Chronological
Methods 12 - Luminescence Dating
Scientists
in North America first developed thermoluminescence dating of rock minerals in
the 1950s and 1960s, and the University of Oxford, England first developed the
thermoluminescence dating of fired ceramics in the 1960s and 1970s.During the
1970s and 1980s scientists at Simon Frasier University, Canada, developed
standard thermoluminescence dating procedures used to date sediments. In 1985,
they also developed optically stimulated luminescence dating techniques, which
use laser light, to date sediments.
The
microscopic structure of some minerals and ceramics trap nuclear radioactive
energy. This energy is in constant motion within the minerals or sherds. Most
of the energy escapes as heat, but sometimes this energy separates electrons
from the molecules that make up the minerals or ceramics. Usually the electrons
will reconnect with the molecules, but some will not. The electrons that dont
reconnect eventually encounter imperfections in the microscopic structure of the
ceramics or minerals, and they become trapped by these imperfections. Over time
energy (in the form of more and more trapped electrons) is stored in these
structural imperfections. By heating the ceramic or mineral to above 500
degrees Celcius, these trapped electrons are released, creating a flash of
light called thermoluminescence. When a laser light source is used to stimulate
the release of electrons, the process is called optically stimulated
luminescence.
Luminescence Profile
In the
process of making a ceramic vessel, the soft clay vessel must be heated in a
kiln to harden it. The process of firing the vessel releases the trapped
electrons (energy), and resets the thermoluminescence clock to zero. The
process of accumulation of electrons (energy) and then release when heated
occurs every time the ceramic vessel is reheated. What an archaeologist would
be able to measure using this technique is the last time the vessel was heated
above 500 degrees Celcius, either at the time the vessel was first fired or the
last time it was heated if it was used as a cooking vessel. In the laboratory,
the release of electrons can be induced through heating or the use of a laser
beam. The intensity of the light emmisions (luminescence) can be measured to
determine the amount of time that has passed since the vessel was last heated
and the present laboratory heating of the vessel.
How is a Luminescence
Sample Processed?
A small
sample is cut out of the artifact being dated. An equivalent dose (DE) of nuclear
radiation is determined for every artifact through the application of
artificial doses of nuclear radiation (through heating or exposure to a laser
light beam) to subsamples of the artifact to scale the signal. Next the burial
dose rate (DR) is determined by measuring the radioactivity in portions of
the sample grains and surrounding sediments. Lastly, the age of the sample is
determined by dividing the equivalent dose by the burial dose rate (DE/DR = Age).
This
method is applicable to samples that range in age from a few hundred years to
several hundred thousand years.
The Limitations of
Luminescence Dating
Using this technique, almost any rock mineral or ceramic sample
can be directly dated. However, it works best when dating heated grains in
ceramics, obsidian, burned flint, and burned sediments.
Links
The Dalhousie Thermally and Optically Stimulated
Luminescence and ESR Laboratory
The Research Laboratory for Archaeology and the
History of Art, Oxford University
Aberystwyth Luminescence Dating Laboratory
The Sheffield Centre for International Drylands Research (SCIDR),
England
Archaeometry Research Group Heidelberg, Germany
The University of Washington Luminescence Dating
Laboratory
References
Aitken,
Martin J. 1985. Thermoluminescence Dating.
Aitken, Martin J. (in press) Optical Dating: A Review for Non-Specialists. Quaternary
Science Reviews
Ollerhead, Jeff, David J. Huntley, and Glenn W. Burger. 1994. Luminescence
Dating of the Buctouche Spit, New Brunswick. Canadian Journal of Earth
Sciences 31: 523-531.
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