By radiocarbon dating
Using this measurement also corrects for any mass-dependent fractionation within the AMS system.
The Fraction Modern corrected for δC of a sample 10 separate times over the course of a run.
One other situation that limits the age (if not already limited by the background age) is the error of the AMS result.
If twice the reported error of the Fraction Modern (let's call this 2sigma) is larger than the sample Fraction Modern, then a limiting age is reported.
However, limiting ages or "backgrounds" are also determined by process blanks which correspond to the method used to extract the carbon from the sample.
» NOSAMS General Statement of C from contamination introduced during chemical preparation, collection or handling.
In AMS, the filiamentous carbon or "graphite" derived from a sample is compressed into a small cavity in an aluminum "target" which acts as a cathode in the ion source.
The surface of the graphite is sputtered with heated, ionized cesium and the ions produced are extracted and accelerated in the AMS system.
It is expected then, for a 5,570 year (1 half-life) or 11,140 year old (2 half-lives) sample that 125 or 63 counts per second would be obtained.The limiting age is then calculated as -8033 * ln(2sigma) and rounded according to conventions outlined above. These techniques are made possible by sensitive electronic instruments developed in the late twentieth century. A computer program for radiocarbon age calibration. The rate of bombardment is greatest near the poles, where the Earth's magnetic field is dipping into the Earth and therefore does not deflect incoming cosmic rays. Once the radiocarbon atom is produced, it rapidly combines with oxygen (OC (7 neutrons) atoms.