Carbon 14 - the post-bomb effect

As early as the late 1940s, American researchers began using the properties of natural carbon-14 radioactivity to date organic matter. In the 50s, Nobel Prize-winning chemist Williard Franck Libby successfully dated Egyptian samples.

Today, carbon-14 dating (also known as radiocarbon dating) determines the time elapsed since the death of a living organism. This method has revolutionized archaeometry thanks to its ability to date wood, ivory, bone, teeth, linen, straw and all other organic materials.

The principle of carbon-14 dating

Carbon-14 is a radioactive isotope of carbon that disappears over time, so this method is based on measuring this quantity of carbon-14 present in the organism.

A living organism contains a constant quantity of carbon-14. When it dies, exchanges with the outside world cease and the quantity of carbon 14 decreases according to a known exponential law. We know that carbon-14 concentration is halved every 5730 years. Dating is therefore based on measuring the ratio of C14 to total carbon. It is possible to date organisms up to 60,000 years after their death; beyond that, the quantity of carbon 14 will be too low to be measured.

The measurement is carried out by mass spectrometry coupled to a particle gas pedal (AMS). This technique requires very little material (around 0.01g vs. 1g previously), little analysis time (less than an hour vs. several weeks previously) and brought more accurate measurements than before.

The calibration of results

The amount of carbon 14 remaining in a material is proportional to its age. Radiocarbon age is expressed in BP (before present) years. What is known as "carbon 14 present" was fixed by Libby in 1950, and radiocarbon age is calculated on the assumption that C14 concentration has been constant over time, which is not the case.

In fact, carbon-14 content varies according to several factors such as solar activity, climate change or industrial activity. The results obtained must therefore be corrected, a process known as calibration. CIRAM laboratories use calibration curves to transform the BP age into a calibrated date interval, which is then associated with a probability percentage. Only this result will have any scientific significance.

The recent past, the post-bomb effect

The nuclear bombs on Japan in 1945 and atmospheric nuclear testing in the following years led to massive radiocarbon production from 1954 onwards, until its concentration almost doubled in 1964 and 1965. This makes it possible to draw a strict line between organisms that lived before and after 1954. This boundary is ideal for spotting recent fakes. The main problem encountered concerns living beings that lived after 1954, with abnormally high 14C concentrations far exceeding the "normal" concentration. This is why carbon-14 dating is so relevant for recent objects, even very recent ones. Indeed, for the post-1954 period, dating accuracy is only a few years.

But there is an ambivalence to the accuracy of the results. Concentration rises more rapidly until 1965, after the 1963 partial nuclear test ban treaty. After this date, radiocarbon content decreases again, with a gradual return to natural isotope ratios. Calibration is therefore complex, depending on the date of the materials to be analyzed. This phenomenon also biases the a posteriori probability distribution, as the "slope" before the maximum is "steeper" than after. This fact therefore induces a probability distribution artificially more in favor of the post 1964-1965 part.