What is OSL?
OSL is an acronym for Optically-Stimulated Luminescence.
Optically-Stimulated Luminescence is a late Quaternary dating technique used to date the last time quartz sediment was exposed to light. As sediment is transported by wind, water, or ice, it is exposed to sunlight and zeroed of any previous luminescence signal. Once this sediment is deposited and subsequently buried, it is removed from light and is exposed to low levels of natural radiation in the surrounding sediment. Through geologic time, quartz minerals accumulate a luminescence signal as ionizing radiation excites electrons within parent nuclei in the crystal lattice. A certain percent of the freed electrons become trapped in defects or holes in the crystal lattice of the quartz sand grain (referred to as luminescent centers) and accumulate over time (Aitken, 1998).
In our laboratory, these sediments are exposed to an external stimulus (blue-green light) and the trapped electrons are released. The released electrons emit a photon of light upon recombination at a similar site. In order to relate the luminescence given off by the sample to an age, we first need to obtain the dose equivalent to the burial dose. Following the single-aliquot regenerative (SAR) method of Murray and Wintle (2000), the dose equivalent (De) is calculated by first measuring the natural luminescence of a sample. Then, the bleached sample is given known laboratory doses of radiation, referred to as regenerative doses. The regenerative dose data are fit with a saturating exponential to generate a luminescence dose-response curve. The De is calculated by the intercept of the natural luminescence signal with the generated curve. A curve is generated for each aliquot (subsample), multiple aliquots are needed to obtain an accurate De.
Example of a SAR signal-dose plot for one aliquot of one sample.
Finally, the age of the sample is calculated by dividing the equivalent dose (De) by the dose rate of the environment surrounding the sample:
Age (kyr) = Equivalent Dose (Gy) / Dose Rate (Gy/kyr)
Aitken, M.J., 1998, An introduction to optical dating: The dating of Quaternary sediments by the use of photon-stimulated luminescence: Oxford,University Press, 267 p.
Murray, A.S., and Wintle, A.G., 2000, Luminescence dating of quartz using an improved signle-aliquot regenerative-dose protocol: Radiation
Measurements, v. 32, p. 57-73.