Cosmic rays striking the atmosphere create carbon-14, a radioactive isotope. Living organisms continuously exchange carbon with the air and therefore contain the same 14C/12C ratio as the atmosphere. When an organism dies this exchange stops and 14C decays with a 5,730-year half-life by beta emission. Libby measured the beta particles in samples using gas proportional counters and calculated the time since death—their age. Radiocarbon dating revolutionized archaeological chronology and studies of geological strata and ice cores, allowing a precise reconstruction of human and environmental history.

Radiocarbon dating assumes an initially constant atmospheric Δ14C ratio and applies the decay law N(t)=N0·e^(−λt) with λ=ln2/5730 yr⁻¹ to estimate sample ages. Libby calibrated his gas proportional counters with modern wood standards and validated the method on known-age tree rings. Subsequent advances—AMS mass spectrometry, H₂/CO₂ gas conversion for higher counting efficiency, rigorous pretreatment (cellulose extraction, removal of carbonates), and atmospheric–oceanic 14C cycle models—led to dendrochronological and coral calibration curves, improving accuracy to ±20-40 years. Beyond archaeology the method underpins paleoclimatology, volcanic eruption chronology, and tracing of ocean circulation.

Libby converted CO₂ to solid carbon, sealed it in 2-inch Al proportional counters, and measured the 156 keV β spectrum under low-background conditions (lead shielding, active charcoal guard). Using Poisson statistics he derived maximum-likelihood ages under the assumption of a constant initial atmospheric 14C/12C ratio (Λ). Subsequent recognition of the Suess effect and nuclear-bomb 14C perturbations necessitated interlaboratory calibration. AMS with Cs⁺ sputter sources and combined magnetic/electrostatic analyzers boosted sensitivity by 10⁴-10⁵, enabling μg-scale compound-specific dating. Modern protocols apply Bayesian statistics (OxCal, BCal) with the IntCal20 tree-ring curve to produce probabilistic age distributions.