Radiocarbon, or ¹⁴C, is the unstable, radioactive isotope of carbon having atomic number 6 and atomic mass 14. It is continuously produced in Earth's upper atmosphere by cosmic ray bombardment. In space, ¹⁴C is produced when cosmic rays interact with oxygen in silicate minerals in rocks. The half-life of ¹⁴C is 5730 years, which makes this radioactive isotope suitable for dating rocks and archaeological items as old as about 75,000 years.