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Lecture 15

Anthropology 2229F/G Lecture Notes - Lecture 15: Radiocarbon Dating, Benzene, Issf 10 Meter Air Rifle


Department
Anthropology
Course Code
ANTH 2229F/G
Professor
Christopher Ellis
Lecture
15

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Lecture 15 Laboratory Methods for Radiocarbon Dating
Conventional/traditional radiocarbon date generation by:
o 1. Gas Proportional Counting
Convert the sample to carbon dioxide and measure the amount
of carbon
Measure by the radioactive decay
Count the number of decays in a certain timeframe
Not a direct measurement of C-14
o 2. Liquid Scintillation Counting
Convert sample into a benzene liquid (C6H6)
Add a scintillant gives off light every time a radioactive decay
occurs
Count number of decays occur over a period of time
Allows for an estimate of how much C-14 is in the sample
o Methods only good for about +/- 100 years, no less, cannot be very
precise
o Need a fairly large sample (e.g. 10-25 grams of charcoal, 3000 grams
of bone), often fairly difficult to get that much
Nuclear Accelerators
o Revolutionized radiocarbon dating
o AMS dates
o Run isotopes past powerful magnets, those with a higher mass bend
differently past the magnets
o Advantages:
Measures the radiocarbon in the sample directly, not an
estimate
Do not need as big a sample (e.g. use a piece of charcoal the
size of a pinhead (milligrams) instead of large pieces, single
seeds, etc.)
Less standard deviation (+/-), can be within 60 years (gives
you a 120 year window)
Much more precise
o Disadvantages:
Very expensive to do, about $1500 - $2000 each; now about
$600 USD
Cost of conventional dates is going up, AMS going down
Archaeologists concerned with dating objects
o Many are inorganic
o Have to date objects in association with other objects, need to be in
good association to be precise
Need to be careful, try and be precise as possible
o Chance of contamination very low, easy to decontaminate with acids
Most widely used to date things within the past 12,000 years
o In order to date things in the past 50,000 years, there are other
isotopes to use but they are often hard to find in the record
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