Geology Reference
In-Depth Information
1500
75000
A
B
Optically Stimulated
Luminescence
at 340 nm
Thermo-
Luminescence
Glow Curve
1000
50000
500
25000
0
0
0
100
200
300
400
0
5
10
15
20
25
Temperature (°C)
Time (seconds)
Fig. 3.16
Thermal and optically stimulated luminescence.
A. Example of a thermoluminescence “glow curve” that results from the progressive heating at a rate of a few degrees
per second. In this quartz sample, most of the traps are emptied by 400
°
°
C results from optical
stimulation for 25 s at 420-560 nm (shown in B) and is not seen in the natural sample. B. Luminescence signal
recorded at
C. The peak at 100
∼
340 nm due to optical stimulation of the same sample as in A using a light source with wavelengths
ranging from 420 to 560 nm. Modified after Duller (1996).
wavelength and the emitted luminescence is
measured as a function of time (Fig. 3.16B). OSL
has some important advantages over TL that are
making it a generally preferable dating tech-
nique (Aitken, 1998). In particular, TL measures
luminescence signals that are sensitive to light,
heat, and any other types of energetic stimula-
tion, whereas OSL measures only signals that
are sensitive to light. In addition, TL destroys the
signal of interest during measurement and,
therefore, cannot be repeated, whereas OSL can
be applied in short bursts that modify the total
luminescence only slightly, such that multiple
measurements can be made.
Both the number of electrons residing in
“traps” in a crystal and the intensity of
luminescence released in the laboratory due to
exposure to heat or light are a function of the
total dose of radiation received by the sample
over time. If one can quantify that past radiation
exposure,
P
, termed the “paleodose,” and the
rate (
DR
) at which the sample was irradiated,
then an age,
t
, for the sample can be calculated:
or by laboratory measurements of the sample
itself. Ideally, samples should be collected from
the center of homogeneous beds that are at least
60 cm thick, because a 30-cm radius defines the
approximate volume that will produce most of
the radiation received by a sample. Collecting
from such thick, homogeneous beds is espe-
cially important when the dose rate is to be
determined in the laboratory.
The paleodose can be calculated in several ways
(Duller, 1996). The additive dose method relies on
measuring the luminescence resulting from differ-
ent levels of irradiation in the laboratory. A curve
drawn through these points and through the point
defined by the sample's natural luminescence is
extrapolated to zero to estimate the paleodose
(Fig. 3.17A). In the regenerative method, several
subsamples are measured for their natural lumi-
nescence and the remainder are zeroed through
exposure to light, then exposed to known doses of
radiation and remeasured for their luminescence.
The curve developed from these measurements
can be matched against the observed lumines-
cence to estimate the paleodose (Fig. 3.17B). In the
partial bleach method, some subsamples are sub-
jected to the additive dose method, whereas others
are exposed to a short burst of light (partial
bleaching) in order to remove a proportion of
their light-sensitive luminescence prior to meas-
urement. The paleodose is defined by where
these two extrapolated curves cross (Fig. 3.17C).
t
=
P
(Gy)/
DR
(Gy/kyr)
(3.3)
where Gy (grays) is the SI unit for radiation.
The dose rate is typically measured either
in situ
, by leaving a radiation detector for an
extended time (one year, for example) in the
sediment from which the sample was collected,
