The Method
Radiocarbon dating : what is Carbon 14?
The chemical element carbon is one of the major components of all living organisms;
it is present in almost all the molecules that combine carbon atoms to oxygen,
hydrogen, nitrogen , etc. Each atom is composed of a nucleus, around which co-exixts
protons ( positively charged) , neutrons ( no charge) and a number of electrons
(negatively charged). The negative electron charge compensates the positive charge
of the protons. Quantic mecanic laws are responsible for the gravitation of the
electrons around the nucleus: an electron does not emit energy when it follows
a stable orbital path at a fixed given energy level. When this electron shifts
jumps from a stationary state to another, it either acquires or looses energy.
In this last case , it emits a ray.
The Same chemical element may posess different nucleus configurations, distinguishable
through the number of neutrons. Those are called Isotopes. Carbon has three isotopes:
– Carbon 12 (stable isotope) : its nucleus is formed of 6 protons and 6 neutrons.
– Carbon 13 ( Stable isotope) : Its nucleus is formed of 6 protons and 7 neutrons.
– Carbon 14 ( unstable Isotope) : Its nucleus is formed of 6 protons and 8 neutrons.
The isotope14C, , unstable, gets transformed according to the following
reaction:
146C >147N + ß- : gets
created nitrogen 14, stable, with an energy emission in In the atmosphere, the
carbon isotope content distribution is approximately equivalent to
– Carbon 12 : 98,89%
– Carbon 13: 1,108%
– Carbon 14: 1,2.10-12%
Datation by means of Carbon 14 : the principle
During their lifetime, plants and animals maintain continuous exchanges with their
environment, absorbing and transforming a partial amount of the total Carbon dioxide
(CO2) in their tissues. Therefore, the percentage content
of the Carbon isotope 14 in their organism, would be sensibly equivalent to that
of the atmosphere surrounding them, at the time of their existence. Those exchanges
stop with the death of those living organisms. The Isotope 14, unstable, gets
transformed through the reaction mentioned above, proportionally with time. Half
of the14C desintegrated in 5730 years (time called the "half-life"
of a radioactive element). We can , consequently, estimate the age of a sample
by measuring the amount of 14C available in the matter. When Edward
Libby by 1950, implemented the method of datation by use of radiocarbon, the content
in Isotope 14 was estimated constant in the atmosphere through the ages. This
has been revised since then. It has been necessary to correct those measurements,
through a calibration method that takes this matter into consideration. A calibration
curve has been established through cross age observation between various means
of measurements and the method of radiocarbon, and essentially by the dendrochronology
method.This curve is constantly revised and affined.
Datation by means of Carbon 14 : the methods
Two methods are used:
1. The Indirect method : measurement of the radioactivity with liquid scintillation
counters.
2. The Direct method : direct measurement of the isotope 14 content with an
accelerator-mass spectrometer duetto instrumentation.
1. The Indirect method : measurement of the radioactivity with liquid scintillation
counters.
The most preliminary method, the most simple and that was the most used: The samples
get transformed into a compound which radioactivity is measured. After an appropriate
pretreatment to eliminate eventual chronologically polluting elements, the sample
is burned into a combustion chamber. During this combustion, carbon gets transformed
into carbon dioxide (CO2).The sample gets purified from
other combustion products and the carbon dioxide is then, transformed into acetylene
(C2H2), and finally into benzene (C6H6)
which 14C radioactivity is measured by means of a liquid scintillation counter.
In that instrument, the ß rays are converted into photons (or light emission).
It is that very slight scintillation that is measured.
2. The Direct method : direct measurement of the isotope 14 content with
an accelerator-mass spectrometer duetto instrumentation(AMS: Accelerated mass
Spectrometry).
Once the sample has been transformed into carbon dioxide or graphite, it is ionised
by beeing collided with Cesium ions. After a first selection of ions of similar
mass 14, the beam they form in the instrument is accelerated, until it collides
with a gas in order to separate the C3+. Those are then accelerated
in turn into the second part of the instrument for accurate separation of ions 14C3+,
13C3+ et 12C3+.
Advantages and disadvantages of both methods:
The Direct method, with its great sensitivity, allows measurement of weights in
Carbon of less than a milligram, a thousand times less than that required by the
indirect method. For the small samples, there is therefore no choice. However,
the mobility of the little particules in the sediments and the great ratio between
surface/weight, does not permit a clear identification of the polluting elements
in many a situation. This method requires heavy instrumentation. The Indirect
method, on the contrary, permits the same accuracy, bif the adequate sample weight
is available.