How much do X-rays and CT scans affect ESR dating?

The electron spin resonance (ESR) method of geological dating relies upon the idea that newly-formed solids (such as tooth enamel or igneous rock) are affected by natural radiation over time, resulting in unpaired electrons that give rise to a paramagnetic signal. Groundwater and sediments contain low levels of uranium and other radioactive elements, many of which are part of the uranium decay chain. Also, cosmic radiation impacts materials that are buried at shallow depths. Laboratory equipment can measure the signature that results from this natural radiation over long periods of time, and if the rate of natural radiation is known, the present signature can be used to estimate the age.

More and more, ESR has become important to understanding the ages of hominin fossils. It is one of the few methodologies that can apply to biological material older than around 50,000 years. But the technique is complicated for many reasons. The level of uranium in the immediate environment of a fossil tooth can vary greatly over time. The uranium that is absorbed within a fossil matters a lot, and this can be absorbed quickly or slowly.

So any estimate of geological age is based on an assumption that the present-day measurement of radiation is relevant to the past. To understand the uranium history, ESR analysis is usually carried out in conjunction with direct measurements of uranium and its decay products (usually thorium) in the fossil tooth. Results are often reported using a series of different statistical models, including “early uptake”, “linear uptake”, or others.

But ESR on teeth is not just a method for geological dating. It’s also a method for measuring radiation doses experienced by people who have been exposed to X-rays and other artificial forms of radiation.

For example, Ishii and colleagues (1990) examined teeth extracted from people who lived near Chernobyl and were exposed to radiation. Similar approaches have been used to examine the accumulated dose from dental X-rays. Scientists have even applied ESR to assess irradiation of food.

Those applications hint at a big problem for archaeologists who want to know the geological age of fossils. Newly-uncovered fossils straight from the ground are one thing. But when scientists uncover fossils, they often start to subject the fossils to human-induced high-energy radiation. The most common and important of these is X-ray radiation from radiography and CT-scanning. These methods have been very important to studying the morphology of fossil material, but they have strong effects on the overall radiation dose suffered by a fossil.

How much?

Recently, scientists have been concerned whether X-ray radiation from CT scanning may damage the preservation of DNA or other ancient biomolecules. Medical devices have long been tuned to lower radiation levels, but examination of fossils has often been done with industrial or research microCT devices that exert higher radiation doses. Some fossil tooth enamel has been visibly darkened in color by such scanning.

Alexander Immel and coworkers in 2016 investigated the effects of such scanning on ancient DNA preservation in subfossil material. They found that conventional exposures to X-ray radiation used in synchotron scanning caused significant degradation of ancient DNA. Doses in those cases could exceed 2000 Gray. Doses below 200 Gray did not have a substantial effect, and they judged that micro-CT devices would not normally result in doses that damage ancient DNA to an appreciable degree.

That’s good news for most studies of ancient DNA. But it’s sobering to realize that a high enough dose of radiation used in morphological analysis can actually chemically damage ancient DNA. ESR relies upon a much more subtle signal of energy in ancient fossil remains. What effect will smaller radiation doses have on this signal?

A couple of months ago, Science published an exchange of comments responding to a paper published early last year on the hominin fossil from Misliya Cave, Israel. This maxilla was published by Israel Hershkovitz and coworkers with an ESR age estimate between 177,000 and 194,000 years: “The earliest modern humans outside Africa”.

Misliya maxilla, mirrored in a CT rendering
Misliya-1 maxilla, the rendering based on CT data from Gerhard Weber, distributed to the media.

That result was challenged by Warren Sharp and James Paces, who wrote a “Comment on ‘The earliest modern humans outside Africa’”. They raised their challenge based upon a sampling of carbonate crust on the fossil used to calibrate the U-series age and thereby the ESR age.

