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Analysis

Materials measured by the radiometric technique are analyzed by synthesizing sample carbon to benzene (92% C), measuring for 14C content in a scintillation spectrometer, and then calculating for radiocarbon age. If the Extended Counting Service is used, the 14C content is measured for a greatly extended period of time. AMS results are derived from reduction of sample carbon to graphite (100 %C), along with standards and backgrounds. The graphite is then sent for 14C measurement in an accelerator-mass-spectrometer and verification, isotopic fractionation correction, calendar calibration, and reporting.

The Radiocarbon Age and Calendar Calibration

The "Conventional C14 Age" is the result after applying C13/C12 corrections to the measured age and is the most appropriate radiocarbon age. Applicable calendar calibrations are included for organic materials and fresh water carbonates between 0 and 20,000 BP. If certain calibrations are not included with a report, the results were either too young, to old, or inappropriate for calibration.

The 14C Method

    There are three principal isotopes of carbon which occur naturally - C12, C13 (both
    stable) and C14 (unstable or radioactive). These isotopes are present in the following
    amounts C12 - 98.89%, C13 - 1.11% and C14 - 0.00000000010%. Thus, one carbon 14 atom
    exists in nature for every 1,000,000,000,000 C12 atoms in living material. The radiocarbon
    method is based on the rate of decay of the radioactive or unstable carbon isotope 14
    (14C), which is formed in the upper atmosphere through the effect of cosmic ray
    neutrons upon nitrogen 14. The reaction is:

                               14N + n => 14C + p

    (Where n is a neutron and p is a proton).
    The 14C formed is rapidly oxidised to 14CO2 and enters the earth's plant and animal
    lifeways through photosynthesis and the food chain. The rapidity of the dispersal of C14
    into the atmosphere has been demonstrated by measurements of radioactive carbon
    produced from thermonuclear bomb testing. 14C also enters the Earth's oceans in an
    atmospheric exchange and as dissolved carbonate (the entire 14C inventory is termed
    the carbon exchange reservoir (Aitken, 1990)). Plants and animals which utilise carbon
    in biological foodchains take up 14C during their lifetimes. They exist in equilibrium with
    the C14 concentration of the atmosphere, that is, the numbers of C14 atoms and
    non-radioactive carbon atoms stays approximately the same over time. As soon as a
    plant or animal dies, they cease the metabolic function of carbon uptake; there is no
    replenishment of radioactive carbon, only decay. There is a useful diagrammatic
    representation of this process given here

    Libby, Anderson and Arnold (1949) first discovered that this decay occurs at a constant
    rate. They found that after 5568 years, half the C14 in the original sample will have
    decayed and after another 5568 years, half of that remaining material will have
    decayed, and so on (see figure 1 below). The half-life (t 1/2) is the name given to this
    value which Libby measured at 5568±30 years. This became known as the Libby half-life.
    After 10 half-lives, there is a very small amount of radioactive carbon present in a
    sample. At about 50 - 60 000 years, then, the limit of the technique is reached (beyond
    this time, other radiometric techniques must be used for dating). By measuring the C14
    concentration or residual radioactivity of a sample whose age is not known, it is possible
    to obtain the countrate or number of decay events per gram of Carbon. By comparing
    this with modern levels of activity (1890 wood corrected for decay to 1950 AD) and
    using the measured half-life it becomes possible to calculate a date for the death of the
    sample.

    As 14C decays it emits a weak beta particle (b ), or electron, which possesses an
    average energy of 160keV. The decay can be shown:

                                 14C => 14N + b

    Thus, the 14C decays back to 14N. There is a quantitative relationship between the
    decay of 14C and the production of a beta particle. The decay is constant but
    spontaneous. That is, the probability of decay for an atom of 14C in a discrete sample is
    constant, thereby requiring the application of statistical methods for the analysis of
    counting data.

    It follows from this that any material which is composed of carbon may be dated.Herein
    lies the true advantage of the radiocarbon method, it is able to be uniformly applied
    throughout the world. Included below is an impressive list of some of the types of
    carbonaceous samples that have been commonly radiocarbon dated in the years since
    the inception of the method:

        Charcoal, wood, twigs and seeds.
        Bone.
        Marine, estuarine and riverine shell.
        Leather.
        Peat
        Coprolites.
        Lake muds (gyttja) and sediments.
        Soil.
        Ice cores.
        Pollen.
        Hair.
        Pottery.
        Metal casting ores.
        Wall paintings and rock art works.
        Iron and meteorites.
        Avian eggshell.
        Corals and foraminifera.
        Speleothems.
        Tufa.
        Blood residues.
        Textiles and fabrics.
        Paper and parchment.
        Fish remains.
        Insect remains.
        Resins and glues.
        Antler and horn.
        Water.

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