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THERMOLUMINESCENCE
Thermoluminescence literally means the
emission of light during heating. Many different materials including
glass, gemstones, pottery, rocks and sand can show thermoluminescence
if they have previously been exposed to radiation. They absorb energy
from the radiation which is released in the form of light if they are
heated up.
The amount of light that any of these
materials gives out is proportional to the amount of radiation it has
absorbed. This fact allows thermoluminescence to be used as a tool for
several professions.
For example, the radiation badges worn by
medical staff, nuclear power station workers and some scientific
researchers are analysed using thermoluminescence. The badges contain a
powder that gives out a strong light signal when heated - even if it
has only absorbed a very small dose of radiation. By regularly testing
these badges, it's possible to accurately record how much radiation a
worker is receiving. There are set amounts of radiation that are
considered 'safe' for various types of worker to receive over any given
year, so the badges help make sure no-one ends up exceeding the safety
limit.
By contrast, archaeologists use
thermoluminescence to date pottery and other artefacts, and in the same
way geologists use it to work out the age of sediments. This is
possible because everything on Earth constantly receives a small amount
of radiation known as background radiation. Background radiation comes
from cosmic rays - which continually bombard the Earth - and from the
decay of naturally occurring radioactive elements in rocks and soil. It
is possible to estimate the radiation dose this causes during each
year.
So if we take some grains of sand, say,
and heat them up, the resulting thermoluminesence reveals the total
amount of radiation the sand has absorbed over its entire lifetime. But
as we can estimate how much dose it has received each year, we can work
out how old the sand is simply by dividing the total amount of
radiation the grains have absorbed by the dose they receive each year.
For archaeological artefacts such as
pots, it's useful to know where they were buried. This is because the
amounts of natural radioactive elements in rocks and soil differ from
place to place, so knowing where an artefact was buried enables a
better estimate to be made of the radiation dose it has received each
year. Pottery is a particularly easy thing to date, because when the
pot is fired, the temperature in the kiln is so hot that any
thermoluminescence from the clay the pot is made from is emitted there
and then. This means any light given out when the pot is heated up in a
laboratory can only correspond to the radiation the pot has received
since it was fired.
Thermoluminescence can also be used to
detect art forgeries. A good forger can make a figurine or pot or bowl
look exactly as it should, but it won't have absorbed enough radiation
to give a thermoluminescence signal that corresponds to its supposed
age. The one disadvantage of using thermoluminescence on works of art
is that you need a sample of the thing, so you need a steady hand to
drill very carefully into the base of the object and just extract a few
tiny grains of material.
Finally, thermoluminescence can be used
in scientific research. This is because the light given out in the
thermoluminescence process tells you more than just how much radiation
something has absorbed. Looking at the colour of the light emitted,
what temperature the bursts of light are emitted at and how intense
each burst of light is, can reveal information about the arrangement of
the atoms inside the substance and what impurities are present. For
this sort of analysis, samples of the material under study are
deliberately irradiated - usually with X-rays - in the laboratory prior
to being heated up. This ensures a strong light signal is produced.
For my doctorate I analysed a selection
of different coloured zinc sulphide crystals, some glass from an
Italian company that is used for making car windows, and a variety of
minerals known as fluorites. The thermoluminescence experiments I
carried out were on laboratory based equipment costing hundreds of
thousands of pounds, but the first experiments on thermoluminescence
were somewhat lower tech.
In 1663, Robert Boyle gave the Royal
Society one of the first sensible accounts of thermoluminescence. He
described some experiments he'd carried out on a diamond, saying 'I
also brought it to some kind of glimmering light, by taking it into bed
with me, and holding it a good while upon a warm part of my naked
body'. Primitive though his experiment may sound by today's standards,
it was a huge step forward, as before work by Boyle and his 17th
Century contemporaries, people had thought luminescence was magic!
Copyright Sharon Ann Holgate |
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