But Hershkovitz and colleagues, “Response to Comment on ‘The earliest modern humans outside Africa’”, responded more broadly to issues besides the narrow question of the carbonate. In their comment, they observe that the radiation dose of CT-scanning was a significant element of doubt that they did not previously report:

Direct dating of Misliya-1 provided a U-series age of 70.2 ± 1.6 ka and a combined U-series and electron spin resonance (US-ESR) age of 174 ± 20 ka. Because uranium uptake may be delayed after the death of the organism, and because it is difficult to accurately evaluate the radiation effects from previous computed tomography (CT) scanning, these two ages should be regarded as the minimum and maximum age brackets for the fossil, respectively. Misliya-1 was CT-scanned three times prior to the ESR dating analysis (1). The x-ray dose during CT scanning is highly variable (6–8). On the basis of our recent study (7), we could roughly estimate that the total x-ray dose was approximately 30 Gy, resulting in an effective equivalent dose (DE) value of 98.3 ± 5.3 Gy. The corresponding US-ESR age would thus be 152 ± 24 ka (i.e., agreeing within error with the TL results). However, considering the significant uncertainty in the true x-ray dose absorbed by the tooth sample, regarding the cited date of 174 ± 20 ka as a maximum age is the most straightforward and reasonable interpretation of the combined US-ESR result.

That’s an interesting analysis of error, and one that deserves attention. In this case, three CT scans of the Misliya-1 fossil were done. Multiple scans are not uncommon in morphological analysis. It is not at all uncommon to do a medical-resolution CT and then later follow up with higher-resolution micro-CT. Also, it is often difficult to estimate what radiation level will be effective in producing images, because the circumstances of mineralization and preservation vary within a fossil. If the first scan doesn’t produce good results, it may take several trials.

Here, the exact dose of X-rays could not be determined. So Hershkovitz and coworkers provide a “rough estimate” of 30 Gray. The “effective equivalent dose” is an attempt to examine how damaging the actual radiation dose was for the purposes of ESR – this is higher than the total dose because the energy of the X-rays is disproportionately damaging to the ESR signal. In their estimation, the CT scanning results in an overestimate of the age of the fossil by some 15 percent (from 152 ± 24 ka to 174 ± 20 ka).

In this case, even though the ESR estimate must be younger that originally reported, the biological interpretation is not very different. The ESR age estimate is augmented by thermoluminescence results on stone artifacts from the Misliya site that have roughly similar and overlapping age estimates as the maxilla. The interpretation of the fossil depends on the accuracy of its association with this archaeological layer, and the exact value of the ESR estimate makes little difference.

That being said, Misliya-1 is a much better situation than many fossils. Many of the fossils in today’s textbooks were unearthed in the early twentieth century. At that time X-rays themselves were cutting-edge science, and not well understood. Early radiographs used high radiation doses and very slow film, which required long exposures to record an image.

Some of those early fossils were subjected to radiographs many, many times. As the specimens have retained their importance in morphological comparisons, every new advance in measuring morphology required that they be re-studied with the newest methodologies. Countless radiographs from the dawn of roentgenography, medical radiographs, early medical CT-scans, high-resolution medical CT-scans, micro-CT scans.

We’re talking about the pre-1960 era when shoe salesmen used X-rays to fit their customers, and ended up with horrible radiation consequences.

I’m surprised that some fossils aren’t glowing by now. Most scientists do realize this, and it’s why no one has attempted to do ESR dating on some important fossils. The context and subsequent history of them after excavation just has not been recorded to a standard that would allow any accuracy at all.

But no doubt in the future some scientists will irresponsibly attempt to apply ESR methods on fossils without accurate context or full records of radiation subsequent to discovery. As the Misliya case shows, a dose from modern CT-scanning results in the overestimate of the age by 15% or more. A history with many X-rays and CT-scans of unrecorded dosage would have a much greater effect.

I take from this another lesson: It is tremendously important to the future of the science that we share CT data widely and freely. There should be no excuse to replicate unnecessary high radiation doses on fossil material. Institutions should demand that CT scans be lodged in repositories where other researchers can use them freely.