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.