Carbon Dating (Simplified)

8/13/2019 Carbon Dating (Simplified)

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Carbon Dating

Carbon dating is a variety of radioactive dating which is applicable only to matter which was once

living and presumed to be in equilibrium with the atmosphere, taking in carbon dioxide from the air

for photosynthesis.

Cosmic ray protons blast nuclei in the upper atmosphere, producing neutrons which in turn

bombard nitrogen, the major constituent of the atmosphere . This neutron bombardment producesthe radioactive isotope carbon-!. The radioactive carbon-! combines with oxygen to form carbon

dioxide and is incorporated into the cycle of living things.

The carbon-! forms at a rate which appears to be constant, so that by measuring the radioactiveemissions from once-living matter and comparing its activity with the equilibrium level of living things, a

measurement of the time elapsed can be made.

Index

Hyperhysics!!!!! "uclear # "ave

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Carbon Dating

resuming the rate of production of carbon-'( to be constant, the activity of a sample can be directly

compared to the equilibrium activity of living matter and the age calculated. )arious tests of reliability have

confirmed the value of carbon data, and many examples provide an interesting range of application.

Carbon-'( decays with a halflife of about *+ years by the emission of an electron of energy .' /e).

0his changes the atomic number of the nucleus to +, producing a nucleus of nitrogen-'(. 1t equilibrium

with the atmosphere, a gram of carbon shows an activity of about '* decays per minute.

0he low activity of the carbon-'( limits age determinations to the order of *, years by countingtechniques. 0hat can be extended to perhaps ', years by accelerator techniques for counting the

carbon-'( concentration.

2rigin of carbon-'(

Index

%eta decay concepts


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Carbon-'( 3quilibrium 1ctivity

4ince living organisms continually exchange carbon with the atmosphere in the form of carbon dioxide, theratio of C-'( to C-'5 approaches that of the atmosphere.


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6rom the &nown half-life of carbon-'( and the number of carbon atoms in a gram of carbon, you can

calculate the number of radioactive decays to be about '* decays per minute per gram of carbon in a living

organism.

#adioactive carbon is being created by this process at the rate of about two atoms per second for every

square centimeter of the earth7s surface.8 9evin

0he rate of production of carbon-'( in the atmosphere seems to be fairly constant. Carbon dating of ancient

bristlecone pine trees of ages around years have provided general corroboration of carbon dating and

have provided some corrections to the data.

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#eliability of Carbon Dating

%ristlecone ine 0rees

6rom the dating of ancient bristlecone pine trees from the western :.4., a correction curve for the carbon

dating over the range bac& to * %C has been developed. 0rees dated at ( %C show the maximum

deviation of between and + years too young by carbon dating.

$lacier /easurements

rior to carbon dating methods, the age of sediments deposited by the last ice age was surmised to be about

5* years. 8#adiocarbon dates of a layer of peat beneath the glacial sediments provided an age of only'',( years.8

0hese examples are from 0he 3arth 0hrough 0ime, 5nd 3d. by Harold 9. 9evin

;rane points out that future carbon dating will not be so reliable because of changes in the carbon isotopic

mix. 6ossil fuels have no carbon-'( content, and the burning of those fuels over the past ' years has

diluted the carbon-'( content. 2n the other hand, atmospheric testing of nuclear weapons in the '


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1ccelerator 0echniques for Carbon Dating

1ccelerator techniques for carbon dating have extended its range bac& to about ', years, compared to

less than half that for direct counting techniques. 2ne can count atoms of different masses with a mass

spectrometer, but that is problematic for carbon dating because of the low concentration of carbon-'( and

the existence of nitrogen-'( and CH5 which have essentially the same mass. Cyclotrons and tandemaccelerators have both been used to fashion sensitive new mass spectrometer analyses. 0he tandem

accelerator has been effective in removing the nitrogen-'( and CH5, and can be followed by a conventionalmass spectrometer to separate the C-'5 and C-'. 1 sensitivity of '-'* in the '(C='5C ratio has been

achieved. 0hese techniques can be applied with a sample as small as a milligram.

Index

#eference

;rane

4ec 5.5


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Carbon Dating (Simplified)

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