Towards the development of a preheat procedure for OSL dating of quartz

Isothermal decay data for the natural optically stimulated luminescence (OSL) of a quartz sample from the Kimberley region of Western Australia are presented. These show that >99% of the initial OSL signal measured using a broad-band blue/green-light stimulation source is derived from a single trap. A lifetime at 20 degrees C of about 850 Ma is predicted for this OSL trap. Since these measurements may be affected by charge transfer or sensitivity changes, both of which are thought to occur as a result of heating, the effects of prior heating (preheating) on the shape of the quartz OSL decay curve have been investigated. The relationship between the initial OSL (first 0.4 s), the OSL at the end of a 100 s stimulation period, and the integrated OSL (0-100 s), is presented at a constant stimulation temperature of 125 degrees C, but following preheats at temperatures between 110 degrees C and 380 degrees C. It is deduced that both the natural and regenerated OSL are dominated by a single trap/luminescence centre combination for preheats up to above 300 degrees C. There is probably a small contribution [about 1% of the natural OSL integral (0-100 s) without preheating] from traps which are not emptied by heating to 340 degrees C, and which have a longer lifetime. Based on the observation that the initial OSL signal from 0.1 s stimulation correlates well with the net integral OSL (0-100 s), the effect of preheat on the natural and regenerated OSL is examined again using single aliquots. These curves are then corrected for changes in luminescence efficiency with preheating obtained using the 110 degrees C thermoluminescence from a 0.1 Gy test dose, before fitting with a simple two trap model. We draw conclusions regarding the implications of this study for dating. (C) 1998 Elsevier Science Ltd. All rights reserved.