Crustal Imaging and Characterization Team

USGS Luminescence Dating Laboratory

Information for Prospective Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) Users

Introduction | Section I | Section II | Section III | Section IV | Section V | Other U.S. Labs

Section I: General TL and OSL Methodology (and Assumptions)

If a sample of sediment is heated rapidly to 500°C, there is weak but measurable emission of light. This light is known as thermoluminescence (TL) and is based on time-dependent accumulation of radiation damage in minerals. Optically Stimulated Luminescence (OSL) is measured by shining a beam of light onto mineral grains and measuring the resulting luminescence back. A low level of ionizing radiation, which we measure from 40K , 238U, 235U, 232Th and daughter products, 87Rb and cosmic rays is omnipresent in nature. Almost exclusively, luminescence from quartz and feldspar grains is used in dating.

The interaction between this radiation and the atoms of minerals results in gradually increasing radiation damage. The intensity of the radiation damage in crystal lattices is a measure of the Equivalent Dose (DE), which the mineral has received since formation or last "resetting" by exposure to sunlight or heat over 300°C. The mineral is used as a natural dosimeter. DE is measured in Gray (Gy) or absorbed radiation energy per unit mass. Once one has "read" the DE by means of a TL or OSL measurement, the Dose Rate (DR) is obtained via measurement of K , U, Th, Rb and cosmic rays as dose per unit time or Gy/Ka. The equation for obtaining an age is:

Age (Ka) = DE (Gy)/DR (Gy/Ka)

To evoke the emission of TL and OSL, additional thermal or optical stimulation, respectively, has to be supplied to the crystal. Luminescence is created by the ionizing radiation freeing electrons that wander through the crystal lattice until they encounter a trapped hole or recombine and become trapped at electron traps that are lattice defects with negative charge deficits. During luminescence measurement, "traps" are emptied and luminescence centers destroyed. The longer the crystal has been exposed to ionizing radiation, the more "traps" can be filled, resulting in an increased luminescence signal, with a practical time limit of 800 Ka (Berger, G.W., 1994, Thermoluminescence Dating of Sediments Older than 100 Ka, Quaternary Geochronology, Quaternary Science Reviews, 13, 445-456).

Key assumptions are:

1. Materials have uniform and definable dose rates.
2. Moisture content of the sample and its environment can be determined.
3. Depth, altitude and intensity of cosmic rays on site can be calculated or are known.
4. The radiation-induced signal has to be thermally or optically reset by the event to be dated. The rate and completeness of "resetting" can be reliably obtained.
5. The TL or OSL must have been stable during the time span in question. Any spurious "fading" of TL or OSL can be measured and compensated for in age calculations.
6. The TL and OSL growth characteristics have to follow a mathematical function.

The principal minerals used in OSL are quartz and K feldspar, although use of volcanic glass and some forms of calcite and zircon have been used. The principle minerals used in TL are polymineralic or separation of the quartz or feldspar. M.J. Aitken (Thermoluminescence Dating, 1985, Academic Press, 359 pages and An Introduction to Optical Dating, 1998, Oxford University Press, 267 pages) presents a very able discussion of the complete TL and OSL techniques.

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