Counting atoms to tell time
The Oxford Radiocarbon Accelerator Unit (ORAU) is a world-leading laboratory for determining the age of objects containing organic material. The facility uses a technique called Accelerator Mass Spectrometry (AMS), which has changed the field of radiocarbon dating. Before AMS, scientists had to wait for radioactive carbon-14 (¹⁴C) atoms to decay, a slow process requiring large samples, often several grams of material. This older method, known as beta-counting, was destructive to small or precious artifacts.
AMS operates on a different principle. Instead of waiting for decay, it directly counts the individual ¹⁴C atoms in a sample. The process begins by converting a tiny sample, often as small as a few milligrams—into graphite., This graphite is placed in an ion source, where it is bombarded with caesium ions to create a beam of fast-moving carbon ions. A powerful tandem accelerator, operating at millions of volts, speeds these ions up to extremely high energies. Magnetic and electric fields then act like a giant prism, separating the carbon ions by mass. This allows detectors to count the rare ¹⁴C atoms and compare their number to the stable ¹²C and ¹³C atoms. This direct-counting method is about a thousand times more sensitive than decay-counting, enabling analysis of samples weighing less than a milligram.
From holy relics to ancient humans
The ORAU’s most famous analysis occurred in 1988, when it was one of three laboratories selected to date the Shroud of Turin. In a carefully controlled, blind-test experiment with labs at the University of Arizona and ETH Zurich, Oxford scientists analyzed a small sample of the linen cloth. The results were conclusive: the linen dated from between 1260 and 1390 AD with 95% confidence, indicating a medieval origin.
Beyond this single, high-profile project, the unit has helped clarify countless historical and prehistoric timelines. Researchers at ORAU have dated the remains of King Richard III, recovered from a car park in Leicester, confirming his death in the 15th century. They have also worked on material from extraordinary Bronze Age burials, such as the one discovered on Whitehorse Hill, Dartmoor.
The technology is particularly effective for dating very old, fragmentary remains. The ORAU was part of a major international project analyzing sediment cores from Lake Suigetsu in Japan. This work helped create a more precise calibration curve for radiocarbon dating, extending the reliable timescale back to about 53,000 years. This refined timeline allows archaeologists to more accurately date events like the extinction of Neanderthals and the spread of early modern humans.