You probably have seen or read news stories about fascinating ancient
artifacts. At an ar chaeological dig, a piece of wooden tool is unearthed and
the archaeologist finds it to be 5,000 years old. A child mummy is found high
in the Andes and the archaeologist says the child lived more than 2,000 years
ago. How do scientists know how old an object or human remains are? What
methods do they use and how do these methods work? In this article, we will
examine the methods by which scientists use radioactivity to determine the age
of objects, most notably carbon-14 dating.
Carbon-14 dating is a way of determining the age of certain archeological
artifacts of a biological origin up to about 50,000 years old. It is used in
dating things such as bone, cloth, wood and plant fibers that were created in
the relatively recent past by human activities.
How Carbon-14 is Made
Co smic rays enter the earth's atmosphere in large numbers every day. For
example, every person is hit by about half a million cosmic rays every hour. It
is not uncommon for a cosmic ray to collide with an atom in the atmosphere,
creating a secondary cosmic ray in the form of an energetic neutron, and for
these energetic
neutrons to collide with nitrogen atoms. When the neutron collides, a
nitrogen-14 (seven protons, seven neutrons) atom turns into a carbon-14 atom
(six protons, eight neutrons) and a hydrogen atom (one proton, zero neutrons).
Carbon-14 is radioactive, with a half-life of about 5,700 years.
For more information on cosmic rays and half-life, as well as the process of
radioactive decay, see How Nuclear Radiation Works.
Carbon-14 in Living Things
The carbon-14 atoms that cosmic rays create combine with oxygen to form carbon
dioxide, which plants absorb naturally and incorporate into plant fibers by
photosynthesis. Animals and people eat plants and take in carbon-14 as well.
The ratio of normal carbon (carbon-12) to carbon-14 in the air and in all
living things at any given time is nearly constant. Maybe one in a trillion
carbon atoms are carbon-14. The carbon-14 atoms are always decaying, but they
are being replaced by new carbon-14 atoms at a constant rate. At this moment,
your body has a certain percentage of carbon-14 atoms in it, and all living
plants and animals have the same
percentage.
