Investigation of the optically stimulated luminescence dating method ...

Essentials of luminescence dating 13
In nature, the filling rate of the traps is low. As the shallow traps loose their electrons
rather quickly, this means that the luminescence signal measured from a natural sample
will be primarily associated with deep traps. Thermal fading will become visible and will
lead to an age shortfall when the age of the sample is significant compared to the lifetime
of the electrons in the traps that are being sampled during the measurement of the natural
luminescence signal.
For reasons outlined in Chapter 3, however, it is also necessary to irradiate the sample in
the laboratory, and to compare the artificial luminescence signals so induced with the
natural signal. In the laboratory, the doses are administered to the samples at a much
higher rate than in nature. Now, the shallow traps will be filled and, owing to the short
time scale over which the experiments are carried out, they may contribute significantly to
the artificial signal. Therefore it is necessary to remove this unstable “contaminating”
luminescence by emptying the shallow traps before the signal is measured. This emptying
is usually accomplished by heating the sample prior to measurement. This treatment is
called preheating and it will be further discussed in Chapter 3, together with the additional
reasons for why it is necessary.
1.4. Anomalous fading
Equation 1.5 describes the expected mean lifetime of an electron in a trap of depth E and
escape frequency s at a storage temperature T. For deep traps and at low temperatures, the
lifetime will consequently be quite large and leakage of electrons from these traps will be
low. However, it has been observed for many materials that the electrons are released
from their traps at a much faster rate than predicted by equation 1.5. This fading of the
luminescence signal is therefore termed ‘anomalous’ (abnormal) fading.
For natural minerals relevant to dating, the result of anomalous fading is an age shortfall,
regardless of whether TL or OSL signals are being used. The effect was first observed by
Wintle et al. (1971) and Wintle (1973), when trying to date feldspars extracted from
volcanic lava with TL. The ages obtained were significantly lower than the accepted ages
for the lava flows. The effect has subsequently been observed and investigated in a
number of studies such as by Wintle (1977), Clark and Templer (1988), Spooner (1992;
1994a), Visocekas (2000) and Auclair et al. (2003).