24 results on '"Helen M. Roberts"'
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2. Exploring sources of variation in thermoluminescence emissions and anomalous fading in alkali feldspars
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Anthony M. T. Bell, Mayank Jain, Nicholas J. G. Pearce, Helen M. Roberts, Geoff A. T. Duller, Svenja Riedesel, Adrian A. Finch, and Georgina E. King
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010302 applied physics ,Radiation ,Thermoluminescence dating ,Thermoluminescence ,Perthite ,Framework disorder ,Emission spectra ,Feldspar ,Fading rate ,01 natural sciences ,Molecular physics ,030218 nuclear medicine & medical imaging ,03 medical and health sciences ,0302 clinical medicine ,visual_art ,0103 physical sciences ,visual_art.visual_art_medium ,Fading ,Emission spectrum ,Luminescence ,Instrumentation ,Alkali feldspar - Abstract
Alkali feldspar is routinely used in retrospective dosimetry using luminescence methods. However there is a signal loss over time, termed ‘anomalous fading’, which results in age underestimation if uncorrected. Although significant improvements have been made in recent years, luminescence dating of feldspars remains challenging. This paper investigates the relationships between chemistry, structural state and the scale of exsolution with thermoluminescence (TL) emission spectra and infrared stimulated luminescence (IRSL) fading rates. We measure TL emission spectra, where possible linking the recombination site to physical features of the feldspar crystals. We show that fading rates are lowest in ordered end-member Na- and K-feldspars but significantly greater in disordered end-members, showing that Al–Si order influences fading. As well as having very low fading rates, ordered end-member samples have distinctive TL emission spectra, with the yellow-green emission dominant, while all other samples have a dominant blue emission. Perthite, i.e. exsolved members of the (Na,K)-feldspar solid solution, show greater fading than disordered end-members and fading is greatest in semi-coherent macroperthite. We propose that the state of Al–Si-order, and the occurrence of defects and dislocations at the perthite lamellar interfaces influence anomalous fading rates in feldspar. more...
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- 2021
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3. Age determination using feldspar: Evaluating fading-correction model performance
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Christoph Burow, Nicholas J. G. Pearce, Georgina E. King, Helen M. Roberts, King G.E., Burow C., Roberts H.M., and Pearce N.J.G.
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010506 paleontology ,Radiation ,feldspar, luminescence, fading ,Contrast (statistics) ,Sample (statistics) ,Function (mathematics) ,010502 geochemistry & geophysics ,Feldspar ,01 natural sciences ,Exponential function ,visual_art ,Statistics ,visual_art.visual_art_medium ,Range (statistics) ,Fading ,Saturation (chemistry) ,Instrumentation ,0105 earth and related environmental sciences ,Mathematics - Abstract
The recent introduction of post-IR IRSL measurement protocols has prompted a resurgence in luminescence applications using feldspar, some of which are affected by anomalous fading related signal loss. Many fading-corrected feldspar ages are reported in the literature, however few of those ages have been corrected using the model of Huntley (2006) [Huntley, D.J., 2006. An explanation of the power-law decay of luminescence. Journal of Physics: Condensed Matter 18(4), 1359–1365]. Here we present a new R function that calculates fading-corrected ages using the model of Huntley (2006), implemented with either a single-saturating exponential (1EXP) or general-order kinetic (GOK) fit. We evaluate the performance of the model through (i) contrasting measured and modelled field saturation values for a suite of 41 published saturated samples, and (ii) through using the model to fading-correct feldspar ages of samples with independent age control. Our results indicate that when implemented with 1EXP this model has an accuracy of 10% for predicting sample saturation, but that independent ages may be overestimated when the model is used to fading-correct samples across a range of timescales. In contrast, providing that the dose response curve has been characterised beyond 600 Gy, implementing the Huntley (2006) model with a GOK fit yields accurate age estimations. Modelled age overestimation for 1EXP is associated with dose response curve deviation from a single-saturating exponential. Finally we contrast the laboratory measured light levels of a suite of 50 saturated samples with their corresponding fading rates. We show that these saturated samples may yield De values below 2D0, and thus that 2D0 is not an effective screening criterion for sample saturation where no anomalous fading correction is made. more...
