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Carbon Dating

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Daniel Lyon-Brown

on 28 April 2014

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Transcript of Carbon Dating

One of the most crucial parts of any archaeological find, is giving amazing new artifacts approximate dates!

But how do archaeologists date an item that could be 4 million years old!? (..)
With Carbon Dating!
Carbon Dating is a widely used technique, that works with all artifacts that were made from plant matter.

Don't forget that animals, and humans, consume plant matter. Therefore bones and leather can be carbon dated too!

The first carbon dating experiments can be attributed to Willard Libby, who conducted the tests his colleagues at the University of Chicago, in 1949. (1)

Modern carbon dating can date products effectively that are up 60,000 years old! (1)

Radioactive Decay
Isotopes that have a high ratio of neutrons to protons in their nucleus, are said to be "neutron rich."

Neutron rich atoms undergo one of the three forms of radioactive decay, called Beta decay.

During beta decay, a neutron from the isotope will randomly, and spontaneously decay into a proton, and an electron and anti-neutrino will be emitted. The emitted electron is known as beta radiation.

The isotope does this in an effort to become more stable and realise a more balanced ratio of neutrons to protons.

To appreciate carbon dating, it isn't necessary to fully understand the physics of radioactive decay.
However do note that beta decay introduces a proton, and therefore the parent atom that decayed, has now become of a different element.

At anytime!
Unstable isotopes may decay instantly, or never change. There is no way to say exactly when an isotope of any sort will undergo a form of radioactive decay.

We can however provide a fixed amount of time by which we would expect half of the carbon-14 isotopes to have decayed. This is called the "half life."

5730 years, give or take 30, is the half life of Carbon-14. (1)

If we had one gram of C-14, we would expect after 5730 years to only have half a gram remaining.

Archaeologists use this knowledge to date ancient artifacts.
Carbon 14 Dating
How old is this axe?

Isotopes
To understand Carbon Dating, we must first have an understanding of what an
Isotope
is.
An Isotope is an variation of an atom from a particular element. If two atoms have the same number of protons, but one has extra
neutrons,
they are said to be isotopes of one another.

Neutrons, as the name suggests, are neutral in electromagnetic charge, and therefore an isotope behaves chemically the same as an ordinary atom from that element. Isotopes occur naturally but can be also created artificially. (2)
For example, pictured on the far left is a standard hydrogen atom.

Next to it, is another hydrogen atom. However this atom has a neutron within its nucleus.

Far right, is another hydrogen atom, that as two extra neutrons.
There is however, a
Nuclear
difference
between ordinary atoms, and their isotopes!
But what do isotopes and radioactive decay
have to do with the
age of an axe?
The answer is Radiocarbon Dating!
There are only three naturally occurring isotopes of Carbon on our planet, of which,

"99% of the carbon is carbon-12, 1% is carbon-13, and carbon-14 occurs in trace amounts, (only) making up about 1 part per trillion (0.0000000001%) of the carbon in the atmosphere." (4)

Carbon -12, and 13 have a sustainable balance of protons to neutrons, however Carbon-14 is unstable.

Carbon-14 undergoes the following nuclear reaction.
Here, a carbon-14 isotope undergoes beta decay. The result is a stable atom of nitrogen-14, and the emitted electron and anti-neutrino. Notice how on both sides of the equation, we still have equal atomic and mass numbers (electrons and neutrinos are of neglible mass.)

The decay of an element such as carbon is particularly interesting to archaeologists because carbon is present in not only the atmosphere, but all life forms on Earth.
93% of the C-14 in the atmosphere quickly combines with oxygen to create Carbon monoxide. This is in turn oxidizes at a slower rate, whereby carbon dioxide is created. Atmospheric gasses mix and disperse and the C-14 isotope is rapidly distributed across the globe. (4)

Plants absorb and store concentrations of the carbon isotope during photosynthesis. Herbivores eat this plant matter, and naturally, the carnivores eat the herbivores. In time, carbon dioxide also dissolves in the world's oceans. Thus the carbon content in marine life forms is also constant.

