Carbon dating is a variety of radioactive dating which is applicable only to matter which was once living and presumed to be in equilibrium with the atmosphere, taking in carbon dioxide from the air for photosynthesis. Cosmic ray protons blast nuclei in the upper atmosphere, producing neutrons which in turn bombard nitrogen, the major constituent of the atmosphere.
This neutron bombardment produces the radioactive isotope carbon The radioactive carbon combines with oxygen to form carbon dioxide and is incorporated into the cycle of living things. The carbon forms at a rate which appears to be constant, so that by measuring the radioactive emissions from once-living matter and comparing its activity with the equilibrium level of living things, a measurement of the time elapsed can be made.
Presuming the rate of production of carbon to be constant, the activity of a radioactive isotopes in carbon dating can be directly compared to the equilibrium activity of living matter and the age calculated. Various tests of reliability have confirmed the value of carbon data, and many examples provide an interesting range of application. Carbon decays with a halflife of about years by the emission of an electron of energy 0.
This changes the atomic number of the nucleus to 7, producing a nucleus of nitrogen Radioactive isotopes in carbon dating equilibrium with the atmosphere, a gram of carbon shows an activity of about 15 decays per minute. The low activity of the carbon limits age determinations to the order of 50, years by counting techniques.
That can be extended to perhapsyears by accelerator techniques for counting the carbon concentration. Since living organisms continually exchange carbon with the atmosphere in the form of carbon dioxide, the ratio of C to C approaches that of the atmosphere. From the known half-life of carbon and the number of carbon atoms in a gram of carbon, you can calculate the number of radioactive decays to be about 15 decays per minute per gram of carbon in a living organism. Radioactive carbon is being created by this process at the rate of about two atoms per second for every square centimeter of the earth's surface.
The rate of production of carbon in the atmosphere seems to be fairly constant. Carbon dating of ancient bristlecone pine trees of ages around years have radioactive isotopes in carbon dating general corroboration of carbon dating and have provided some corrections to the data. From the dating of ancient bristlecone pine trees from the western U.
Trees dated at BC show the maximum deviation of between and years too young by carbon dating. Prior to carbon dating methods, the age of sediments deposited by the last ice age was surmised to be about years. Krane points out that future carbon dating will not be so reliable because of changes in the carbon isotopic mix.
Fossil fuels have no carbon content, and the burning of those fuels over the past years has diluted the carbon content. On the other hand, atmospheric testing of nuclear weapons in the s and s increased the carbon content of the atmosphere. Krane suggests that this might have doubled the concentration compared to the carbon from cosmic ray production. Accelerator techniques for carbon dating have extended its range back to aboutyears, compared to less than half that for direct counting techniques.
One can count atoms of different masses with a mass spectrometerbut that is problematic for carbon dating because of the low concentration of carbon and the existence of nitrogen and CH 2 which have essentially the same mass. Cyclotrons and tandem accelerators have both been used to fashion sensitive new mass spectrometer analyses. The tandem accelerator has been effective in removing the nitrogen and CH 2and can be followed by a conventional mass spectrometer to separate the C and C These techniques can be applied with a sample as small as a milligram.
Index Beta decay concepts. Carbon Equilibrium Activity Since living organisms continually exchange carbon with the atmosphere in the form of carbon dioxide, the ratio of C to C approaches that of the atmosphere.