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- 2018
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4. Testing single aliquot regenerative dose (SAR) protocols for violet stimulated luminescence
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Geoff A. T. Duller, Helen M. Roberts, Melissa S. Chapot, and Debra Colarossi
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010506 paleontology ,Radiation ,Materials science ,business.industry ,Equivalent dose ,fungi ,Mean value ,01 natural sciences ,030218 nuclear medicine & medical imaging ,Background level ,body regions ,03 medical and health sciences ,0302 clinical medicine ,Optics ,skin and connective tissue diseases ,business ,Luminescence ,Instrumentation ,Sensitivity (electronics) ,0105 earth and related environmental sciences - Abstract
Basic assumptions of the single aliquot regenerative dose (SAR) protocol are tested using the violet stimulated luminescence (VSL) signal from quartz. The VSL signal is shown to be reduced to a sufficiently low background level between SAR steps, and the SAR protocol appears to adequately correct for sensitivity changes during measurement. The VSL SAR protocol can recover a large (405 Gy) laboratory beta dose within uncertainties, however the mean value for the dose recovery ratio is commonly 0.8 or less. This poor behaviour is echoed in the measurements of equivalent dose (De) for a sample with an expected De of ∼354 Gy, which underestimates De by 50–70%. Further investigations are required to understand the mechanisms underlying these underestimations in VSL SAR De values. more...
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- 2018
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5. Exploring the behaviour of luminescence signals from feldspars: Implications for the single aliquot regenerative dose protocol
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Debra Colarossi, Helen M. Roberts, and Geoff A. T. Duller
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010506 paleontology ,Radiation ,Test dose ,Materials science ,Optically stimulated luminescence ,Infrared stimulated luminescence ,Analytical chemistry ,010502 geochemistry & geophysics ,01 natural sciences ,Signal ,Luminescence ,Instrumentation ,Sensitivity (electronics) ,0105 earth and related environmental sciences - Abstract
A series of dose recovery experiments are undertaken on grains of potassium-rich feldspar using a single aliquot regenerative dose (SAR) protocol, measuring the post-infrared infrared stimulated luminescence (post-IR IRSL) signal. The ability to successfully recover a laboratory dose depends upon the size of the test dose used. It is shown that using current SAR protocols, the magnitude of the luminescence response (Tx) to the test dose is dependent upon the size of the luminescence signal (Lx) from the prior regeneration dose because the post-IR IRSL signal is not reduced to a low level at the end of measuring Lx. Charge originating from the regeneration dose is carried over into measurement of Tx. When the test dose is small (i.e. 1%–15% of the given dose) this carry-over of charge dominates the signal arising from the test dose. In such situations, Tx is not an accurate measure of sensitivity change. Unfortunately, because the carry-over of charge is so tightly coupled to the size of the signal arising from the regeneration dose, standard tests such as recycling will not identify this failure of the sensitivity correction. The carry-over of charge is due to the difficulty of removing the post-IR IRSL signal from feldspars during measurement, and is in stark contrast with the fast component of the optically stimulated luminescence (OSL) signal from quartz for which the SAR protocol was originally designed. more...
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- 2018
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6. Natural and laboratory TT-OSL dose response curves: Testing the lifetime of the TT-OSL signal in nature
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Helen M. Roberts, Melissa S. Chapot, Geoff A. T. Duller, and Zhongping Lai
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010506 paleontology ,Radiation ,Materials science ,Optically stimulated luminescence ,Equivalent dose ,Mineralogy ,Central china ,01 natural sciences ,Signal ,030218 nuclear medicine & medical imaging ,03 medical and health sciences ,0302 clinical medicine ,High doses ,Dose rate ,Instrumentation ,0105 earth and related environmental sciences - Abstract
This study compares natural and laboratory generated thermally transferred optically stimulated luminescence (TT-OSL) dose response curves (DRCs) for fine-grain quartz extracts from the Luochuan loess section in central China. Both DRCs saturate at high doses relative to the quartz OSL signal; the natural TT-OSL DRC saturates at about 2200 Gy and laboratory DRCs saturate at about 2700 Gy. However, the natural and laboratory TT-OSL DRCs deviate from one another at circa 150 Gy resulting in TT-OSL equivalent dose underestimation relative to palaeodoses expected from dose rates and independent age control. The lifetime of the TT-OSL signal at 10 °C, calculated from values of trap parameters E and s, is compared against the value for lifetime of the TT-OSL signal in nature at average burial temperature as determined from the age underestimation caused by deviation of the natural and laboratory generated DRCs. These two independent assessments of TT-OSL signal lifetime at Luochuan give similar values, suggesting that laboratory measurements of thermal stability reflect natural burial lifetimes and can potentially be used to correct TT-OSL ages for the difference between natural and laboratory dose response curves. more...
