Radiometric counting determines the amount of 14C present in a sample by measuring its radioactivity. This is done by converting the carbon in the material being dated to a gas such as CO2, methane, or benzene, and placing it in a suitable radiation detector.
There are two types of counting systems in use:
- Gas counting: the sample is converted to methane or CO2 to fill a proportional counter. The decay of a 14C atom triggers an electrical discharge in the gas which is electronically detected. The photo shows the interior of a typical gas counting system, using thick steel plates to shield the counters inside from external radiation.
- Liquid scintillation counting: the sample carbon is converted to benzene, mixed with special organic compounds and placed in a transparent container. The solution emits a pulse of light whenever a 14C atom decays, and the light pulse is detected by sensitive photomultiplier tubes placed close to the container.
Both types of counting system work by measuring the rate of radioactive decay in the sample. The difference is in the technology used to do this.
The radiometric method can detect the presence of a 14C atom only when the atom undergoes radioactive decay. By determining the number of 14C decays that take place in a certain time we can calculate the number of 14C atoms in the detector, and if we also know the total amount of carbon this is enough to determine the concentration of 14C, which in turn allows us to calculate the age of the material.
However, the decay of a radioactive atom is a random event, and by measuring the number of decays that occur in a given time we obtain only an estimate of the decay rate: if we repeat the measurement we will almost certainly get a different answer. To attain a certain degree of precision in the measurement we can calculate how many decays we must count. But since the half life of 14C is 5730 years, we must ensure that there are enough 14C atoms in the detector to give us enough decays in a reasonable time. This typically amounts to several grams of carbon. Since in most cases the carbon is only one component of the material, radiocarbon dating using decay counting may require the consumption of many grams of the sample, and in some cases kilograms.