At any one point in time, scientists can reasonably assume that any life form on Earth will therefore maintain a trillion to one ration of carbon-12 to carbon-14.
Where does C-14 come from?
This following phenomenon is what makes carbon-14 dating possible.

The vast majority of Carbon-14 is created naturally within the atmosphere. The nuclear equation follows:
A neutron collides with a nitrogen-14 atom and causes the ejection of a proton. The resulting particle is the carbon-14 isotope. The incident neutron is a thermal neutron, which is the result of cosmic rays from outside our solar system. High energy protons and nuclei enter the atmosphere and undergo a variety of transformations. After numerous collisions, neutrons present have just the right amount of energy to be absorbed by Nitrogen-14.

What' s important about the production of C-14 is the fact that it is reasonably constant. Fluctuations in the Earth's magnetic field, and the detonation of atomic bombs are just a few ways in which the level of C-14 can vary within the atmosphere. Nonetheless, over time the ratio of C-14 to C-12 maintains the previously stated one in a trillion ratio. (4)
The crux of this is; when an organism dies, it no longer absorbs (or eats!) any new sources of carbon-14!

Remember previously that Carbon-14 decays into Nitrogen-14. Therefore in time, the ratio of C-12 to C-14 in all dead organisms will change..

So when does C-14 decay into N-14?

With our knowledge of Radiocarbon dating we can now find the age of our axe.
That difference is..
We now know that carbon-14 is decaying at a constant rate.

If we assume the level of cosmic radiation over the past 60,000 years has also been constant, we know that carbon-14 has also been produced at a constant rate.
By analyzing the bone in the handle, Archaeologists find that is .09 carbon-14 has decayed, compared to the natural ratio in our environment.

This means .91 is remaining. For this to be true, .13606 of a half life must have passed.
Made in 1234
By observation of the table above, we can establish a general rule for determining the proportion of a radioactive material left behind after
n
multiples of the isotopes half life.

With regards to carbon-14 dating,
n
can be found by dividing time passed, by 5730 years.

For example;

Archeologists find an ancient hand spear. A sample from the wood reveals that since it was taken from a tree, only a quarter of the expected amount of C-14 remains. For this to be possible, the sample must have experienced two half lives. (2),(6)
The sample is 11,460 years old!
If .13606 has passed, it equates to a period of:
780 years! Therefore our axe is made around about ~1234 AD (2014-780)
Thanks for watching!
References and sources, in order of appearance:

(1) Wikipedia. 2014. Radiocarbon Dating, Available at:
http://en.wikipedia.org/wiki/Radiocarbon_dating

(2) Cahill, Jeff, Doug Bail, Keith Burrows, Rob Chapmen and Carmel Fry. 2008. Heinemann Physics Content and Context. Port Melbourne, VIC: Pearson
Education.

(3) Technological Solutions, Inc. 2014. Chemistry for Kids, Isotopes. Available at:
http://www.ducksters.com/science/chemistry/isotopes.php

(4) Wikipedia. 2014. Carbon-14. Para 2, 4, and 13. Available at:
http://en.wikipedia.org/wiki/Carbon-14

(5) Wikipedia. 2014. Radioactive decay. Availiable at:
http://en.wikipedia.org/wiki/Radioactive_decay

(6) Helmenstine, Anne Marie. 2014. About.com-Chemistry. Availiable at:
http://chemistry.about.com/od/workedchemistryproblems/a/c14dating.htm

Images

Loess Hills Archaeological Interpretive Center. 2014. Available at:
loesshillsarchaeology.org

Museum of London. Availiable at:
http://blog.museumoflondon.org.uk/a-history-of-london-in-10-archaeological-objects-object-5/




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By Daniel Lyon-Brown
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