Residual thermoluminescence for sun-bleached quartz: Dependence on pre-exposure radiation dose

Abstract

Quartz grains zeroed by heat and subsequently irradiated -either in the laboratory, or naturally in volcanic deposits -were exposed to sunlight for several months prior to TL measurements. As a result, the residual TL, either Blue or Red, appeared to grow significantly with the radiation-dose experienced by the grains since zeroing. In another experiment, un-heated sedimentary quartz grains, sampled at the very surface of a coastal sand-dune, were irradiated and subsequently exposed to sunlight before TL measurements.The residual Red-TL still exhibited a strong dose-dependence, but at a level which was higher than the natural signal. Finally, it was concluded that the so-called "un-bleachable TL" is not a constant, intrinsic data, for a given quartz sample.Because the residual TL is strongly correlated to the radiation dose experienced since the last zeroing by heat, it can theoretically be quoted in terms of "dose", as sometimes done.However, this raises the question of its correct evaluation, and, on the other hand, there is a risk of misinterpretation of its physical meaning in the case of un-heated sediments.In the context of sediment dating, the experimentally-estimated residual signal that is routinely subtracted from the total signal for palaeodose estimation is therefore higher than the signal that would have been measured prior to burial. Such effect induces an error which can be significant for Red TL. It can be corrected for, at least roughly, from experimental exploration of the dose-dependence of the residual TL.

Thermoluminescence: theory

“…1 Thermoluminescence is luminescence monitored from the pre-illuminated sample when it is heated at a constant rate: it provides information on the depth of the energy stabilization of the charge-separated state(s) initially present and on their amount. Apart from its use in photosynthesis, TL is a popular technique for monitoring the age of the samples (see, for example, Miallier et al 2006). Some substances such as quartz or fluorite store energy when exposed to radiation and this trapped energy can be released while emitting light when heating the sample.…”

An Overview of Red-Thermoluminescence (RTL) Studies on Heated Quartz and RTL Application to Dosimetry and Dating

“…Since a light-sensitive RTL portion was principally employed for such RTL dating, some fundamental and applied studies, involving light-sensitive and isothermally stimulated red emissions, have been carried out (Westaway and Roberts, 2006;Fattahi and Stokes, 2005;Miallier et al, 2006). Additionally, the far-red TL measurement of feldspar has investigated for possible applications to archaeological and sediment dating Hashimoto et al, 2007a), but space limitations prevent their discussion here.…”

Future Directions of Luminescence Dating of Quartz

“…In volcanic quartz, red ITL emission with a peak at 630 nm is observed (Tsukamoto et al, 2007). However, the red TL signal does not reach zero when exposed to sunlight (Miallier et al, 2006;Ganzawa and Maeda, 2009) and thus a dual-aliquot protocol using the red ITL measured at 260°C was developed to take account of the residual levels (Westaway and Roberts, 2006). This approach has been vital in dating sediments in some key sites in southeast Asia (Morwood et al, 2004;Roberts et al, 2009;Westaway et al, 2007Westaway et al, , 2009) and the ages obtained using this procedure have been shown to agree well with independent age evidence (Westaway, 2009).…”