Electron Spin Resonance (ESR) dating method

Crystal lattices, trapped charges and spinning electrons. They all form a part of the ESR dating method.

Electron spin resonance (ESR) dating is a trapped charge dating method. The other trapped charge dating methods are thermoluminescence dating (TL) and optically stimulated luminescence (OSL). However, only ESR can be applied to teeth and is therefore used for the direct dating of human fossils.

Trapped charge refers to the behaviour of the electrons within the material being tested. Here comes the science…

Crystalline materials have their atoms arranged in an ordered structure called a lattice. The structure of these lattices determines many of the physical properties of the material. The lattice has imperfections, or defects, where the arrangement of the molecules is not perfect – atoms may be missing in places, for example, creating holes in the lattice. The surrounding structure is strong enough that this doesn’t cause any collapsing of the crystalline structure.

These defects often have an electric charge. If it’s positive, we can trap negatively charged electrons. When the mineral is formed, all the electrons are where they belong: whizzing around atoms. However, with radioactive irradiation (like the one we use for killing off cancer cells in the hospital), electrons can be removed from the atoms and go on a journey through the crystal lattice. Some of the electrons get trapped at these defects. Here we can measure them with ESR. Now, the number of trapped electrons is proportional to the number of defects, the strength of the radioactivity, and time (= the age of the sample).

We need teeth

For ESR dating, we need teeth. They preserve the ESR signal in the hard minerals of the enamel. When we dig out a tooth at an archaeological site, it has an ESR signal. That signal was generated over the time the tooth was buried, from the radioactivity in the sediment around the tooth (external component) and the radioactivity within the tooth itself (internal component).

So how can we get to the age of the tooth?

We have to measure two things: firstly, all radioactive sources in the tooth and its environment – that is the dose rate. Then we do the same in the laboratory that nature did in the past: we artificially age the tooth by irradiating the tooth at the defined doses of gamma rays (as we do in the hospital with the cancer cells). That tells us the dose that the natural signal presents – the total dose of radiation absorbed by the tooth since it has been buried in sediment. The age results by simply dividing the dose over the dose rate – representing the duration of the exposure of the tooth to natural radioactivity.

Usually, the ESR measurements are carried out on enamel powders. Of course, we can’t take the enamel from rare fossil teeth and turn it to powder. Rainer Grün developed a technique where we separate a cracked piece of tooth enamel, measure the piece and then restore it to the tooth from which it was taken. You’ll see this at work in the video we have put together for you. The tooth enamel fragment is then measured. The orientation of the fragment has an effect on the ESR signal, so the fragment is rotated during measurement. It’s more complicated and time consuming than measuring powder.

It may help to use another analogy – this time we’ll use a bathtub.

Imagine that the tooth is a bathtub and the radiation flowing into the tooth is the flow of water into the bathtub from a tap. Over time, the constant flow of water will fill the bathtub. We can measure the volume of water in the bathtub when we find the bathtub. We can also measure the flow of water into the bathtub. If we then divide the volume of water in the bathtub (V) by the flow (F) – we can calculate how long it took to fill the bathtub.

Similarly, if we can determine the total dose of radiation in the tooth (the volume of water in the bathtub), and the dose rate (the flow of water into the bathtub) – then we can calculate how long the tooth has been exposed to natural radioactivity.

Of course, we are making some assumptions with our bathtub:

• Was the bathtub empty when the process began? (Were there electrons already trapped in the tooth at the death of the animal?)
• Did the flow of water into the bathtub change at any time? (Did the dose rate into the tooth vary over the millennia?)
• Are there any leaks in the bathtub? (Was the tooth subjected to processes that changed the level of electrons trapped within the lattice – such as heating?)
• Did anyone add water to the bathtub later? (Has the tooth sample been irradiated after sampling?)

Using the analogy of a bathtub – potential issues appear on the column to the right

Complicated calculations

The calculation of the dose rate is actually quite complicated. The vast majority of radioactivity in nature derives from three elements: uranium, thorium and potassium (we use the radioactive isotope K-40 for dating as well, but this is another story). The challenge here is that the concentrations of radioactive elements within the sample are usually very different from its surroundings – which means that we need to assess the internal dose rate and the external dose rate separately.

The external dose rate needs to be calculated from any sediment attached to the fossil, along with samples from the site where the fossil was located, which is not always possible. Even the fact that the teeth may be located within their original jaw adds complications, as the jawbone will both shield the tooth from environmental radiation – and add its own radiation to the tooth. In many cases we are forced to reconstruct this external dose rate from museum samples, which adds the possibility of very large errors.

The internal dose rate is calculated using the same principles as U-series dating – the equilibrium between the uranium and the thorium. And with that we inherit the same challenges that require us to model the uranium uptake of the fossil. We do this by combining ESR and U-series dating results on the tooth. Both dating methods depend on U-uptake, but to a different extent. As we know from school, if we want to solve for two unknowns, we need two independent equations. Here the two unknowns are the age of the sample and the way the uranium migrates into the sample (that is described by a one-parameter diffusion equation). By putting everything together we can solve for the age and the diffusion parameter. Does that sound complicated? It should – the equations run over several pages. Don’t worry, we have computer programs that do the hard math.

We asked Mathieu to give us a quick summary of the process of ESR dating.

Of course, the dates only make sense if all our assumptions are correct. This is tricky when we work, for example, on museum material to reconstruct the environmental radioactivity of the tooth. Does it really come close to the original environment of the tooth?

You can see once again that the science of the dating method is asking a lot of questions and posing many challenges. Our theories raise questions with implications that we understand and acknowledge – but are not always able to answer (yet).

In the laboratory video we show you the preparation of two samples. One is a powdered ancient horse tooth, and the other is a piece of enamel from a human tooth. You will see how Mathieu prepares both samples and uses the ESR spectrometer to measure them.

Your task

Well, you made it through a lot of science here. As you probably suspect, there is a lot more underpinning the process and the modelling.

As we said when we looked at the limitations of dating methods, when it comes to the general reliability of dating methods other than radiocarbon, we still have a long way to go. But that’s the theory behind ESR dating, and where we are at with its application.

What do you think of the ESR dating method?

Have any questions for Rainer or Mathieu?

Select the comments link below and share your thoughts.

References

Aitken, M.J. (1990). Science-based dating in Archaeology. Longman Inc., New York.

Duval, M. (2014). “Dating fossil teeth by electron paramagnetic resonance: how is that possible?” Spectroscopy Europe 26(1): 6-13.

Grün, R. (1989). “Electron Spin Resonance (ESR) Dating”, Quaternary International, 1: 65-109.

Grün, R. (2006). “Direct Dating of Human Fossils”. Yearbook of Physical Anthropology, 49: 2-48.