterisation from a mineral collection were used. After beta-irradiation with 180 Gy and storage in the dark for several weeks feldspar aliquots were bleached down to a residual level of ≈1% of the initial level, first with IR and subsequently with GL. For both stimulations, detection of the stimulated luminescence was carried out in the near ultraviolet region (around 260–360 nm, peaked at 340 nm). Other aliquots were bleached in reverse order (1st GL, 2nd IR). These bleaching sequences were also applied to recuperation measurements after 30 days storage time under controlled room-temperature conditions. Results of these experiments will be presented and a possible interpretation of the observed recuperation signals in terms of a two-trap optically simulated luminescence model will be given. The different interaction of the traps, depending on the specific feldspar, will be shown.     

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.     

Study of sensitivity change of OSL signals from quartz and feldspars as a function of preheat temperature

Optically stimulated luminescence (OSL) signals from feldspar and quartz samples were studied using infrared (860 nm) and green light (420–575 nm) stimulation. A serious problem connected with the regeneration technique used for dating is associated with a change of OSL sensitivity to radiation in the course of the measurement process. A typical effect seen is a large increase of the apparent strength of our beta source when calibrated against a gamma source. If the regeneration procedure is used, it is shown that the sensitivity increases up to 50% during the measurement process and as a result, the equivalent dose (ED) would be underestimated. A study of sensitivity changes in feldspars and quartz was carried out with emphasis on the effect of preheat and annealing on the OSL signal. Measurement results obtained are presented, and possible elimination of errors in dating caused by sensitivity changes is discussed.     

Limits to depletion of blue-green light stimulated luminescence in feldspars: implications for quartz dating

Feldspar contaminants in quartz aliquots, either as micro-inclusions or as remnant grains (due to inadequate etching) can affect the accuracy and precision of paleodose estimates based on blue-green light stimulated luminescence (BGSL). Such contamination could also alter the shape of the BGSL stimulation curve of otherwise pure quartz. In this study, the functional relationship between the infra-red stimulated luminescence (IRSL) and BGSL of feldspars, (1) at different preheats, and (2) with IR bleaching at different stimulation temperatures and durations, is examined. The results suggest two trap populations participate in the feldspar BGSL process. These are: (1) Type (A) trap populations that can be stimulated by both the infra-red and the blue-green light at 125°C and, (2) Type (B) trap populations that respond only to blue-green-light stimulation at 125°C. However, infra-red stimulation at elevated temperature (220°C) (ETIR) permits depletions of charges in Type (A) and Type (B) to the extent that the feldspar BGSL can be reduced by up to 97% in 5 min.

These results offer prospects for (1) improved precision in paleodose estimates based on quartz; (2) BGSL dating of quartz in a polyminerallic fine grain samples; (3) age estimates based on both quartz and feldspars from the same aliquots, and (4) dating based on feldspar micro-inclusions.     

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.     

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.     

Luminescence sensitivity changes of polymineral fine grains during IRSL and [post-IR] OSL measurements

Sensitivity changes during measurement sequences of infra-red-stimulated luminescence (IRSL) and post-IR optically stimulated luminescence (OSL) are presented for a sample of loess from northwestern China. Together with a dose recovery experiment, the results are used to investigate the ratio of 2.4 found for the D_e values (for IRSL and [post-IR] OSL) obtained when using a single-aliquot regenerative-dose (SAR) protocol in which both D_e values are obtained in a single sequence of measurements. Responses to test doses for both the IRSL and [post-IR] OSL show progressive luminescence sensitivity changes with repeated measurement cycles, with a slight dose dependence for the IRSL. In addition, five modified SAR procedures were used, varying preheats, filter combinations and method of measuring the luminescence signal. The D_e values for all IRSL measurements were at least 50% greater than those for the [post-IR] OSL signal. A modified SAR sequence was also applied in which 0.1 s stimulations (using both IR and blue light sources) were made between all sample treatments. A lack of consistency in the measured luminescence sensitivity of the natural IRSL signal suggests that the [post-IR] OSL signal provides the more reliable value of D_e.     

The Dependence of IRSL Intensity on Radiation Dose for Samples of Chlorides Contained in Feldspars

The intensity of the luminescence generally increases with radiation dose and measurement of these phenomena can be used to characterise the degree of dependence on beta doses. In this study, in order to test whether this is a significant problem on the optically stimulated luminescence (OSL) studies, the radiation dose response of the OSL signal from samples of chlorides contained in feldspars have been investigated by irradiating the samples with beta doses. The infrared-emitting diodes were used with a wavelength of (880±80) nm, and an IRSL (infrared stimulated luminescence) intensity parametres, m, was described and found m = 1±0.03.     