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- 2016
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7. Single grain infrared photoluminescence (IRPL) measurements of feldspars for dating
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Helen M. Roberts, Geoff A. T. Duller, and Matthew Gunn
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010302 applied physics ,Photomultiplier ,Radiation ,Materials science ,business.industry ,Equivalent dose ,Infrared ,Stray light ,Detector ,01 natural sciences ,Signal ,030218 nuclear medicine & medical imaging ,03 medical and health sciences ,Time resolved data ,0302 clinical medicine ,Optics ,0103 physical sciences ,business ,Instrumentation ,Image resolution - Abstract
Existing infrared photoluminescence (IRPL) systems have used pulsed infrared stimulation (~830 nm) and measured IRPL emission (at 880 or 955 nm) using time resolved data collection with photomultipliers. Breakthrough of the infrared stimulation light overwhelms the IRPL, but the delayed emission during the laser-off period has been used instead. This paper describes a system for measurement of the IRPL signal from single sand-sized grains of feldspar. The attachment uses an electron-multiplying charge-coupled device (EMCCD) imaging system, and has two innovations that make it possible to use such a detector to obtain IRPL data. First, the optical detection system has been designed to minimise stray light and maximise the efficiency with which filters reject the stimulation light. This acts to reduce, but not eliminate, the breakthrough. Second, by placing the sample to be measured in a clearly defined sample grid, the spatial resolution provided by the EMCCD has been used to differentiate between regions of the image where IRPL is emitted and adjacent regions where only breakthrough is expected. This allows quantification of the breakthrough and effective subtraction to isolate the IRPL signal from the grains of interest. The attachment has been used to measure IRPL from single sand-sized grains of feldspar from an aeolian dune from New Zealand. A 1W UV LED (365 nm) is also added to the system and this is effective at resetting the IRPL signal, permitting a single aliquot regenerative dose (SAR) protocol to be used to measure equivalent dose (De). Measurement of a known laboratory dose (104 Gy) demonstrates the reproducibility of the attachment, with no overdispersion observed in the resulting single grain De values. The recovered dose is within 10% of the given dose. The natural IRPL signal yields De values from single grains with low overdispersion (22%) and giving a weighted mean value (103 ± 5.8 Gy) that is consistent with that obtained using post-IR IRSL measurements (105 ± 3.8 Gy). The attachment described here provides IRPL measurements on single grains suitable for exploring the potential of this novel and exciting signal for dating geological sediments. more...
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- 2020
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8. Strategies for equivalent dose determination without heating, suitable for portable luminescence readers
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Geoff A. T. Duller, Claire R. Cousins, D. P. Langstaff, Matthew Gunn, Rachel Elizabeth Cross, Helen M. Roberts, Science & Technology Facilities Council, University of St Andrews. School of Earth & Environmental Sciences, and University of St Andrews. St Andrews Centre for Exoplanet Science more...
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010506 paleontology ,Materials science ,Optically stimulated luminescence ,110C TL peak ,Sample (material) ,NDAS ,01 natural sciences ,Signal ,030218 nuclear medicine & medical imaging ,Ionizing radiation ,03 medical and health sciences ,0302 clinical medicine ,Optics ,Thermal ,Irradiation ,Instrumentation ,QC ,0105 earth and related environmental sciences ,Radiation ,GE ,business.industry ,Equivalent dose ,De determination ,Quartz ,QC Physics ,Preheating ,Dating ,business ,Luminescence ,GE Environmental Sciences - Abstract
This work was supported by the UK Space Agency CREST3 program under grant ST/P001998/1. Research in Next Generation Luminescence methods in Aberystwyth is supported by NERC grant CC003, and by HEFCW infrastructure funding for SPARCL. In recent years a number of portable instruments have been built for measuring the optically stimulated luminescence (OSL) signal from naturally occurring minerals. Some of these instruments have incorporated ionising radiation sources, giving the possibility of determining an equivalent dose (De), but little use has been made of these. One challenge has been that heating samples in this type of equipment is a major engineering challenge, yet methods for De determination use thermal pretreatments to remove charge from unstable traps, making signals arising from irradiation in nature and the laboratory comparable. This paper explores three strategies for obtaining accurate estimates of the De of samples in situations where thermal treatments are not possible: (1) deriving a correction factor based on comparing De values obtained using protocols with and without heating; (2) removing the contribution from the 110 °C TL peak and other unstable defects by component fitting the unheated OSL signal; and (3) adding a small beta dose to the sample prior to measurement of the natural luminescence signal so that the 110 °C TL peak is filled, making this measurement comparable with regeneration measurements where this peak is also populated. All three methods are promising when applied to quartz that has been physically separated from samples using standard laboratory procedures. The next step in this work will be to explore whether such methods can be applied to mixed mineral assemblages as would be encountered in the field. Postprint more...
