碳-14测年Radiocarbon dating

宇宙射线产生的中子在高层大气中与氮反应生成 ¹⁴C：¹⁴N + n → ¹⁴C + p。¹⁴C 氧化为 ¹⁴CO₂ 进入碳循环，活体生物中 ¹⁴C/¹²C 与大气平衡（约 1.2 × 10⁻¹²）。生物死亡后停止交换，¹⁴C 衰减，年龄 t = 8267·ln(A₀/A)（使用 5730 年半衰期时；常规放射性碳年龄按惯例用利比半衰期 5568 年计算）。由于大气 ¹⁴C 浓度历史上有波动，测得结果需用树轮等建立的 IntCal 曲线校正为日历年龄。

Cosmic-ray neutrons make ¹⁴C in the upper atmosphere: ¹⁴N + n → ¹⁴C + p. It is oxidised to ¹⁴CO₂ and enters the carbon cycle, so living organisms keep a ¹⁴C/¹²C ratio in balance with the air (about 1.2 × 10⁻¹²). After death exchange stops and ¹⁴C declines; the age is t = 8267·ln(A₀/A) with the 5730-year half-life, although conventional radiocarbon ages are by convention calculated with Libby's 5568-year value. Because atmospheric ¹⁴C has varied, results are calibrated to calendar ages with the IntCal curve built from tree rings and other records.

1940 年卡门和鲁本发现 ¹⁴C。1946–1949 年芝加哥大学的利比提出并建立了放射性碳测年法，用已知年代的埃及法老墓木材验证，获 1960 年诺贝尔化学奖。早期用气体正比计数器测 β 衰变，需几克碳；1977 年起加速器质谱（AMS）直接计数 ¹⁴C 原子，只需不到 1 毫克样品。

Kamen and Ruben discovered ¹⁴C in 1940. In 1946–49 Willard Libby at the University of Chicago devised radiocarbon dating and tested it on wood of known age from Egyptian tombs, receiving the 1960 Nobel Prize in Chemistry. Early laboratories counted beta decays in gas proportional counters and needed grams of carbon; since 1977 accelerator mass spectrometry (AMS) has counted ¹⁴C atoms directly, needing less than a milligram.

该方法广泛用于考古学（如都灵裹尸布测定为 13–14 世纪）、古气候研究和海洋环流示踪。化石燃料燃烧释放不含 ¹⁴C 的 CO₂，使大气 ¹⁴C 比例下降（休斯效应）；1950–1960 年代大气核试验又使其几乎翻倍（“核弹峰”），后者被用来测定 1955 年后的生物样品和人体细胞年龄。

It is used widely in archaeology (the Shroud of Turin, for example, dated to the thirteenth or fourteenth century), palaeoclimate research and as a tracer of ocean circulation. Fossil-fuel CO₂, which has no ¹⁴C, has diluted atmospheric ¹⁴C (the Suess effect), while atmospheric nuclear tests in the 1950s and 1960s nearly doubled it (the bomb pulse), which is now used to date biological samples and human cells formed after 1955.