Radiocarbon dating (or simply carbon dating) is a radiometric dating technique that uses the decay of carbon-14 (14C) to estimate the age of organic materials, such as wood and leather, up to about 58,000 to 62,000 years Before Present (BP, present defined as 1950). Carbon dating was presented to the world by Willard Libby in 1949, for which he was awarded the Nobel Prize in Chemistry. Since its introduction it has been used to date many items, including samples of the Dead Sea Scrolls, the Shroud of Turin, enough Egyptian artifacts to supply a chronology of Dynastic Egypt, and Ötzi the Iceman.
The Earth's atmosphere contains various isotopes of carbon, roughly in constant proportions. These include the main stable isotope (12C) and an unstable isotope (14C). Through photosynthesis, plants absorb both forms from carbon dioxide in the atmosphere. When an organism dies, it contains the standard ratio of 14C to 12C, but as the 14C decays with no possibility of replenishment, the proportion of carbon 14 decreases at a known constant rate.
The time taken for it to reduce by half is known as the half-life of 14C. The measurement of the remaining proportion of 14C in organic matter thus gives an estimate of its age (a raw radiocarbon age). However, over time there are small fluctuations in the ratio of 14C to 12C in the atmosphere, fluctuations that have been noted in natural records of the past, such as sequences of tree rings and cave deposits. These records allow fine-tuning, or "calibration", of the raw radiocarbon age, to give a more accurate estimate of the calendar date of the material.
One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites.
Presuming the rate of production of carbon-14 to be constant, the activity of a sample 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-14 decays with a halflife of about 5730 years by the emission of an electron of energy 0.016 MeV. This changes the atomic number of the nucleus to 7, producing a nucleus of nitrogen-14. At equilibrium with the atmosphere, a gram of carbon shows an activity of about 15 decays per minute.
The low activity of the carbon-14 limits age determinations to the order of 50,000 years by counting techniques. That can be extended to perhaps 100,000 years byaccelerator techniques for counting the carbon-14 concentration.
How Carbon-14 is Made
Cosmic rays enter the earth's atmosphere in large numbers every day. For example, every person is hit by about half a million cosmic rays every hour. It is not uncommon for a cosmic ray to collide with an atom in the atmosphere, creating a secondary cosmic ray in the form of an energetic neutron, and for these energetic neutrons to collide with nitrogen atoms. When the neutron collides, a nitrogen-14 (seven protons, seven neutrons) atom turns into a carbon-14 atom (six protons, eight neutrons) and a hydrogen atom (one proton, zero neutrons). Carbon-14 is radioactive, with a half-life of about 5,700 years.
For more information on cosmic rays and half-life, as well as the process of radioactive decay, seeHow Nuclear Radiation Works. Carbon-14 in Living Things
The carbon-14 atoms that cosmic rays create combine with oxygen to form carbon dioxide, which plants absorb naturally and incorporate into plant fibers by photosynthesis. Animals and people eat plants and take in carbon-14 as well. The ratio of normal carbon (carbon-12) to carbon-14 in the air and in all living things at any given time is nearly constant. Maybe one in a trillion carbon atoms are carbon-14. The carbon-14 atoms are always decaying, but they are being replaced by new carbon-14 atoms at a constant rate. At this moment, your body has a certain percentage of carbon-14 atoms in it, and all living plants and animals have the same percentage.
Carbon-14 Equilibrium Activity
From the known half-life of carbon-14 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." Levin
The rate of production of carbon-14 in the atmosphere seems to be fairly constant. Carbon dating of ancient bristlecone pine trees of ages around 6000 years have provided general corroboration of carbon dating and have provided some corrections to the data.
Bristlecone Pine Treescarbon dating
Accelerator Techniques for Carbon Dating
magnetic field magnetic force centripetal force
Mass Spectrometer Analysis
A Geiger–Müller counter, also called a Geiger counter, is a type of particle detector that measures ionizing radiation. It detects the emission of nuclear radiation — alpha particles,beta particles, and gamma rays — by the ionization produced in a low-pressure gas in aGeiger–Müller tube, which gives its name to the instrument. In wide and prominent use as a hand-held radiation survey instrument, it is perhaps one of the world's best-known radiation instruments.
The original operating principle was discovered in 1908 in early radiation research.
Since the subsequent development of the Geiger-Müller tube in 1928 the Geiger-Müller counter has been a popular instrument for use in radiation dosimetry, health physics, experimental physics, the nuclear industry, geological exploration and other fields, due to its robust sensing element and relatively low cost.
However there are limitations in measuring high radiation rates and in measuring the energy of incident radiation.