Investigation of the optically stimulated luminescence dating method ...

Essentials of luminescence dating 9
conditions of impurity content and heat treatment that a more complete picture can be
obtained. Nevertheless, the main features of the mechanism by which luminescence is
produced, and of how the phenomenon can be exploited for dating purposes, can be
understood from the simplified representation given above. It can be added that other
types of insulators (such as glasses), as well as semi-conductors also exhibit
luminescence; metals, on the other hand, do not. For more complex and detailed accounts
on the physical theory, reference is made to McKeever (1985), Chen and McKeever,
(1997) and Bøtter Jensen et al. (2003a).
1.3. Signal growth and trap stability
From the previous section it is clear that the intensity of the luminescence signal is
proportional to the number of electrons trapped: the longer the irradiation time, the more
electrons will have become trapped and the higher the luminescence intensity. There are
however two limitations. First of all, the total number of traps that is available for storing
the charge is limited. Consequently, under continuous irradiation, the available traps are
gradually filled and eventually saturation will be reached. A second limitation is that
electrons are also spontaneously evicted from their traps, a process which is termed
‘thermal fading’ (see later).
Taking both effects of thermal fading and saturation into account, the growth of the
luminescence intensity (I) as a function of time can be described by the following
equation (Wagner, 1995):
with:
I (t)
•
= S×
D × τ ×
( 1 − e
S = the sensitivity; the amount of luminescence per unit of dose
•
D = the dose rate; the dose received per unit of time
τ = the apparent mean lifetime
t = the time span of signal accumulation
t
−
τ
)
(1.1)