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Using the "report your findings" worksheet, notify the museum of your results.
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BRSP-15
Page 2
Graph of M&M Decay
100
90
80
70
60
Number of
Undecayed
M&Ms
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
10
Number of Runs (Half-Lives)
In mathematics, a smooth, declining curve of the type just drawn is called, not coincidentally,
exponential decay.
Discussion points: Which graph line – the individual or class average – most closely follows the
theoretical line? Why?
The declining number of M&Ms after each succeeding run is like the radioactive decay of atoms
after each half-life has passed. Eventually, the number of remaining atoms from the original
quantity approaches zero, as almost all of the atoms have decayed into a different atomic form.
Why Carbon-14?
Carbon dating is performed on objects containing bits of organic matter, carbon-based substances
that were once included in the bodies of living plants and animals. Not all carbon atoms are alike.
Approximately 99 percent of carbon atoms have atomic nuclei with six protons and six electrons,
giving them an atomic mass of 12. About one percent of carbon atoms have an extra neutron in
the nucleus, giving them an atomic mass of 13. A very small number of carbon atoms – about
one in every trillion – have two additional neutrons, giving them an atomic mass of 14. These
different types of carbon atoms are called isotopes (see figure on next page).
The carbon-14 atoms are unstable and will disappear, or decay, on their own in time. This fact is
central to understanding how carbon dating is used to establish the age of an object.
BRSP-15
Page 3
http://www.academic.rccd.cc.ca.us/~freitas/
Because all living things are made from organic molecules, they must have a carbon source in
order to exist. For most organisms, that source is carbon dioxide (CO2) in the atmosphere. The
carbon atoms in the atmosphere include the three isotopes in the proportions stated above. The
same proportions are found in the organisms that obtain their carbon from the atmosphere.
Those proportions will stay the same in their bodies for as long as they are living, because the
carbon in living organisms is continuously being replenished.
When an organism dies, however, the carbon is no longer replaced. It remains there until it is
eventually returned to the ecosystem through decomposition processes. The radioactive
carbon-14 in the dead organism decays in a known way, similar to the decay curve for M&Ms.
Imagine a pollen grain that is picked up by the wind
and carried aloft. After traveling hundreds of miles,
the pollen grain settles onto a glacier and becomes
buried under many annual snowfalls. Thousands of
years later, a geologist removes an ice core from
the glacier and finds that same pollen grain in his
sample of ice. Can he determine the age of the
pollen grain? If so, he will also know how old that
part of the glacier is.
Using carbon dating, the geologist can closely
estimate the age of the sample. By measuring the
ratio of carbon-14 to all carbon atoms in the pollen
grain, he will know how much carbon-14 decay has
occurred. The extent of decay will reveal the age of
the pollen grain.
Radioactive decay of carbon-14
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/cardat.html
Shallow ice core drilling, Greenland
http://www.glaciology.gfy.ku.dk/ngrip/billeder01_eng.htm
BRSP-15
Page 4
Graphing the Decay of Carbon-14
Knowing that the half-life of carbon-14 is 5,730 years, it is easy to construct a decay curve like the
one for M&Ms. Complete the following table and make a line graph of the data on the chart
provided. The result should be a smooth, curving line through all points.
Decay of Carbon-14
Years from Present
0
Percent of Original
C14 Remaining
100
5,730 11,460 17,190 22,920 28,650 34,380 40,110 45,840 51,570
100
90
Decay of Carbon-14
80
70
60
Percent of
Original
Carbon-14
50
40
30
20
10
0
0
10,000
20,000
30,000
40,000
50,000
Years from Present
Carbon dating is generally useful for dating objects up to about 50,000 years old. Why is this
method limited for objects of greater age? Fortunately, for scientists who need larger “measuring
sticks,” there exist other elements whose radioactive isotopes have half-lives greater than that of
carbon-14 and can be used in a similar manner as dating tools.
BRSP-15
Page 5
Applying the Concepts
Use the graph of carbon-14 decay to solve the following real-life puzzles.
