Radioactive decay carbon 14 dating

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technology columnist David Pogue explores how isotopes of carbon can be used to determine the age of once-living matter. The difference between them is the number of neutrons in the nucleus.

Learn how variations in atomic structure form isotopes of an element and how the three natural isotopes of carbon differ from each other. Neutrons are electrically neutral particles that act as glue to hold atoms together. And that rare version of carbon has proven to be a crucial tool for unlocking the past.

Meet paleoclimatologist Scott Stine, who uses radiocarbon dating to study changes in climate. What we think of as normal carbon is called carbon-12: six protons plus six neutrons. Several times a year, scientist Scott Stine travels to the shores of Mono Lake, near Yosemite National Park. He's studying the long history of droughts in California, trying to determine how frequently they occur and how long they last.

Find out what it means for an isotope to be radioactive and how the half-life of carbon-14 allows scientists to date organic materials. But about one percent of carbon atoms have an extra neutron, giving them seven. Over the millennia, the water level has risen and fallen, as the area has cycled between wet periods and dry times. During times when the climate was dry, Mono Lake dropped down, exposed the shore lands, and allowed trees and shrubs to grow.

Scientists now know that most elements come in more than one version. When the dry periods ended and the water level rose, the trees drowned, marking the end of the droughts.

Since then, the remains of those trees have been well preserved by the arid climate. To determine how long ago these droughts occurred, Scott is using carbon-14 to date the trees.

Unlike the other natural isotopes of carbon, carbon-14 is unstable. One of its neutrons turns into a proton and spits out an electron.

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The amount of carbon-12 stays the same, but the carbon-14 decays away, at a constant rate, making carbon-14 a ticking atomic clock.

Co­smic 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.

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