Use of radioactivity in carbon dating

Uranium has a very long half-life and so by measuring how much uranium is left in a rock its approximate age can be worked out.

Dating Methods Using Radioactive Isotopes

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How Does Radiocarbon Dating Work? - Instant Egghead #28

This page has been archived and is no longer updated. This information is then related to true historical dates.

Radioactive Tracers

Before deciding on using carbon dating as an analytical method, an archaeologist must first make sure that the results of radiocarbon dating after calibration can provide the needed answers to the archaeological questions asked. The implication of what is represented by the carbon 14 activity of a sample must be considered.

The sample-context relationship is not always straightforward. Date of a sample pre-dates the context it is found. Some samples, like wood, already ceased interacting with the biosphere and have an apparent age at death and linking them to the age of the deposits around the sample would not be wholly accurate.

Radioactive dating

There are also cases when the association between the sample and the deposit is not apparent or easily understood. Great care must be exercised when linking an event with the context and the context with the sample to be processed by radiocarbon dating. An archaeologist must also make sure that only the useful series of samples are collected and processed for carbon dating and not every organic material found in the excavation site. It is important that the radiocarbon scientists and archaeologists agree on the sampling strategy before starting the excavation so time, effort, and resources will not be wasted and meaningful result will be produced after the carbon dating process.

It must be stressed that archaeologists need to interact with radiocarbon laboratories first before excavation due to several factors. Laboratories have limitations in terms of the samples they can process for radiocarbon dating. Some labs, for example, do not date carbonates.

Radiocarbon dating

Laboratories must also be consulted as to the required amount of sample that they ideally like to process as well as their preference with certain samples for carbon dating. Other labs accept waterlogged wood while others prefer them dry at submission. Contaminants must not be introduced to the samples during collection and storing. Hydrocarbons, glue, biocides, polyethylene glycol or polyvinyl acetate PVA must not come in contact with samples for radiocarbon dating. Other potential contaminants include paper, cardboard, cotton wool, string and cigarette ash. Samples must be stored in packaging materials that will protect them during transport and even during prolonged storage.

Labels attached to the packaging materials must not fade or rub off easily.

Radiocarbon dating

Glass containers can be used when storing radiocarbon dating samples, but they are susceptible to breakage and can be impractical when dealing with large samples. The cosmic rays originating from the Sun collide with nuclei in the upper atmosphere and are capable of breaking off individual neutrons. These neutrons, once freed, can interact with atoms of nitrogen 14 in air, causing the expulsion of a proton and the formation of carbon One naturally assumes that the cosmic bombardment responsible for this transmutation remains constant over the millennia.

The rate of cosmic rays which hit the Earth depends on two very slowly changing factors: This latter serves as a shield against all cosmic radiation - when its strength goes down, the bombardment increases, as does the number of carbon 14 atoms. All living beings assimilate carbon dioxide molecules, a fixed but very small fraction of which contains carbon This assimilation stops upon the death of the organism, thus halting the absorption of any more carbon The atoms of carbon 14 then proceed to decay exponentially, with a half life of 5, years.


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When much later, an archaeologist examines the remains fireplace ashes, bones, plant remains , he can date the fossil by comparing the fraction of remaining radiocarbon nuclei to the fraction existing at the time the organism stopped absorbing carbon. The fundamental hypothesis in these estimations is that the rate of radioactive carbon existing when the organism was living would have been the same as the rate in a similar organism alive today.

The ratio of the activities of the fossilized and living bodies then provides an age.