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- 2018
9. Spatially-resolved thermoluminescence from snail opercula using an EMCCD
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Myung Ho Kook, Rosemary Jane Stirling, Andrew S. Murray, Helen M. Roberts, and Geoff A. T. Duller
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Physics ,Radiation ,biology ,Equivalent dose ,Spatially resolved ,Mineralogy ,Tentaculata ,Snail ,Astrophysics ,biology.organism_classification ,Thermoluminescence ,Bithynia tentaculata ,biology.animal ,visual_art ,visual_art.visual_art_medium ,Instrumentation ,Operculum (gastropod) - Abstract
In recent years opercula of the snail species Bithynia tentaculata have been shown to emit thermoluminescence (TL) signals that can be used to determine equivalent dose, and may be capable of dating events throughout the entire Quaternary period. Concentric growth lines are a notable feature of almost all B. tentaculata opercula, but it is not known whether the luminescence emitted by the opercula is influenced by these structures. This study uses a newly developed EMCCD imaging system to measure the TL signals from opercula. A combination of microscopic analysis of the opercula using visible imagery, and measurement of the TL using the EMCCD system has been undertaken. Variations in TL intensity and equivalent dose (De) are seen, but the two are not correlated. Changes in TL intensity broadly mimic the concentric growth structures, but the largest variations in intensity are between different margins of the opercula, not individual growth bands. The EMCCD system makes it possible to produce a two dimensional map of the De measured from an operculum. Dose recovery experiments give De values that are consistent with each other across the whole opercula. Measurement of the De arising from irradiation in nature shows significant variability across a single operculum, but at present the reason for this variability is unknown. more...
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- 2015
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10. A method for routinely monitoring the reproducibility of thermal pretreatment prior to optically stimulated luminescence measurements
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Helen M. Roberts, R.C. Pinder, and Geoff A. T. Duller
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010302 applied physics ,Reproducibility ,Radiation ,Materials science ,Thermal lag ,Optically stimulated luminescence ,Equivalent dose ,Analytical chemistry ,01 natural sciences ,Thermoluminescence ,030218 nuclear medicine & medical imaging ,03 medical and health sciences ,0302 clinical medicine ,0103 physical sciences ,Calibration ,Sample preparation ,Luminescence ,Instrumentation - Abstract
Thermal pretreatments are used prior to optically stimulated luminescence (OSL) measurements in methods of dose determination, but the reproducibility of such heating appears to be poor within laboratories, and between laboratories. Monitoring the position of the 110 °C thermoluminescence (TL) peak in quartz can be used to routinely assess the reproducibility of heating within a repeated set of measurements on an aliquot (e.g. during the single aliquot regenerative dose protocol) providing feedback on whether changes in sample preparation can improve reproducibility. Poor reproducibility between measurements from a single aliquot are likely to arise from grains between the hotplate and sample holder. Variability between aliquots may also be affected by deformed sample holders. Differences between readers may also be caused by differences in the calibration of the hotplate. Reproducibility of heating can be improved within runs, and between instruments, by taking care to avoid loose grains, by removing deformed sample holders, and if necessary by calibrating hotplates. It is suggested that individual aliquots should only be accepted if the apparent 110 °C TL peak temperature within a sequence of measurements varies by 5 °C or less. Improving the reproducibility of thermal treatment is important in reducing scatter in equivalent dose determinations based on OSL from quartz, but also for signals such as TT-OSL from quartz, and the various feldspar luminescence signals. Temperature control is also critical for accurate kinetic analyses. more...
