Lighter, faster, cheaper detection of radiocarbon

Carbon-14, carbon’s only naturally occurring radioactive isotope, is rare. It makes up about one part per trillion of Earth’s atmospheric and biogenic carbon stores, and the trace amount found in organic samples forms the basis for accurately dating them. Since the late 1970s, accelerator mass spectrometry (AMS) has served as the benchmark method for measuring such tiny concentrations, but the spectrometers can cost millions of dollars and require a dedicated facility. Four years ago Iacopo Galli and his colleagues at Italy’s National Institute of Optics and the European Laboratory for Non-Linear Spectroscopy adapted an optical tabletop method for the job. Called saturated-absorption cavity ring-down (SCAR) spectroscopy, the method entails first burning the sample to produce carbon dioxide. An optical cavity is then filled with the gas and illuminated with IR laser light tuned to a vibrational transition in ¹⁴C¹⁶O₂. When the light is turned off, the intensity of the light stored in the cavity decreases in time, or “rings down.” After the researchers measure how much of the decrease comes purely from mirror leakage and how much comes from molecular absorption, they can determine the concentration of ¹⁴C in the sample. Galli and colleagues’ latest implementation of SCAR achieves a radiocarbon sensitivity of just 5 parts in 10¹⁵ because of higher-reflectivity mirrors, which increase the effective path length and thus the overall absorption, and an operating temperature of 170 K, among other features. Although SCAR’s sensitivity falls short of that of AMS by a factor of two, its potential portability and short acquisition times—an hour, typically—could make the method practical for use in the field. (I. Galli et al., Optica 3, 385, 2016.)