1. In 1991, hikers in the Tyrolean Alps of Europe made a remarkable discovery. They found an
almost perfectly preserved body of a prehistoric man, whom scientists named Ötzi. The
discovery was made possible because recent warming of the atmosphere had caused glaciers
in the region to retreat, exposing objects that had been buried under the ice for millennia.
Ötzi’s fate was matched by a variety of well-preserved plant and animal species that were
found close by. As discoveries of such quality are rare, the event was a genuine treasure
trove for scientists. They reasoned that Ötzi and the other organisms must have been trapped
by a sudden snowfall and virtually “flash frozen.” This singular event was followed
immediately by an extended cold period that preserved the specimens until the present glacial
retreat. Carbon dating of samples from the site established the time of Ötzi’s demise at
approximately 5,300 years ago. What percentage of the original carbon-14 in Ötzi’s body was
remaining in 1991?
Many websites tell the history and science of Ötzi. This one is a good place to start:
http://www.explorenorth.com/library/weekly/more/bl-iceman.htm
2. Scientists have been rethinking the nature of past climates. A 1998 study provided evidence
that the tropics were much colder during the last glacial maximum than previously thought.
Prior understanding had been that tropical regions were mostly unaffected by past ice ages.
Constructing an accurate history of ancient climates is important, since the knowledge gained
may have relevance to global climate change today. In the study just mentioned, investigators
used a solar-powered drill to bore through the ice cap at the summit of an extinct Bolivian
volcano named Sajama. They retrieved two ice cores at the bottom of the glacier, more than
132 meters (433 feet) deep. Trapped within the cores were insects and bark fragments from
local trees. Carbon from organic material near the bottom of the cores dated to the coldest
period of the last ice age. If those samples had 5.5 percent of their original carbon-14,
approximately how many years ago did the glacier atop Sajama begin to form?
Read a more detailed description of the Sajama investigation at this website:
http://researchnews.osu.edu/archive/sajama.htm
3. The authenticity of the Shroud of Turin had long been debated. In 1988, scientists received
permission to remove small samples for carbon dating. Three different laboratories in
Arizona, U.S.; Oxford, England; and Zurich, Switzerland analyzed the samples. All three
laboratories came to the same conclusion: The shroud had lost about 8 percent of its carbon14 atoms to radioactive decay. Given this result, what was the approximate date of origin of
the Shroud of Turin? (Note: Despite these and other scientific investigations, the origin and
date of the Shroud of Turin remains a subject of controversy.)
For a detailed account of the carbon-14 dating of the Shroud of Turin, visit this website:
http://www.shroud.com/nature.htm
Student directions Radioactive Dating Game activity
Learning Goals: Students will be able to:




Identify isotopes that are commonly used to determine how old matter might be.
Explain how radiometric dating works and why different elements are used for dating different
objects.
Use the percent of an isotope measured in an object to estimate its age.
Identify types of nuclear reaction used for dating; include how elements change and balanced
reaction.
Directions:
1. Explore Radioactive Dating Game . Try all the tabs to figure out why there is more than one
element used to estimate how old things might be.
2. What elements’ isotopes are used to estimate how old something is? Why do scientists use more
than one type? (Be specific, it is not just to get repeated results)
3. Pretend you are a scientist and have a tool like the one on Dating Game tab:
a. How do you decide which to use: Carbon-14 or Uranium-238?
b. How does the percentage
help you estimate the age?
c. If you can’t get a reading on one object like the fish fossil
, what else can
you try? Determine the approximate age of the fish fossil and explain what you did to
estimate the fossil age.
4. If you were a forensic scientist and found a dead buried body, could you use one of the isotopes
in the simulation to figure out how long ago the person died? Explain.
5. What type of reaction do Carbon -14 and Uranium- 238 undergo? Explain how you figured this
out and write the reaction for each.
2/19/2013 Loeblein
http://phet.colorado.edu