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The test dose was chosen to be 16 Gy and it was chosen based on our previous studies e. The inset shows an enlargement for the dose region up to Gy. A similar test is not necessary when the analyses focus on feldspar grains, because the quartz grains which may be present in the aliquots are insensitive to infrared stimulation; iv a measurement of anomalous fading for feldspar. However, their precision is restricted due to methodological limits the use of several aliquots to obtain only one equivalent dose, obtained by extrapolation of the natural signal on a curve derived from various laboratory doses.
Osl dating limits and fits - Pré-publication
Quartz is also present in sediments trapped in karstic caves on Eivissa Fiol et al. Thus, well bleached quartz grains of different sizes are likely to be present in the carbonate dune sands at deposition. Samples and analytical facilities The three aeolianite samples were collected as blocks ca. The samples were checked for radioactive equilibrium in uranium series. U activity was determined by measuring the Th emissions, namely the As such, we used the emissions of As these emissions have relatively low yields and are on the Compton edge of the spectrum, it remains very difficult to use them with great confidence for measuring such low activities.
Pb is a good indicator for radon loss; however the problems mentioned above apply to this radionuclide as well due to its low energy Moreover, these samples are fairly dense and consequently this low energy emission is subjected to attenuation within the sample.
We have nevertheless quantified these emissions; the values obtained for all samples were lower than in the case of Ra, but due to reasons stated above, we consider that this more likely reflects inaccuracies in the measurement itself of these low energy emissions than a systematic radium gain accompanied with radon loss.
In the case of radium measurement, Ra activity was determined from the well defined Pb peaks at keV with a yield of Samples were measured after they have been stored for one month to allow for equilibrium between radium and radon. We consider that due to the nature of the aeolianite material and its formation, radioactive equilibrium is a fair assumption. The cosmic ray contribution was estimated using published formulae Prescott and Hutton, Water content estimation was based on the difference between the as found and the oven-dried weight of material from the inner part of the blocks.
The blocks were highly cemented, allowing little water penetration; the presence of rhizoconcretions pedodiagenetic mineral accumulations around living or dead plant roots; Klappa, in the investigated profile indicates a quick cementation process. Considering the various sea-level fluctuations during the dated period see del Valle et al. Moreover, identifying an upper limit for water content is not critical, since the sea-level during the dated period was mostly lower than today del Valle et al.
The information relevant for annual dose estimation is given in Table 1. The luminescence and dosimetry data are indicated along with the random uncertainties; the uncertainties mentioned with the OSL ages are the overall uncertainties.
Small aliquots 2 mm diameter have been used for the measurements performed on coarse quartz grains. For all grain sizes, the error bars often fall within the data point on the graph. The OSL signal used for analyses was the initial 0 to 0. Inset shows a typical normalised natural OSL signal and response to , and Gy compared with that for calibration quartz in response to 4.
The error bars often fall within the data point on the graph. The test dose was chosen to be 16 Gy and it was chosen based on our previous studies e. Timar-Gabor and Wintle, ; Timar-Gabor et al. The growth of the luminescence signal with dose was best described by the sum of two saturating exponential functions. Within error limits, the values obtained for both recycling and IR depletion ratio tests fell in the range of 0.
No systematic variation in equivalent dose with preheat treatment was observed over the employed temperature range Fig. Pulse annealing experiments and isothermal decay experiments were also performed and the results confirm the thermal stability of the OSL signal from the samples used in this study see Text S1, Fig.
We consequently describe in the present paper its general principles and its application to the case-study samples LUM and LUM It includes the measurement of several OSL signals for a single aliquot tab. Basically the comparison of the natural signal with the artificial luminescence signals should make it possible to interpolate the palaeodose directly. The procedure is however more complicated, as artificial irradiation leads to the trapping of electrons in unstable traps of the mineral crystal lattice.
Since the measurement of the luminescence signal occurs immediately after irradiation, these electrons are very likely to be released during the optical stimulation, leading to an overestimate of the signal. The unstable traps must consequently be emptied before the measurement.
The temperature of the preheat has to be chosen by a preliminary test that requires exposing the aliquots to different preheat temperatures or by exposing the aliquots to a given temperature during various time-spans. Agrandir Original jpeg, k The aliquots are placed on a wheel in the reader black box on the right.
The black tube above the reader is the photomultiplier tube used to record the photons emitted by quartz and feldspar during a stimulation i. The heater element is located in the lower part of the reader.
The results can be seen on the screen left of the picture photo: The SAR protocol applied to coarse-grain quartz 1. Give various regenerative doses s, s, s and s and repeat step 1 to 6. Checking of recuperation and recycling: Give a repeated regenerative dose and repeat step 1 to 6.
Checking of feldspar contamination: Give the same regenerative dose Repeat step 2 to 5. The SAR protocol applied to coarse-grain feldspars 1. Give a repeated regenerative dose and repeat step 1 to 5.
Laboratory treatments may actually induce sensitivity changes in the grains, which means that the signal obtained after a subsequent stimulation may be affected by the preheat and irradiation conditions. At this step, the SAR protocol can be summarised as follows tab. Each measurement is followed by the administration of the test dose and measurement of the test signal Tx. The palaeodose value for this aliquot from sample LUM is These tests may vary from one laboratory to the other but basically correspond with: This shows that sensitivity changes were corrected using the test dose.
A recycling ratio significantly different with 1 means that for a similar dose the two signals are not the same: If the normalised signal is theoretically equal to zero, a weak signal is often induced by the transfer of electrons during the preheat process. This detection is undertaken using infrared diodes. This test is important because feldspars are not only stimulated by infrared light, but also by the blue or green light used for quartz.
Hence, the presence of feldspar contaminates the luminescence one wishes to record from quartz. A similar test is not necessary when the analyses focus on feldspar grains, because the quartz grains which may be present in the aliquots are insensitive to infrared stimulation; iv a measurement of anomalous fading for feldspar. This test may be performed using a SAR protocol including variable delays between irradiation and measurement of the signal to estimate the fading to be estimated.
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