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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
All text and photo copyright: Sharon Ann Holgate, unless otherwise stated.
