Nuclear Time Shift

by Nicholas Mee on September 3, 2020

The White Horse of Uffington has been galloping over the rolling hills of southern England for many years. But for just how long? 

The White Horse of Uffington.

The age and origin of this elegant abstract horse cut into the Oxfordshire chalk downs provoked a heated debate that dragged on for centuries. Some claimed it was carved on the instructions of Alfred the Great to celebrate his victories over the Danes. Some claimed it was the work of an even earlier Anglo-Saxon king. Others argued it must date to the century before the Roman invasion, and pointed to its similarity to images on pre-Roman gold coins found nearby. But without contemporary written records, how could such a question be answered?

Radiocarbon dating has revolutionised archaeology. But it is only suitable for dating organic material. If we had a sample of the white horse’s droppings, then it would certainly help. Unfortunately, chalk horse droppings are as rare as rocking horse droppings. The answer lies elsewhere in nuclear physics and it is tied to a handy little device designed to protect workers in the nuclear industry.

Radiation Exposure

Many occupations involve an increased risk of exposure to potentially damaging radiation. These include workers at nuclear power plants or nuclear fuel reprocessing facilities and some research physicists, but also dentists, radiographers and other medical staff who operate X-ray machines, and doctors and nurses who deliver nuclear medicines. Many mine workers are exposed to increased levels of radon gas, and this does not just apply to those mining uranium. The technique of neutron logging used in the oil industry also has associated risks of exposure to radiation. Even airline pilots and their cabin crew receive additional exposure to cosmic rays when flying above the protective cloak of the atmosphere. Radiation exposure has a cumulative effect, but it is invisible, so monitoring this exposure is a serious challenge.

In 1954 Farrington Daniels at the University of Wisconsin-Madison invented an ingenious device that would solve the problem. Crystal lattices are formed of regular arrays of vast numbers of atoms, but they always include some impurities and defects that disrupt the regularity of the crystal. When a gamma ray photon hits a crystal it collides with numerous electrons within the crystal boosting them into excited states associated with impurity atoms where they remain trapped. Daniels realised that subsequent heating of the crystal to somewhere in the region of 100-200 degrees Centigrade enables the trapped electrons to fall back into lower energy states releasing energy as photons of visible light. By counting these photons it is possible to determine how many electrons were trapped and therefore the amount of ionising radiation the crystal has been subjected to. This is the physical basis for the thermo-luminescent dosimeter (TLD) invented by Daniels that is used by workers in many industries. The TLD is usually supplied as a small clip-on device, as shown here.

Clip-on thermo-luminescent dosimeter (TLD).

The TLD uses crystals such as calcium fluoride that are doped with small quantities of manganese, magnesium or other atoms. Different crystals may be used to monitor exposure to specific types of radiation. (Lithium fluoride crystals are used to monitor exposure to neutrons. A lithium-6 nucleus will absorb a neutron and split into a helium-4 nucleus and a tritium nucleus, which subsequently decays into helium-3. The energy released in these reactions promotes electrons into trapped states.)

Crystal Clocks

Farrington Daniels

Farrington Daniels and his colleagues Charles A. Boyd and Donald F. Saunders suggested in the 1950s that thermo-luminescence might also help determine the age of pottery shards and other long-buried archaeological artefacts. Applying such methods to the dating problem faced by archaeologists is like the flip-side of monitoring modern-day radiation hazards. When a worker is exposed to radiation we want to know the total radiation dose over a period of time. But if we could measure the total radiation exposure of an archaeological specimen and we knew the rate at which radiation damage occurred, then the time taken to reach the measured total could be calculated. Exposure to daylight resets the clock as photons in sunlight de-excite any trapped electrons. So this method could potentially determine the time that has elapsed since an artefact was buried.

By the 1980s this idea was developed into a technique known as Optically Stimulated Luminescence Dating. This method uses light rather than heat to de-excite the electrons in sample rock crystals such as quartz and feldspar. The number of emitted photons is then measured with a photo-multiplier tube to determine the accumulated radiation dose of the archaeological artefact. 

The background rate at which radiation damage has accumulated is determined by measuring the radiation due to radioactive elements, such as uranium, thorium and potassium, within the artefact and its surroundings, and by estimating the additional contribution due to cosmic rays. Dividing the accumulated total by the background rate gives the length of time since the artefact was last in direct sunlight. Exposure to daylight for as little as one hundred seconds will reset the clock, so the samples must be taken using opaque core tubes and then analysed in dark room conditions to determine their radiation exposure. This technique now has widespread archaeological applications and is reliable for materials that are up to 150,000 years old.

Looking Back in Time

In 1995 the Oxford Archaeological Unit decided to see whether Optical Stimulated Luminescence Dating would settle the debate about the White Horse of Uffington. Over the centuries, the horse has been cleaned and maintained by many generations of local people, so samples were taken from the lowest disturbed layers of chalk beneath the horse.

When the results came back they were a surprise to everyone. The white horse is much older than had been imagined. The laboratory analysis revealed that the chalk horse was originally dug some time between 1400 BC and 600 BC, so 3000 years ago plus or minus 400 years. This places its construction some time in the late Bronze Age or early Iron Age, making it easily the oldest chalk figure in England.

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