-counting method) or by directly counting the radiocarbon atoms using a method called Accelerator Mass Spectrometry (AMS).
Measurement of the radioactivity of the sample works very well if the sample is large, but in 9 months less than 0.01% of the radiocarbon ions will decay, so in a reasonable measurement time (typically a few weeks) only a very small proportion of the radiocarbon atoms are detected by this method.
Since carbon is fundamental to life, occurring along with hydrogen in all organic compounds, the detection of such an isotope might form the basis for a method to establish the age of ancient materials.
Libby, a Professor of Chemistry at the University of Chicago, predicted that a radioactive isotope of carbon, known as carbon-14, would be found to occur in nature.
Buried beneath about 2.7-3.4 m (8.8-11.2 ft) of calcareous loess were the remains of four oval to circular huts, with surface areas of between 12 to 24 square meters (120-240 square feet) each.
The ensuing atomic interactions create a steady supply of c14 that rapidly diffuses throughout the atmosphere.
Plants take up c14 along with other carbon isotopes during photosynthesis in the proportions that occur in the atmosphere; animals acquire c14 by eating the plants (or other animals).
AMS, on the other hand, can in principle detect a much higher proportion (typically about 1% of the total) allowing sample sizes to be smaller by a factor of about a thousand.
The method is relatively new because it needs very complicated instruments first developed for Nuclear Physics research in the late 20th century.
The sample is put into the ion source either as graphite or as carbon dioxide.