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Radiometric Dating

Radioisotopes are present in most elements. These are unstable forms of elements which will spontaneously decay to another form of the same element, an element close to it in the periodic table, or split into two or more decay products much lower in atomic number.

All transuranic elements (i.e. Z > 92) are highly unstable, and consequently do not occur naturally on the Earth. Other radioactive isotopes, such as uranium-235 and uranium-238, were formed prior to the coalescence of the solar system, during solar formation after a supernova. Hence, the original quantities are at least as old as the solar system. Since they have decayed at different rates, over time the proportions of the two isotopes has changed, providing a useful and remarkably accurate measurement of the age of the planet.

Other radioisotopes measure shorter spans of time.

Carbon-14

A very useful radioisotope for archeology is carbon-14. Radiocarbon dating was invented by Willard Libby in the 1940s, for which he received the Nobel Prize in Chemistry in 1960.

Carbon-14 is formed when nitrogen in the atmosphere is bombarded by cosmic rays.

$ {\table {1};{0}}$n + $ {\table {14};{7}}$N $ → {\table {14};{6}}$C $ + {\table {1};{1}}$H$^+$

The rate of transmutation is effectively constant, so carbon-14 exists in a natural balance with the more stable carbon-12. Hence, a living organism absorbs the two isotopes in proportions equal to their relative environmental abundances. However, upon death, if the carbon is not released through oxidation back into the atmosphere, but remains locked within a specimen, then the radioactive carbon-14 will decay according to its half-life. The residual proportions of carbon-12 and 14 will therefore provide an accurate yardstick to the age of dead organic matter.

${\table {14};{6}}$C   ${→}↖{β-} $   $ {\table {14};{7}}$N   $ + \ov{ν}_e$

The half-life (t1/2) of carbon-14 is 5730 years. After 5730 years, there will be half of the carbon-14 remaining compared to the time of death. After another 5730 years, there will a quarter, and so on. the limits of accuracy are reached after about 9-10 half-lives (beyond about 50,000 years), at which point not enough of the carbon-14 remains for there to be sufficient certainty about the age. This timespan neatly covers modern human history. Archeologists refer to the introduction of radiocarbon dating as a 'revolution' in archeology.

The technique has been refined gradually, to take into account variations in C-14 through time, which can be used to form a calibration curve. Different types of organisms and biosphere reservoir effects also induce variations in the radiometric record. Since the nuclear testing of the 1940s and 1960s, the atmosphere has been dosed with radionuclides, doubling the amount of C-14 in the air we breathe, further complicating the technique. The industrial age has added fossil fuel emissions containing carbon.

The latest instruments use accelerator mass spectrometry (counting the actual atoms) rather the previous beta decay detectors, to obtain more accurate measurements.

Other radioisotopes, such as beryllium, are useful for measuring the age of air in pockets trapped in ancient ice cores. This is used by climatologists to date samples to determine the changes to atmospheric gases through the ages.

Thorium

There are two types of radiometric dating possible involving thorium: uranium-234 to thorium-230 (ionium), and thorium-232 to thorium-230.

Thorium-232 (half-life = 14 billion years (age of the universe)) is a primordial radioisotope. Thorium-230 (half-life = 75,400 years) occurs only as part of the decay chain of U-238 (on its way down to stable lead).

Uranium-thorium dating is used for measuring the age of calcium carbonate materials, such as corals. Uranium is soluble in water, but thorium is selectively deposited as sediment on ocean floors, where it is measured. The range of the dating method is up to half or more million years. There are necessary assumptions about the presence of thorium in sediments before the precipitation of carbonate residues.

Content © Andrew Bone. All rights reserved. Created : December 24, 2015 Last updated :December 24, 2015

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