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- 2020
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11. A comparison of natural- and laboratory-generated dose response curves for quartz optically stimulated luminescence signals from Chinese Loess
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Melissa S. Chapot, Zhongping Lai, Helen M. Roberts, and Geoff A. T. Duller
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Dose–response relationship ,Radiation ,Materials science ,Optically stimulated luminescence ,Loess ,Mineralogy ,Loess plateau ,Irradiation ,Growth curve (biology) ,Instrumentation ,Quartz ,Exponential function - Abstract
It has previously been observed that laboratory-generated quartz optically stimulated luminescence (OSL) signals from different samples have similar dose response curves (DRCs) after they are normalized using a test dose. It therefore seems likely that growth of the normalized signal due to natural irradiation of quartz may also follow a general dose response curve. The existence of such a curve is investigated by constructing a natural DRC from the test dose-normalized natural OSL signals of seven samples from the Luochuan section of the Chinese Loess Plateau. The same aliquots are then used to build single aliquot regenerative (SAR) DRCs, making it possible to compare the natural and laboratory constructed curves. Two main differences are observed. Firstly, the laboratory-generated DRCs are best fitted with double saturating exponential functions whereas the natural DRC is equally well fitted with a single saturating function. Secondly, in the laboratory-generated DRCs the normalized OSL signal continues to increase at high laboratory doses (>500 Gy), whereas no growth is seen at these doses in the equivalent natural DRC. These differences between natural- and laboratory-generated DRCs are still apparent even if data are manipulated to isolate the fast component, or if a sensitivity corrected multiple aliquot regenerative (SC-MAR) dose procedure is used. This suggests that the observed differences are not due to the influence of different components or inter-regenerative dose cycle sensitivity changes. The divergence between the natural- and laboratory-generated DRC means that the current maximum limit of quartz OSL dating at the Luochuan section is 150 Gy, as D e estimates above this value are likely to be underestimations. more...
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- 2012
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12. Testing Post-IR IRSL protocols for minimising fading in feldspars, using Alaskan loess with independent chronological control
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Helen M. Roberts
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Radiation ,Fine grain ,Thermoluminescence dating ,Equivalent dose ,Loess ,Single factor ,Mineralogy ,Fading ,Fission track dating ,Instrumentation ,Thermoluminescence ,Geology - Abstract
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. more...
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- 2012
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13. Determining the K-content of single-grains of feldspar for luminescence dating
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Geoffrey A. T. Duller, Nicholas J. G. Pearce, Rachel Smedley, and Helen M. Roberts
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Radiation ,Materials science ,Thermoluminescence dating ,Mineralogy ,Feldspar ,Thermoluminescence ,Spectral line ,visual_art ,visual_art.visual_art_medium ,Signal intensity ,Dose rate ,Luminescence ,Instrumentation ,Quartz - Abstract
Feldspars form a solid-solution series whereby the K-content may range from 0 to 14%. LA-ICP-MS measurements for density-separated single-grains of feldspar yielded realistic concentrations of K within the range of those naturally occurring, and also highlighted the difficulty in isolating the pure end members during density-separation. No direct relationship was found between the thermal stability of the infrared-stimulated luminescence (IRSL) signal and measured K-content of individual grains. However, the brightest IRSL and post-IR IRSL signals originated from grains with ∼12% K-content. All grains giving a measurable signal had K-content between 6 and 13%, therefore it is suggested that an internal K-content of 10 ± 2% can be assumed for routine single-grain dating of density-separated K-feldspars. more...
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- 2012
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14. Improving the TT-OSL SAR protocol through source trap characterisation
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Ann G. Wintle, Helen M. Roberts, Geoffrey A. T. Duller, and Grzegorz Adamiec
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Radiation ,Optically stimulated luminescence ,Chemistry ,Mineralogy ,Trap (plumbing) ,Luminescence ,Instrumentation ,Thermoluminescence ,Quartz ,Thermal quenching ,Signal ,Optical dating - Abstract
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. more...
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- 2010
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15. A simplified SAR protocol for TT-OSL
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N. Porat, Geoffrey A. T. Duller, Ann G. Wintle, and Helen M. Roberts
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Protocol (science) ,Measure (data warehouse) ,Radiation ,Materials science ,Equivalent dose ,Optical stimulation ,Sand sample ,Mineralogy ,Instrumentation ,Signal ,Biomedical engineering - Abstract
A new single-aliquot regenerative-dose (SAR) protocol for measuring the equivalent dose using the thermally-transferred OSL (TT-OSL) signal is proposed. Characterisation of the Basic Transferred OSL (BT-OSL) signal measured using a previous protocol showed that it was unnecessary to measure the BT-OSL thus allowing simplification of the SAR protocol for TT-OSL. To remove the BT-OSL, holding the aliquot for 100 s at 300 °C without optical stimulation at the end of each SAR cycle gave the most reproducible results. A simplified protocol incorporating this heat treatment was tested by recovering doses up to 700 Gy which had been given to a modern sand sample with an equivalent dose of less than 5 Gy. more...
