From thermodynamics to GPS, from social systems theory to studies of consciousness, time plays an essential role in how we study, interpret, and understand the natural universe and the peoples and technologies that occupy it. Keeping time in particular allows us to understand its passage and the change by which that passage is characterised. The technique called radiocarbon dating brought the first verifiable way to do this to many fields of science, transforming them – and our world – to a significant degree.
What is radiocarbon dating?
‘Dating’ is a method by which the age of an object can be determined. Radiocarbon dating refers to a method that does this using radiocarbon, a name for the isotope carbon-14.
Carbon-14 is created in the earth’s atmosphere when cosmic rays – energetic streams of charged particles coming from sources in outer space – slam into the atoms of the gases and release neutrons. When these neutrons interact with the nitrogen-14 nitrogen isotope, they can produce carbon-14. Since cosmic rays are ceaselessly passing through the earth’s atmosphere, carbon-14 is created constantly there.
Carbon-14 readily combines with atmospheric oxygen to form radioactive carbon dioxide. This compound then enters the bodies of plants (via photosynthesis), animals (when they consume plants), and other biomass through the carbon cycle.
In the early 1940s, the American chemists Martin Kamen and Sam Ruben found a way to synthesise carbon-14 in the lab as well as that its half-life – the time taken to decay to half its original mass – was around 5,000 years, and not a few hours as expected. In 1939, the Finnish-American physicist Serge Korff had found that it’s possible to produce carbon-14 by bombarding nitrogen-14 with neutrons – as cosmic rays do. Inspired by these findings, the American physical chemist Willard Libby is credited with conceiving the idea of using carbon-14 to date organic materials, which he published in the journal Physical Review in 1946.
Notably, Libby’s idea made two assumptions that weren’t exactly known to be true at the time. First, the concentration of carbon-14 in the earth’s atmosphere doesn’t change across thousands of years. If it did, radiocarbon dating – which dates organic materials by measuring the amount of carbon-14 they contain – wouldn’t work.
Second, carbon-14, in the form of carbon dioxide and other carbon compounds, would have to be able to diffuse into the earth’s various ecosystems such that the concentration of carbon-14 in the atmosphere was comparable to the concentration of carbon-14 in the planet’s other biospheres. Some preliminary studies Libby conducted at the time, with his student Ernest Anderson, indicated this was the case.
Fortunately for Libby, scientific studies that came later proved both these assumptions to be valid.
How does radiocarbon dating work?
When an organic entity – like the human body – is ‘alive’, it constantly exchanges carbon with its surroundings by breathing, consuming food, defecating, shedding skin, etc. Through these activities, carbon-14 is both lost from the body as well as replenished, so its concentration in the body is nearly constant and in equilibrium with its surroundings.
When this individual dies, the body no longer performs these activities and the concentration of carbon-14 in the body begins to dwindle through radioactive decay. The more time passes, the more the amount of carbon-14 lost, and the less there will remain. This decay rate can be predicted from theory.
Radiocarbon dating dates an object by measuring the amount of carbon-14 left, which scientists and/or computers can use to calculate how long ago the body expired.
In the late 1940s, Libby and chemist James Arnold tested this technique by dating objects whose ages were already known through other means – including redwood trees (age estimated from tree rings) and a piece of the funerary boat of an Egyptian pharaoh (whose death had been recorded at the time). They found the technique could indeed estimate their ages correctly, and published their findings in the journal Science in 1949.
Since carbon-14 decays with a half-life of around 5,730 years, its presence can be used to date samples that are around 60 millennia old. Beyond that, the concentration of carbon-14 in the sample would have declined by more than 99%.
What are the tools of radiocarbon dating?
The instrument of choice in Libby’s time to study radioactive decay was the Geiger counter. It consists of a Geiger-Muller tube connected to some electronics that interpret and display signals.
The Geiger-Muller tube contains a noble gas, such as helium or neon, and a rod passing through the centre. A high voltage is maintained between the tube’s inner surface and the rod. The gas is insulating, so no current can pass between the two. But when energetic particles (including gamma radiation), such as those emitted during radioactive decay, pass through the gas, they can energise electrons in the gas’s atoms and produce an electric discharge. The persistent voltage could also encourage these electrons to knock off electrons in more atoms, producing a bigger discharge (called the Townsend discharge). This electric signal is relayed to the electronics, where, say, a light may come on in response, indicating that radioactive decay is happening nearby.
Libby and his colleagues built on the Geiger counter to create a device called the ‘anti-coincidence counter’: a sample was surrounded by Geiger counters that had been tuned to ignore the background level of radiation, and the setup was housed inside thick shielding that further subtracted background radiation. To further improve results, the team also purified the sample.
How does modern radiocarbon dating work?
The modern radiocarbon dating setup is more sophisticated, of course. For example, one of the most sensitive dating setups uses accelerator mass spectrometry (AMS), which can work with organic samples as little as 50 mg.
Scientists use ‘regular’ mass spectrometry to isolate ions that have the same mass-to-charge ratio. They begin with a sample – say, a minuscule fragment of bone – and bombard it with electrons to ionise its atoms. Next, they subject the ions to different physical conditions that cause them to separate according to their mass-to-charge ratio. For instance, they can be energised by being accelerated and then deflected by electric or magnetic fields. Ions with different mass-to-charge ratios are deflected to different extents.
AMS adds one more filter to this setup: a particle accelerator that energises the ions a thousand-times more. As a result, isotope ions of the same mass and different ions with the same mass-to-charge ratio also become more separable. In this way, all the carbon-14 from a sample can be isolated and examined to estimate the bone fragment’s age.
An accelerator mass spectrometer at Lawrence Livermore National Laboratory, California, August 2007. | Photo Credit: Public domain
Geiger counters are available to purchase for a few tens of thousands of rupees and can be operated by hand. Particle accelerators require specialised training and skill as well as a few crore rupees, but their utility is equally disproportionate.
For example, AMS has allowed geologists to date rocks by measuring the relative amounts of the strontium-87 isotope. Naturally occurring rubidium-87 decays to strontium-87 with a half-life of 49.2 billion years. Strontium-87 is one of four strontium isotopes and the only one to not also be produced by stars. So measuring the ratio of strontium-87 to any of the other isotopes could yield a rock’s age.
How did radiocarbon dating change science?
According to the American Chemical Society, “radiocarbon dating provided the first objective dating method – the ability to attach approximate numerical dates to organic remains”. For this reason, its effects on the fields of archaeology and geology have come to be called the “radiocarbon revolution”.
Radiocarbon dating allowed researchers to date sites of archaeological importance, check whether two objects found at the same time are equally old, and compare the ages of objects found at far-flung sites.
In essence, it allowed scholars a clearer and measurable view of the past, opening the door to findings whose importance resonate to this day – including the history of human migration, the rise and fall of civilisations, the birth of languages and religions, the evolution of human-animal interactions, and undulations of the earth’s climate.
Radiocarbon dating is also of political significance in India, where researchers and politicians alike have invoked its use to date objects retrieved from temples and mosques.
Scientists have also continued to refine the technique and account for any remaining flaws. For example, in 2018, archaeologists at Cornell University, New York, reported evidence of the radiocarbon cycle deviating from the expected version at certain points between 1610 and 1940. As a result, they said, radiocarbon dating to these periods could be off by around 19 years.
In 2020, researchers from Cyprus, the Netherlands, and Russia reported a way to improve the time resolution of radiocarbon dating – the smallest period of time to which it could date objects – from decades to specific points within a year, using “recent developments in atmospheric science“.
Published - January 01, 2024 10:30 pm IST
