Investigation of the optically stimulated luminescence dating method ...

OSL from quartz 27
Figure 2.7: OSL signal (logarithmic scale) as a function of stimulation time (linear scale). The shine-down
measured is shown as the solid line. The dashed curve shows what a single exponentially decaying signal
would look like, having the same depletion rate as during the first 1 s of stimulation. The signal is the
natural OSL signal from sample G7 (Grubbenvorst) and was measured at 125°C after a preheat of 10 s at
240°C, using the blue diodes for stimulation.
The slower-than-exponential decay can be explained by a number of causes (Aitken,
1998):
OSL signal (counts / 0.16s)
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1000
100
1) two or more traps are contributing to the observed OSL signal. These traps have
different eviction probabilities and hence different depletion rates. In other words:
the signal consists of different components. It is not necessary that each of these
components then decays exponentially, as other processes (see e.g. #2 or #3) might
still be operative;
2) some of the evicted electrons get trapped again in other OSL traps, instead of
recombining directly. They are evicted later on, together with electrons released
for the first time, as the stimulation process continues;
3) instead of being retrapped in the OSL traps, the electrons can get trapped
temporarily in other traps. Subsequently they can be thermally evicted and give
rise to a delayed signal;
5 10
Stimulation time (s)
4) the probability that an electron produces a photon changes during the stimulation
(= the luminescence efficiency changes). This can be due to the retrapping of