Opening

BACKGROUND

A decisive contribution

Lucile BeckLucile Beck is head of the CEA Carbon 14 Measurement Laboratory (Climate and Environmental Sciences Laboratory) Philippe DillmannPhilippe Dillmann is research director at the CNRS and deputy director of the UMR3685 “Nanosciences and innovation for materials, biomedicine and energy” (CEA/CNRS) Karine FromentKarine Froment is Director of ARC-Nucléart

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By Lucile Beck, Philippe Dillmann, Karine Froment

(CEA - CNRS)

  • Artemis

    Preparation of a sample for dating in ARTEMIS, the accelerator mass spectrometer of the LMC14 - © L.Godart/CEA.

  • Artemis 2

    ARTEMIS, the accelerator mass spectrometer of the LMC14 - © L. Godart/CEA.

  • Spectrometre de masse

    Isotopic analyses of samples - © F.Rhodes/CEA.

As Etienne Anheim explains so well in his editorial, heritage is not only a matter of culture, nor is it the preserve of historians, archaeologists or restorers. Physicists and chemists also contribute to the preservation and showcasing of this heritage. This is also the case with the CEA, whose staff have for many years been developing and implementing a range of innovative techniques and technologies on behalf of art, history and archaeology. Their contribution is decisive in advancing knowledge of ancient civilisations, preserving the vestiges of the past, helping with archaeological searches but also understanding the future behaviour of materials, more particularly by obtaining reliable predictions of their alteration.

Most of the techniques employed at CEA (dating, microprobe metallographic analyses and nanochemistry, gamma irradiation, etc.) are derived from its nuclear expertise. They are supported by cutting-edge R&D, intended not only to constantly improve their performance, but also to conceive of new approaches, whether for dating or for treatment. n

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Dating methods

The dating methods used at CEA are based on the measurement of naturally present radionuclides

14C


Carbon 14 (14C) is produced by the interaction between cosmic radiation and nitrogen 14 (14N) in the atmosphere. The 14C atom thus formed oxidises to form 14CO2 carbon dioxide which mixes with the CO2 in the air. 14C is thus part of the composition of all living carbon-based matter and all precipitates of calcium carbonate. At the death of the organism, the exchanges cease and the 14C absorbed throughout its lifetime begins to decay, with a half-life of 5,730 years. The current limit of this method is about 50,000 years and can be used to date the death of carbon-based matter or the formation of speleothems.

238U/230Th


Uranium 238 (238U) is a natural radioactive isotope present in rock and water for example. When dissolved in magma or in water, it is incorporated into the minerals formed from them, such as zircons and apatites (rocks); carbonates both marine (corals, shells) and continental (stalagmites, flowstones, travertines) and even minerals in contact with water, such as the hydroxyapatite of fossil teeth and bones. Once the system is closed, the 238U decays into thorium 230 (230Th) with a half-life of 75,200 years. The limit for this method is about 650,000 years.

40K/40Ar and 40Ar/39Ar


Minerals and rocks contain a small quantity of naturally radioactive potassium 40 (40K). This potassium decays into a noble gas, argon 40 (40Ar). After crystallisation, the 40Ar produced by the 40K contained in the rock accumulates in the crystalline lattice. The half-life of 40K is very long, at 1.25 billion years. This method is thus applicable to samples of any age (1000 years to 4,556 billion years) but is limited to minerals rich in potassium and/or igneous rocks. The 40Ar/39Ar method is based on the same radioactive phenomenon. The difference is that the rock samples are irradiated in order to transform the 39K into 39Ar and allow more precise measurement, with less material.

Thermoluminescence (TL)


The luminescence and electron spin resonance (ESR) methods, here simplified as (TL) are based on the accumulation over time of electrons trapped in flaws in the crystalline lattice of the sample under the effect of natural ionising radiation. The total number of trapped electrons depends on the radiation dose absorbed by the sample, also called the paleodose. It is proportional to the intensity of the natural radioactivity (natural dose rate) and the irradiation time. In the simplest case, when the natural dose rate does not vary over time, the age of the sample is equal to the ratio of the paleodose to the annual dose (age = paleodose / annual dose).