Modelling the thermal bleaching of OSL signal in the case of a competition between recombination centres

The thermal bleaching of the optically stimulated luminescence (OSL) has been investigated by computer simulations for a model including three traps and two luminescence centres. The deepest trap is active only during the OSL process. Two other traps are active only during the thermal bleaching. The thermal bleaching effects on the OSL intensity as well as on the OSL curve shape are presented for the wide range of trap and luminescence centre parameters and for the different settings of optical detection window. The conventional OSL curve analysis consisting in decomposition of the OSL curve into first order components is applied to the simulation results and the optical cross section spectra obtained as a result of this analysis are compared with the model assumptions. The simulations show that OSL signal can decrease to undetectable level even when the traps related to this signal are not emptied during thermal bleaching. The residual level of the OSL signal after bleaching process, however, depends strongly on centre parameters and concentrations. The modifications of optical detection spectral window lead to significant changes of bleaching effects. The thermal bleaching influences also the optical cross section spectra obtained as a result of the OSL curve decomposition.     

The estimation of basic experimental parameters in the fine-grained quartz multiple-aliquot regenerative-dose OSL dating of Chinese loess

The sensitivity-corrected multiple-aliquot regenerative-dose (MAR) protocol provides a reliable approach for fine-grained quartz optically stimulated luminescence (OSL) dating. For reliable estimation of the equivalent dose (D_e), we investigated certain basic experimental parameters in the fine-grained quartz MAR OSL dating of Chinese loess. (1) For suitable bleaching of the natural OSL signal of the regenerative-dose aliquots, the effect of bleaching duration using sunlight, SOL2 and blue LEDs on D_e was studied, and it is found that the appropriate method is a short-duration SOL2 (e.g. 5 min) or blue LEDs (e.g. 60 s) bleaching. (2) To select the appropriate test dose, the relationship between the test dose and D_e was investigated based on three samples having D_e values of approximately 11, 31 and 137 Gy respectively. It is suggested that the test dose for sensitivity correction may be limited to less than approximately 10–20 Gy. (3) Three commonly used fitting modes for quartz OSL growth curve were compared at three regenerative-dose scales. The results indicate that the mode of two saturating exponential functions plus a constant is appropriate and universal. (4) The comparison of D_e values derived using OSL approach with those obtained using the recuperated OSL (ReOSL) protocol shows that the reliable D_e estimation in the fine-grained quartz MAR OSL dating of Chinese loess may be limited to less than approximately 300 Gy. (5) The comparison of growth curves for 18 samples from the Weinan, Xifeng and Jingyuan sites shows that it is feasible to construct a standardized growth curve (SGC) for fine-grained quartz OSL signal in the Chinese Loess Plateau (CLP).     

Testing Post-IR IRSL protocols for minimising fading in feldspars,using Alaskan loess with independent chronological control

Concern over anomalous fading has been the biggest single factor responsible for deterring the widespread use of the infra-red stimulated luminescence (IRSL) or thermoluminescence (TL) signal from feldspars for luminescence dating. There has therefore been great interest in the use of the recently proposed Post-IR IRSL signal, because it has been shown to significantly reduce the degree of anomalous fading observed in feldspars and therefore potentially provides a means of circumventing the issue. This study undertakes a systematic investigation into various preheat and Post-IR IRSL measurement conditions proposed in the literature, by using two samples from the Halfway House loess section in Alaska which bracket the Old Crow tephra which has been dated using fission track methods. Preheat plateau tests show a dramatic change in equivalent dose with Post-IR IRSL measurement conditions, and further tests reveal that these changes are driven by preheat temperature rather than Post-IR IR stimulation temperature. Dose recovery tests on laboratory-bleached material mimic the findings of the natural preheat plateau test data, and sensitivity change between the first and second Single Aliquot Regenerative dose (SAR) measurement cycle is found to be responsible. Comparison of the Post-IR IRSL ages with the independent age control shows that, for the samples in this study, the Post-IR IR signal stimulated at 290 °C is inappropriate for dating. However, use of lower preheat (250–300 °C) and Post-IR IR stimulation temperatures from 225 to 270 °C gave rise to ages which were in agreement with the independent age control.     

