Wikipedia:Today's featured article/July 8, 2015

Radiocarbon dating is used to determine the age of carbon-bearing material by measuring its radiocarbon, the radioactive isotope carbon-14 (with six protons and eight neutrons). Invented by Willard Libby in the late 1940s, the method soon became a standard tool for archaeologists. Radiocarbon is constantly created in the atmosphere, when cosmic rays create free neutrons that hit nitrogen. Plants take in carbon, including radiocarbon, through photosynthesis, and after an animal or plant dies, it stops exchanging carbon with its environment. Half of the radiocarbon decays every 5,730 years; the oldest dates that can be reliably measured by radiocarbon dating are around 50,000 years ago. The proportion of radiocarbon in the atmosphere was reduced starting in the late 19th century by fossil fuels, which contain little detectable radiocarbon, but nuclear weapons testing reversed that trend, almost doubling it by 1963. Accelerator mass spectrometry has become the method of choice for radiocarbon dating; it can be used with samples as small as plant grains. Libby received the Nobel Prize for his work in 1960.