Investigation of the optically stimulated luminescence dating method ...

OSL from quartz 23
Figure 2.4: Example of a shine-down curve – the measured luminescence as a function of stimulation time.
The observed signal can be seen to decay with time. The commonly accepted explanation
for this decay is that the number of trapped electrons is progressively depleted during
stimulation. When the stimulation is for a long enough period, all the light-sensitive traps
will be emptied. The total luminescence recorded (the area beneath the shine-down curve,
or ‘light-sum’) is proportional to the number of electrons that was trapped (and hence to
the accumulated dose). Because the probability of a photon interacting with a trapped
electron is proportional to the total number of trapped electrons present, this is also true
for a short exposure to stimulation light. The latter is called a ‘short-shine’ and involves
consequently the sampling of only a fraction of the total population of charge trapped.
The OSL signal is usually recorded at a constant stimulation power, i.e. the rate at which
the stimulating photons arrive is kept constant. As was previously mentioned in Chapter 1,
the rate of electron eviction, and hence the decay rate of the shine-down, is dependent on
the stimulation power. This dependency is illustrated in Figure 2.5. The dashed curve
represents the luminescence measured at half of the stimulation power used to measure
the solid curve. It is clear that doubling the stimulation power initially leads to doubling of
the measured signal, and that the signal decays more rapidly. The light-sum is the same
for both curves.
OSL signal (counts / 0.16 s)
60000
50000
40000
30000
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0
10 20 30 40
Stimulation time (s)