The half-life of 14 C the time it takes for half of a given amount of 14 C to decay is about 5, years, so its concentration in the atmosphere might be expected to reduce over thousands of years, but 14 C is constantly being produced in the lower stratosphere and upper troposphereprimarily by galactic cosmic raysand to a lesser degree by solar cosmic rays. This makes it possible to tell the age of substances that contain carbon. To verify the accuracy of the method, several artefacts that were datable by other techniques were tested; the results of the testing were in reasonable agreement with the true ages of the objects. Archived from the original PDF on 10 August This increase in 14 C concentration almost exactly cancels out the decrease caused by the upwelling of water containing old, and hence 14 C depleted, carbon from the deep ocean, so that direct measurements of 14 C radiation are similar to measurements for the rest of the biosphere. The northern and southern hemispheres have atmospheric circulation systems that are sufficiently independent of each other that there is a noticeable time lag in mixing between the two.
Taylor also suggests that the availability of definite date information freed archaeologists from the need to focus so much of their energy on determining the dates of their finds, and led to an expansion of the questions archaeologists were willing to research.
Many labs now use an Accelerator Mass Spectrometer AMSa machine that can detect and measure the presence of different isotopes, to count the individual 14 C atoms in a sample. In we could only calibrate radiocarbon dates until 26, years. The carbon dioxide is collected and bubbled through various chemicals in the line, which purify it, and the amount of carbon dioxide that has been collected is measured. Computer analysis of data The results from the liquid scintillation spectrometer are carefully analysed and provide a radiocarbon age for the sample. Histories of archaeology often refer to its impact as the "radiocarbon revolution".