Radiocarbon is carbon-14 (14C): 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. 14C decays into 14N with a half-life of 5,730±40 years, which makes this radioisotope suitable for dating rocks and archaeological items as old as 75,000 years. Assuming that the amount of 14C produced is constant, one may deduce that the proportion of 14C atoms present in living material is uniform throughout time. Knowing the half-life of 14C, examination of the amount of 14C remaining in organic material provides a reasonably accurate method dating. Recent advances in dendrochronology have shown that the production of 14C in the atmosphere is not constant, and corrections have accordingly been made to the radiocarbon system.