Thermal assistance in TA – OSL signals of feldspar and polymineral samples; comparison with the case of pure quartz

Thermally assisted optically stimulated luminescence (TA – OSL) is studied for the cases of polymineral, rich in K-feldspar sample as well as one pure sample of K-feldspar. For both cases, the shape of the TA – OSL signal indicates an initial, fast decaying part which is followed by a flat, very slowly decaying part with intensity much larger than the ordinary background noise signal. Thermal assistance characteristics indicate that for the case of pure K-feldspar, the signal originates from a unique very deep trap. The experimental features of the TA – OSL signal in the case of polymineral sample resemble much the corresponding TA – OSL features of pure quartz, in terms of both glow curve shape, especially at high stimulation temperatures, as well as signal intensity and its dependence on the stimulation temperature. Nevertheless, TA – OSL stimulation at low, ambient temperatures provide strong hints towards the contribution of K – feldspars to this signal. Proper selection of the stimulation temperature could possibly discriminate the TA – OSL signal originating from quartz and feldspars.     

Na-rich feldspar as a luminescence dosimeter in infrared stimulated luminescence (IRSL) dating

One of the challenges in dating rock surfaces is the choice of the luminescence mineral. Although quartz is the preferred dosimeter in sediment dating, it is often not sufficiently sensitive when extracted from solid rocks. The intensity of signals from feldspars tends to be much less dependent on geological origin and erosion history, but the dosimetry of K-rich feldspar grains extracted from rocks is complicated because the internal dose rate is very dependent on the original feldspar grain size. The in situ grain size information is lost during the crushing process used to separate the grains for measurement. This latter problem does not apply to Na-rich feldspar because of the absence of internal radioactivity.The potential application of Na-rich feldspar as a luminescence dosimeter for the IRSL dating of rock surfaces is investigated using a variety of sediment samples from different geological settings for which independent age control is available. The blue and yellow luminescence emissions are measured for IR stimulation at 50 °C (IR₅₀), and post-IR IR stimulation at 290 °C (pIRIR₂₉₀). Thermal stability experiments imply that the corresponding signals in both emissions have comparable thermal stabilities and that all signals have similar recombination kinetics and are thermally stable over geological timescales. The IR₅₀ doses measured using blue and yellow emissions are similar to or lower than quartz doses while pIRIR₂₉₀ blue doses are higher than those from yellow emission and quartz doses. The fading rates measured for the IR₅₀ signals are ∼3%/decade larger than those measured for the pIRIR₂₉₀ signals in both yellow and blue emissions. Furthermore the average fading rates of both yellow signals are ∼3%/decade higher than the corresponding fading rates of the blue signals. However, there is no detectable correlation between fading rates and the measured D_e values. The residual doses measured from the laboratory-bleached samples and a modern analogue suggest that the IR₅₀ signals in both blue and yellow emissions bleach to the same degree, as do the corresponding pIRIR₂₉₀ signals, and that there is no significant naturally-unbleachable residual dose observed using these signals. Neither anomalous fading nor incomplete bleaching explains the observed dose discrepancy between the two emissions. Eight uncorrected and fading-corrected ages are calculated for each sample based on all four signals, using the dose rate relevant to Na-rich feldspar extracts (i.e. ∼3% K). The IR₅₀ and pIRIR₂₉₀ blue ages were also calculated assuming a dose rate based on 12.5% internal K (i.e. assuming that the blue signals were mainly derived from contamination by K-rich feldspar). The latter pIRIR₂₉₀ blue ages are in agreement with the expected age control, raising the possibility that this signal originates mainly from K-rich feldspar contamination in our Na-rich fractions, and thus is not so useful in the luminescence dating of rock surfaces. On the other hand, the pIRIR₂₉₀ fading-corrected ages based on the yellow emission are consistent with the independent age controls; higher preheat and stimulation temperatures may result in more stable yellow signals from Na-rich feldspar extracts from rocks, and so reduce the size of the fading correction. We conclude that, because this signal avoids the dosimetry difficulties of K-rich feldspar extracts, it has considerable potential in the IRSL dating of rock surfaces.     