- Published
- 2009
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16. Exploring procedures for the rapid assessment of optically stimulated luminescence range-finder ages
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Geoff A. T. Duller, Helen M. Roberts, and Julie A. Durcan
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Standard sample ,Radiation ,Materials science ,Optically stimulated luminescence ,Analytical chemistry ,Luminescence ,Instrumentation ,Rapid assessment - Abstract
Roberts HM, Durcan JA, Duller GAT. (2009). Exploring procedures for the rapid assessment of optically stimulated luminescence range-finder ages. Radiation Measurements, 44 (5/6), 582-587. Proceedings of the 12th International Conference on Luminescence and Electron Spin Resonance Dating (LED 2008) more...
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- 2009
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17. Dose dependence of thermally transferred optically stimulated luminescence signals in quartz
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Jin Cheul Kim, Yong Il Lee, Helen M. Roberts, S.B. Yi, Ann G. Wintle, and Geoff A. T. Duller
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Radiation ,Materials science ,Optically stimulated luminescence ,Equivalent dose ,Dose dependence ,Analytical chemistry ,Mineralogy ,Linear growth ,Luminescence ,Instrumentation ,Signal ,Quartz - Abstract
The thermally transferred optically stimulated luminescence (TT-OSL) responses of chemically purified fine-grained quartz from seven loess-like samples from Korea are presented. In particular, the experimental procedures used to separate the dose-dependent (recuperated OSL, ReOSL) and dose-independent parts of the signal were explored. The OSL signals used to monitor the sensitivity changes that take place during the measurement sequences used to determine the equivalent dose were investigated. A single aliquot procedure was used for the TT-OSL measurements and resulted in linear growth of the ReOSL with dose up to at least 2 kGy. For this suite of samples, a standardised growth curve (SGC) was constructed for the ReOSL, tested with dose recovery experiments, and was used to obtain De values for the seven samples. more...
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- 2009
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18. Assessment of diagnostic tests for evaluating the reliability of SAR De values from polymineral and quartz fine grains
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Yong Il Lee, Geoff A. T. Duller, Helen M. Roberts, S.B. Yi, and Jin Cheul Kim
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Radiation ,Fine grain ,Materials science ,Optically stimulated luminescence ,Equivalent dose ,Diagnostic test ,Mineralogy ,Thermal transfer ,Mineral composition ,Instrumentation ,Quartz ,Optical dating - Abstract
In this study, we applied optically stimulated luminescence (OSL) dating to two fine grain sediment samples collected at Jeongokri, Korea. A single aliquot regenerative dose (SAR) procedure was applied to both polymineral grains and to chemically isolated (H 2 SiF 6 ) quartz grains of 4–11 μm diameter. For polymineral fine grains, the OSL IR depletion ratio and the equivalent dose (D e ) plateau test appear to be equally sensitive indicators of appropriate IR stimulation time for use in the ‘double SAR’ protocol. Additionally, the OSL IR depletion ratio test gives an indication of the relative mineral composition of the samples, hence providing an assessment of the likelihood of obtaining a quartz-dominated [post-IR] OSL signal. Use of higher preheat temperatures would assist in thermally eroding the non-quartz component of the [post-IR] OSL signal from polyminerals. For the quartz fine grains, data from both natural D e determinations and laboratory dose recovery tests are required to identify the appropriate preheat temperatures for dating, due to problems of thermal transfer. This phenomenon is particularly exaggerated for these samples due to the large D e values (≥350 Gy) and hence low slope of the dose–response curve. The double SAR method cannot be applied ubiquitously, even after careful and rigorous study of one sample from a section. Quartz OSL dating using a range of preheat temperatures is suggested to be the most suitable method for OSL dating of fine grain sediments. more...
