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Radiodating Techniques

Radiometric dating uses knowledge of the decay rates of unstable nuclei to determine, by comparison with the proportion of stable nuclei, the date of the introduction of the radioactive material.

Techniques vary by isotope measured and method of calibration and verification. About forty different dating techniques are used on a variety of substances, from wood to meteorites.

Uses include estimating the age of archeological sites; determining evolutionary rates; and estimating the age of fossils, rocks, and of the Earth itself.

Carbon dating
- The unstable isotope carbon-14 is renewed in the upper atmosphere by cosmic rays, specifically, neutrons striking nitrogen. Living organisms ingest the atmospheric ratio of carbon-12 to carbon-14, and their remains would maintain this ratio if not for the radioactive decay of the carbon-14.
  
  Because the atmospheric ratio of carbon-12 to carbon-14 is not a constant, calibration is required. Tree rings and glacial ice core samples contain atmospheric carbon ratios thousands of years old. Using one or the other as calibration, one can estimate the age of organic remains as far back as 26,000 years ago.
Age of rocks: the uranium-radium-lead connection
- It was noted by Bertram Boltwood in the first decade of the 20th century that the ratio of uranium to lead compares favorably between layers of rock of similar age, and that older layers have a higher proportion of lead.
  
  Uranium-238 decays to the stable isotope lead-206, but there are 13 intermediary decays, the whole chain consisting of 8 alpha emissions and 6 beta emissions. Radium accumulates at an intermediary step. So the ratio of radium to lead and radium to uranium also can be used to estimate and corroborate each other's age.
Age of the Earth: lead-lead dating
- While uranium-lead ratios have been used to determine the age of the Earth, confidence in their accuracy has been limited, due to the probable molten state of early Earth, recycling of the earliest crust, and subsequent erosion. Therefore the focus has instead been on end-products, as well as meteorites, presuming meteorites and the Earth were formed at close to the same time.
  
  The technique uses the fact that uranium-238 decays to lead-206, that uranium-235 decays to lead-207, and that lead-204 is not of radioactive origin, as a normalizing factor in calculations. A technical explanation for the procedure of lead-lead dating can be found in the Talk Origins links referenced below. The animation shows that the technique is sufficiently sophisticated to account for the difference in uptake rates of isotopes by different minerals.
  
  Meteorite dust on the lunar surface has corroborated the findings.

Rocks & Minerals