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Radioactive Half-life

Discusses how rate of radioactive decay is modeled and introduces decay chains.

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How Old Is Ötzi?

How Old Is Ötzi?

Credit: ehecatzin
Source: http://www.flickr.com/photos/ehecatzin/42414394/
License: CC BY-NC 3.0

In 1991, the well-preserved human remains of a mummy were discovered frozen in a glacier.  He was eventually named Ötzi, after the Ötztal Alps where his body was found (please see an image of Otzi here: http://en.wikipedia.org/wiki/%C3%96tzi). Because of the natural mummification that occurred when he was frozen, a simple inspection is not enough to determine how long ago this man lived.  Fortunately, there are clues on the atomic level that allowed scientists to solve this mystery, using a technique called radiocarbon dating.

Amazing But True!

Living organisms contain large amounts of carbon, most of which is the stable isotope carbon-12.  However, a small amount of the radioactive isotope carbon-14 is also present in any living creature.  When the organism dies, its carbon-12 levels stay the same, but the amount of carbon-14 decreases as it undergoes radioactive decay.  By looking at the ratio between these two different isotopes, scientists can estimate the amount of time that has passed since the organism died.  This technique works not only on intact organisms, but also organic substances that are derived from living organisms, such as paper (from trees) and leather (from animals).  Learn more about carbon dating by watching the following video: http://www.teachersdomain.org/asset/nvhe_vid_decay/

Credit: Library of Congress
Source: http://en.wikipedia.org/wiki/File:Psalms_Scroll.jpg
License: CC BY-NC 3.0

There are many methods for Carbon dating and they do not all produce the same results. For example, the Dead Sea Scrolls were carbon dated to various ranges between 408 BCE and 318 CE (common era) [Figure2]

Can You Apply It?

With the links below, learn more about radiocarbon dating. Then answer the following questions.

  1. When Ötzi was found, the amount of carbon-14 still present in his remains was approximately 53% of what it would have been when he died.  Use the calculator linked above to estimate the year of his death.
  2. Can carbon dating be used to determine the age of dinosaur bones?  Why or why not?
  3. If carbon-14 decays over time, why is there a constant ratio between carbon-12 and carbon-14 in the environment?  Shouldn’t this decrease over time?
  4. An archaeologist claims to have uncovered a new religious text from 150 A.D.  A small sample of the paper is analyzed and found to contain 96% of the carbon-14 that would have been originally present.  Is this text likely to be genuine?

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Image Attributions

  1. [1]^ Credit: ehecatzin; Source: http://www.flickr.com/photos/ehecatzin/42414394/; License: CC BY-NC 3.0
  2. [2]^ Credit: Library of Congress; Source: http://en.wikipedia.org/wiki/File:Psalms_Scroll.jpg; License: CC BY-NC 3.0

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