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- 2009
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19. Assessing the effectiveness of the double-SAR protocol in isolating a luminescence signal dominated by quartz
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Helen M. Roberts
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Radiation ,Materials science ,Optically stimulated luminescence ,Treatment duration ,Mineralogy ,Feldspar ,Signal ,Silicate ,chemistry.chemical_compound ,chemistry ,visual_art ,Silicate minerals ,visual_art.visual_art_medium ,Luminescence ,Instrumentation ,Quartz - Abstract
The effect of the presence of feldspars that can be shown to exhibit anomalous fading is investigated using the double-SAR protocol. This protocol has been suggested as a method of isolating an optically stimulated luminescence (OSL) signal dominated by quartz from a polymineralic mixture of grains. The progressive removal of feldspars from the silt-sized fraction of sediments from Bignell Hill, Nebraska, is also investigated using chemical treatment with hydrofluorosilicic acid for different durations; the appropriate treatment duration for these sediments is identified as being 7 days. For untreated material, the double-SAR protocol yields IRSL and [post-IR] OSL signals that are both dominated by feldspars and that give the same value for the equivalent dose ( D e ) determination. However, for material treated with hydrofluorosilicic acid for 7 days, the IRSL and [post-IR] OSL signals have distinctly different characteristics and appear to be dominated by feldspar and by quartz, respectively. Furthermore, the D e values determined for the feldspar-dominated IRSL signal are about half those obtained for the quartz-dominated [post-IR] OSL signal. This demonstrates that the double-SAR protocol is not always capable of isolating a quartz-dominated signal; clearly, in some circumstances it is necessary to apply a chemical pre-treatment to remove the contribution from feldspars prior to any luminescence measurements being made. more...
- Published
- 2007
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20. Optical dating of coarse-silt sized quartz from loess: Evaluation of equivalent dose determinations and SAR procedural checks
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Helen M. Roberts
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Radiation ,Hydrofluorosilicic acid ,Materials science ,Equivalent dose ,Loess ,Radiation dose ,Mineralogy ,Silt ,Instrumentation ,Quartz ,Optical dating - Abstract
This study discusses the values of equivalent dose ( D e ) determined for samples taken for optical dating from a loess section at Loveland, Iowa, North America. A modified single-aliquot regenerative-dose (SAR) procedure was applied to quartz grains of 35– 50 μ m diameter which were isolated using hydrofluorosilicic acid (H 2 SiF 6 ). ‘Preheat plots’ were constructed to select appropriate D e values for use in subsequent age determinations. The D e values of the younger (Peoria loess) samples varied little with preheat temperatures between 160 and 300 ∘ C . However, the older (Pisgah Formation) samples showed a decrease in D e values with increasing preheat temperature, by as much as a factor of two. Interestingly, both younger and older samples could recover a known laboratory administered radiation dose for a range of preheat temperatures. These apparently contradictory findings present a dilemma regarding which D e values are the most appropriate to use for dating the samples. The behaviour of the samples is examined to investigate the cause of the variations in D e values with preheat temperature for older samples. In addition, two modified SAR procedures were tested in an attempt to generate more reproducible D e values. The data from the three SAR methods are examined, and the question of how to discriminate between D e values is discussed. This study demonstrates that preheat temperature can play a significant role in the determination of D e values. The use of plots of normalised luminescence signal ( L x / T x ) versus preheat temperature is proposed to supplement dose recovery tests and preheat plateau tests. more...
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- 2006
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21. determination for young samples using the standardised OSL response of coarse-grain quartz
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Christopher Ian Burbidge, Helen M. Roberts, and Geoff A. T. Duller
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Systematic error ,Accuracy and precision ,Radiation ,Optically stimulated luminescence ,Equivalent dose ,Mineralogy ,Soil science ,Test response ,Growth curve (biology) ,Sensitivity (control systems) ,Instrumentation ,Quartz ,Mathematics - Abstract
It has recently been shown that it is possible to construct standardised curves of the sensitivity corrected growth in optically stimulated luminescence (OSL) with exposure to ionising radiation, and that they may be used in the dating of quartz and polymineral samples. Standardised growth curves are particularly advantageous where measurement time is limited, as once they have been defined, only the natural signal and the response to a subsequent test dose are required in order to determine the equivalent dose of a sub-sample. The present study is concerned with the application of the standardised growth curve approach to OSL dating of Holocene age samples. Systematic changes in the shape of the standardised growth curve of coarse-grain quartz are identified as the size of the test dose is varied, because of non-proportionality between the test dose and the luminescence test response. The effect is characterised by fitting the change in gradient of the standardised growth curve as test dose is varied. An equation is defined to describe standardised growth as a function of regenerative dose and test dose. Regenerative dose responses of other samples in this study are treated as unknowns and recovered through different growth curves to compare precision and accuracy of various methods of D e determination. The standardised growth curve is found to yield similar precision to conventional fits of single aliquot regenerative data, but slightly poorer accuracy. The standardised growth curve approach was refined by incorporating the measurement of one regenerative response for each aliquot as well as its natural signal. Measurements of this additional data point for aliquots of 22 samples were used to adjust the standardised growth equation, improving its accuracy. The incorporation of this additional data point also indicated a systematic uncertainty of 2.4% in the estimates of D e . more...
