The age of the ancient organic materials can be found by measuring the amount of Carbon-14 that is left.
The most common tracer is called Technetium-99 and is very safe because it only emits gamma rays and doesn't cause much ionisation.
As long as something is on the surface of the earth and taking in carbon, the amount of carbon-14 stays the same.
Plants absorb carbon dioxide from the atmosphere and animals eat plants.
This means all living things have radioactive carbon-14 in them.
With paper, or plastic, or aluminium foil, b rays are used, because a will not go through the paper.
We choose a source with a long half-life so that it does not need to be replaced often.
When an organism, eg a tree, dies it stops taking in carbon dioxide.
The amount of carbon-14 in the wood decreases with time as it decays into nitrogen with a half-life of about 5700 years.
Smoke alarms contain a weak source made of Americium-241.
Alpha particles are emitted from here, which ionise the air, so that the air conducts electricity and a small current flows.
These bonds allow carbon to form long chain-shaped molecules, called polymers, such as plastics. A special, man-made, tube-shaped allotrope of carbon is the carbon nanotube. A whole type of Chemistry, organic chemistry, is about carbon and its compounds. Hydrocarbons are molecules with carbon and hydrogen.
When iron is alloyed with carbon, hard steel is formed. The name of carbon comes from Latin carbo, meaning charcoal. Carbon nanotubes are very hard, so they might be used in armor. Methane, Propane, and many other fuels are hydrocarbons.
Much of the information presented in this section is based upon the Stuiver and Polach (1977) paper "Discussion: Reporting of C14 data". 1890 wood was chosen as the radiocarbon standard because it was growing prior to the fossil fuel effects of the industrial revolution.