Site Created by:   Michelle MacPhee

Methods of Calculating Radiocarbon Decay

There are three principle methods used to measure C-14 decay.  They are;

• Gas Proportional Counting (GPC)
• Liquid Scintillation Counting (LSC)
• Accelerator Mass spectrometry (AMS)

 The latter of these, the AMS, is the most commonly used in modern dating.  In the late 1970's Accelerator mass spectrometry, (AMS), became widely used by most archaeologists for radiocarbon dating.  AMS, (shown in the figure to the left), was already a well known nuclear physics technique at the time.  When using AMS the sample to be dated is converted to graphite, and then struck with an infinite number of caesium ions.  The caesium ions free the carbon ions from the sample.  The carbon ions are then accelerated through

a magnetic field at very high speeds.  The magnetic field causes the ions to deflect at an angle proportional to their weight.  Since each of the carbon ions, C-12, C-13, and C-14 have unique weights they will deflect at different angles, and therefore can be detected independently.

 The AMS method allows individual C-14 atoms to be counted.  This means that much smaller samples are needed for dating then the previous methods.  This allows very small items, such as seeds, to be dated.  It also allows the dating of precious materials by removing only insignificant amounts for the sample, leaving the original material essentially unharmed.

Full descriptions of the two other methods of measuring the C-14 to C-12 ratio, the GPC and the LSC, can be found in the links of this web page.