- Published
- 2006
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22. Standardised growth curves for optical dating of sediment using multiple-grain aliquots
- Author
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Geoff A. T. Duller and Helen M. Roberts
- Subjects
Radiation ,Test dose ,Materials science ,Optically stimulated luminescence ,Equivalent dose ,Instrumentation ,Analytical chemistry ,Mineralogy ,Sediment ,Growth curve (biology) ,Luminescence ,Optical dating - Abstract
The single aliquot regenerative dose (SAR) protocol offers the opportunity of exploring, relatively simply, the existence of a ‘universal’ growth curve for use in dating using optically stimulated luminescence (OSL). A test dose is used in the SAR procedure to monitor and correct for sensitivity change occurring either over the burial period, or as a result of thermal pretreatments during the measurement procedure. However, this test dose can also be used to correct for variations in signal intensity between individual aliquots, thus enabling the comparison of growth response with dose for many different aliquots and samples, even for measurements made using different instruments. In this paper, the growth characteristics of coarse-grained quartz and polymineral fine-grains are examined using data obtained as part of the SAR procedure. Following standardisation of these data for differences in the test dose response and the magnitude of the test dose used, distinct and reproducible patterns of growth are observed and mineral-specific standardised growth curves (SGCs) are defined for multiple-grain aliquots. An equivalent dose for a sample can then be determined based only on measurements of the natural luminescence signal intensity (Ln) and the response to an artificial irradiation dose (Tn). This equivalent dose, determined using the SGCs defined for quartz and polymineral fine-grains, is compared to that determined using a conventional SAR measurement procedure, for a large number of samples and aliquots. Using an SGC, accurate estimates of equivalent dose may be made based solely on measurements of Ln and Tn, thus potentially speeding up the measurement process. This has obvious benefits where it is necessary to examine a large number of aliquots. more...
- Published
- 2004
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23. Luminescence sensitivity changes of polymineral fine grains during IRSL and [post-IR] OSL measurements
- Author
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Ann G. Wintle and Helen M. Roberts
- Subjects
Radiation ,Fine grain ,Materials science ,Infrared stimulated luminescence ,Mineralogy ,Luminescence ,Instrumentation - Abstract
Roberts, H. M., Wintle, A. G. (2003). Luminescence sensitivity changes of polymineral fine grains during IRSL and [post-IR] OSL measurements. Radiation Measurements, 37 (6), 661-671.
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- 2003
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24. Bleaching of the post-IR IRSL signal from individual grains of K-feldspar: Implications for single-grain dating
- Author
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Geoff A. T. Duller, Helen M. Roberts, and Rachel Smedley
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GB ,Residual De values ,Radiation ,Materials science ,Luminescence ,pIRIR ,Infrared stimulated luminescence ,Mineralogy ,Feldspar ,Signal ,Bleaching rate ,visual_art ,visual_art.visual_art_medium ,Single grains ,Instrumentation ,Quartz ,K feldspar - Abstract
Post-IR IRSL (pIRIR) signals from K-feldspar grains measured at elevated temperatures are increasingly being used for dating sediments. Unfortunately the pIRIR signal from K-feldspars bleaches more slowly than other signals (e.g. OSL from quartz) upon exposure to daylight, leading to concerns about residual signals remaining at deposition. However, earlier studies have not assessed whether the pIRIR signal bleaches at the same rate in all feldspar grains. In this study laboratory bleaching experiments have been conducted and for the first time the results show that the rate at which the pIRIR signal from individual K-feldspar grains bleach varies. To determine whether grain-to-grain variability in bleaching rate has a dominant control on equivalent dose (De) distributions determined using single grains, analysis was undertaken on three samples with independent age control from different depositional environments (two aeolian and one glaciofluvial). The De value determined from each grain was compared with the rate at which the pIRIR225 signal from the grain bleaches. The bleaching rate of each grain was assessed by giving a 52 Gy dose and measuring the residual De after bleaching for an hour in a solar simulator. There is no clear relationship between the rate at which the pIRIR225 signal of an individual grain bleaches and the magnitude of its De. It is concluded that variability in the bleaching rate of the pIRIR225 signal from one grain to another does not appear to be a dominant control on single grain De distributions. more...
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- View/download PDF
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