radioactive dating

dating, radioactive (radiometric dating) Any of several methods using radioactive decay to assess the ages of archaeological remains, fossils and rocks. The specimens must contain a very long-lived radioisotope of known half-life (time taken for one half of its nuclei to decay), which, with a measurement of the ratio of radioisotope to a stable isotope (usually the decay product), gives the age. In potassium-argon dating, the ratio of potassium-40 to its stable decay product argon-40 gives ages more than 10 million years. In rubidium-strontium dating, the ratio of rubidium-87 to its stable product strontium-87 gives ages to several thousand million years. In carbon dating, the proportion of carbon-14 (half-life 5730 years) to stable carbon-12 absorbed into once-living matter gives ages to several thousand years.

radioactive age dating A technique for measuring the ages of rocks and minerals from the decay of certain radioactive elements within them; also known as radiometric dating. The technique involves comparing the amount of a long-lived radioactive parent isotope in a sample with the amount of the daughter isotope into which it decays. Isotopes used in radioactive age dating include uranium-238, which decays to thorium-230 with a half-life of 4.5 × 10⁹ years; uranium‐235, which decays to protactinium-231 with a half-life of 7.0 7times; 10⁸ years; potassium-40, which decays to argon-40 with a half-life of 1.27 × 10⁹ years; and rubidium-87, which decays to strontium-87 with a half-life of 4.88 × 10¹⁰ years.

radiometric dating (radioactive dating) The most precise method of dating rocks, in which the relative percentages of ‘parent’ and ‘daughter’ isotopes of a given radioactive element are estimated. Early methods relied on uranium and thorium minerals (see URANIUM-LEAD DATING), but potassium-argon, rubidium-strontium, samarium-neodymium, and carbon-14—carbon-12 are now of considerable importance. Uranium-238 decays to lead-206 with a half-life of 4.5 billion (10⁹) years, rubidium-87 decays to strontium-87 with a half-life of 50.0 billion years, and potassium-40 decays to argon-40 with a half-life of 1.5 billion years. For carbon-14 the half-life is a mere 5730 ± 30 years (see RADIOCARBON DATING), and beyond about 70 000 years the amount of carbon-14 remaining in organic matter is beyond accurate measurement. Compare INITIAL STRONTIUM RATIO.