Further investigations of tunneling recombination processes in random distributions of defects

The shape of infrared stimulated luminescence signals (IRSL) from feldspars has been the subject of numerous studies in the field of luminescence dating. Specifically linearly modulated IRSL signals (LM-IRSL) are commonly assumed to consist of several first order components corresponding to distinct optical stimulation cross sections. This paper models the shape of LM-IRSL signals using a recently proposed kinetic model, which describes localized electronic recombination in donor–acceptor pairs of luminescent materials. Within this model, recombination is assumed to take place via the excited state of the donor, and nearest-neighbor recombinations take place within a random distribution of centers. The model has been used previously successfully to describe both thermally and optically stimulated luminescence (TL, OSL). This paper shows that it is possible to obtain approximate solutions for the distribution of donors in the ground state as a function of two variables, time and the distance between donors and acceptors. Approximate expressions are derived for several possible modes of optical and thermal stimulation, namely TL, OSL, linearly modulated OSL (LM-OSL), LM-IRSL and isothermal TL (ITL). Numerical integration of these expressions over the distance variable yields the distribution of remaining donors at any time t during these experimental situations. Examples are given for the derived distributions of donors in each experimental case, and similarities and differences are pointed out. The paper also demonstrates how LM-IRSL signals in feldspars can be analyzed using the model, and what physical information can be extracted from such experimental data. The equations developed in this paper are tested by fitting successfully a series of experimental LM-IRSL data for Na- and K-feldspar samples available in the literature. Finally, it is shown that the equations derived in this paper are a direct generalization of an equation previously derived for the case of ground state tunneling.     

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.     

Tunnel afterglow, fading and infrared emission in thermoluminescence of feldspars

Anomalous fading of TL, OSL and IRSL has been observed in many samples of feldspars and attributed to the tunnel effect. Investigations do show expected tunnel afterglow except for samples with no fading. Its intensitym quite noticeable at LNT, is in proportion with reported rate of fading. The emission is entirely in the red and infrared part of the spectrum. An important thermal quenching is observed. Low temperature storage results in fading of TL. Cathodoluminescence emission spectra, monitored from 200 to 900 nm, do confirm TL observations. They show two well-separated ranges: one, “blue”, from UV to yellow (the only one observed in usual TL dating), displays various emission bands; the other, “IR”, red and infrared, shows a well-characterized narrow Gaussian emission band, with a maximum around 720 nm. Models are proposed, relating fading with disorder in crystals. Tunnel afterglow appears as a good criterion of fading in feldspars.     

Natural variations in the properties of TL and IRSL emissions from metamorphic and volcanic K-feldspars from East Africa: Assessing their reliability for dating

The elevated temperature infrared stimulated luminescence (IRSL) and post-IR IRSL signals of potassium (K)-feldspars have recently garnered attention for their minimal rates of anomalous fading. The post-IR IRSL signal has been used to obtain age estimates for geological deposits, mostly in Europe. Studies on the behaviour of the IRSL and post-IR IRSL signals of K-feldspars from a wider range of geographic regions and depositional contexts are needed, particularly for regions where the OSL signal from quartz is poorly behaved. Discrepancies in the literature regarding the behaviours of the IRSL and TL signals of K-feldspars also highlight the need to characterise the behaviours of samples from a wide variety of contexts. This paper begins to address this problem by characterising and comparing the IRSL signals of a metamorphic and a volcanic K-feldspar sample from two sites in East Africa, a region in which the OSL signal from quartz has generally proven problematic for dating. We demonstrate that the metamorphic and volcanic K-feldspars have substantially different TL glow curves that respond differently to IR stimulation. The sample of metamorphic K-feldspar from Tanzania (MR9) has a peak at 430 °C that is associated with the IRSL signal and an optically less-sensitive peak at 350 °C, while the sample of volcanic K-feldspar from Ethiopia (MB3) exhibits a single broad TL region centred at ～230 °C that responds differently to IR stimulation. Differences in the change of IRSL decay curve shape with stimulation temperature suggest that the processes of IRSL production many vary between the two samples. Using dose recovery tests, we demonstrate that the IRSL (50 °C), IRSL (225 °C) and post-IR IRSL (50 °C, 225 °C) signals of sample MR9 are suitable for dose and age estimation using the single-aliquot regenerative-dose procedure, while those of sample MB3 are less suitable. The post-IR IRSL signal of the latter sample performs poorly in tests of SAR suitability and the three signals exhibit extremely high fading rates over laboratory timescales (g_2days > 19%/decade).     

The problem of dating quartz 2: Synchrotron generated X-ray excited optical luminescence (XEOL) from quartz

The luminescence emission of quartz is used in optically stimulated luminescence dating (OSL), however the precise origins of the emission are unclear. A suite of quartz samples were analysed using X-ray excited optical luminescence (XEOL). Radiation dose effects were observed whereby the UV emissions (3.8 and 3.4 eV) were depleted to the benefit of the red emission (1.9–2.0 eV). Samples were excited at ∼7 keV. Understanding why some quartz emit light more brightly than others will increase the efficiency and precision of OSL analyses.     

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.     

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.