OSL Decay Curves Research Articles

A simple transformation for converting CW-OSL curves to LM-OSL curves

A simple mathematical transformation is introduced to convert from OSL decay curves obtained in the conventional way to those obtained using a linear modulation technique based on a linear increase of the stimulation light intensity during OSL measurement. The validity of the transformation was tested by the IR-stimulated luminescence curves from feldspars, recorded using both the conventional and the linear modulation techniques. The transformation was further applied to green-light-stimulated OSL from K and Na feldspars.

Isothermal decay of optically stimulated luminescence in quartz

Isothermal decay curves are presented for the optically stimulated luminescence from an Australian sedimentary quartz, measured for 0.1 s at 125°C, using a broad band 420–550 nm light source (∼26 mW cm −2). Natural and laboratory irradiated samples were held at 160, 180, 200, 220, 240, 260 and 280°C for up to 22 h. Laboratory irradiated samples showed a marked increase in OSL prior to the decrease expected as a result of thermal detrapping of charge. This increase was caused by thermally induced sensitivity changes, which were monitored using the response to a test dose of the 110°C TL peak after each OSL measurement. After dividing the OSL data by this TL response to remove the effects of these sensitivity changes, the corrected OSL decay curves showed that, in this sample, the natural signal was dominated by a single decay component, whereas those from laboratory irradiated sub-samples were dominated by two clearly separate components. Kinetic analysis suggests that the less thermally stable signal has a half-life of <1000 yr at 20°C, and would be removed by the preheats commonly used in dating applications. The more stable signal, present in both the natural and laboratory irradiated samples, has a lifetime of about 1×10 8 yr at 20°C, confirming its suitability as for dating.

Factors controlling the shape of the OSL decay curve in quartz

The effects of stimulation temperature on the shape of the quartz OSL decay curve have been investigated using a green light stimulation source and a quartz sample from the Kimberley region of Western Australia. The relationship between the initial OSL (first 0.4 s), the OSL at the end of a 100 s stimulation period, and the full OSL integral, is presented as a function of stimulation temperature (25–500°C). It is deduced that a single trap/luminescence centre combination is responsible for the majority of the OSL at stimulation temperatures between 150 and 275°C, and thus that traps giving rise to TL peaks in this temperature range cannot contribute significantly to the OSL signal, either directly or indirectly. Comparison of the initial and integral OSL values in this temperature range permits the effects of thermal assistance and thermal quenching to be separated and the activation energies for each process to be calculated. Below 150°C the relationship changes markedly; this change cannot be explained in terms of only thermal assistance and thermal quenching. After 100 s of stimulation, the contribution to the OSL from deeper traps (>330°C) is deduced to be only about 1.7% of the average OSL over 100 s. All of this luminescence from deep traps appears to be thermally quenched, and it is concluded that the 375°C TL trap in particular does not contribute measurably to the slowly decaying component of the OSL from this material. Furthermore, shallow traps below about 150°C do not contribute a large fraction of the OSL signal by direct stimulation. However, we show that competition for charge by the 110°C trap may be significant during stimulation, and some limits are estimated for this process. The possible implications of our findings for dating procedures are also considered.

Luminescence studies of dunes from North-Eastern Tasmania

Northern Tasmania has a geographically extensive cover of Quaternary aeolian features and although the morphology and stratigraphy of many of these have been studied it is difficult to assign a reliable chronology because of the lack of material suitable for radiocarbon dating. The dunes are primarily composed of quartz and hence are ideally suited for the application of luminescence dating. In this paper a variety of luminescence techniques are applied to samples from the area. Analysis of the multiple aliquot OSL measurements showed that a higher precision could be obtained using the signal from the initial 6 mJ.cm −2 of the optical stimulation rather than the total light sum. The initial, near exponential. decay of the OSL signal is highly reproducible from one sub-sample to another, but the later part of the OSL decay curve, which is dominated by retrapping effects, is highly variable. The multiple aliquot OSL results are consistent with single aliquot additive dose results. The luminescence ages show that the Ainslie Linear Dunes were initially deposited by 44 ka, were mobile again at 30 ka and have been subject to reworking within the last 100–200 years. It appears that the Quaternary history of aeolian activity in this area has been more complex than was previously thought.