Thermoluminescence dating

Thermoluminescence dating (TL) is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated (lava, ceramics) or exposed to sunlight (sediments).

The technique has wide application, and is relatively cheap at some US$300–700 per object; ideally a number of samples are tested.

Subsequent irradiation, for example if an x-ray is taken, can affect accuracy, as will the "annual dose" of radiation a buried object has received from the surrounding soil.

For artworks, it may be sufficient to confirm whether a piece is broadly ancient or modern (that is, authentic or a fake), and this may be possible even if a precise date cannot be estimated.

The flux of ionizing radiation—both from cosmic radiation and from natural radioactivity—excites electrons from atoms in the crystal lattice into the conduction band where they can move freely.

In the process of recombining with a lattice ion, they lose energy and emit photons (light quanta), detectable in the laboratory.

As time goes on, the ionizing radiation field around the material causes the trapped electrons to accumulate (Figure 2).

In the laboratory, the accumulated radiation dose can be measured, but this by itself is insufficient to determine the time since the zeroing event.

Optically stimulated luminescence dating is a related measurement method which replaces heating with exposure to intense light.

Figure 1 : The three stages of thermoluminescence as outlined by Aitken (1985, 1998) and applied to a quartz grain (Keizars, 2008b)
Figure 2 : The process of recharging and discharging thermoluminescent signal, as applied to beach sands. (modified from Aitken, 1998; Keizars, 2008a)
Figure 3 : Thermoluminescence signature lost during migration of two sand grain sizes (Keizars, 2008).
Figure 4 : Illustrated method of passively monitoring sand input (Keizars, 2003).
Typical quartz TL curve measured with a UV filter during routine TL dating.