The carbon stocks are expressed in Gt (Gigatons) and the
number of neutrons per cm
2 per second corresponds to
pre-1950 estimates (modified according to 10):
k
14 C
12 C
f g
¼
Q
12 C
The predicted activity should have been between 1 and 10
decays per minute per gram of carbon, given the uncertainties, in all living matter.
14 C activities of 10.5 disintegrations
per minute per gram were measured from isotopically enriched samples of biomethane in 1947, in good agreement with
the prediction. Libby and collaborators measured then the
specific activities of natural tree samples from different
continents by reducing background counting with lead and
iron shielding. They found worldwide homogeneous
14 C
activities at around 12.5 disintegrations/min/g of carbon
1
(Libby 1981). A precise measurement of the
14 C half-life,
and the specific activity of samples of known ages were
further undertaken to validate the
14 C dating method.
Estimation of the Half-Life and the First
14
C
Dating
The half-life of
14 C was determined through many experiments (Engelkemeir et al. 1949; Olsson et al. 1962). One of
them consisted of measuring, by mass spectrometry, the
isotopic ratio
14 C/
12 C of highly enriched barium carbonates
[BaCO 3 ] (up to 6%) produced by submitting to neutrons
beams, solutions of ammonium nitrate in a cyclotron
(Engelkemeir et al. 1949). Proportional counters were then
filled with the CO 2 released by BaCO 3 hydrolysis. By measuring the number of disintegrations per minute and per gram
of carbon (dN/dt) and knowing the number of atoms of
14 C
(N) in the samples, the half-life was estimated at
5720 ± 47 years. Averaging all the published estimates,
Libby estimated the half-life at 5568 years. Redeterminations
have led to a value of 5730 ± 40 years (Godwin 1962). The
latter value has recently been debated (Chiu et al. 2007).
Libby and collaborators then undertook the dating of
samples of known ages, mostly from the tombs of the
Egyptian kingdoms, and published them as the Curve of
Knowns (Libby 1964).
Principle of the Method
Carbon-14 is formed in the upper atmosphere, where it is
rapidly oxidized to form
14
CO 2 molecules. All living matter
contains carbon, and, thus a very small proportion of
14 C.
The
14 C abundance is of some 1.2 Â 10
−10 % (or
1.2 Â 10
−12 g of
14 C per g of carbon), while those of the
isotopes
13 C and
12 C are respectively 1.108% and 98.892%.
The
14 C exchanges between the living material and their
environment cease at the death of the animals or plants. The
time (t) since the death can be measured by comparing the
residual specific activity in dead organisms to that of the
atmosphere.
Fig. 4.1 Diagram of the
14
C
formation and mixing in the
different terrestrial reservoirs
(Libby 1964)
1
The specific activity is now determined at 13.56 ± 0.07
disintegrations/min/g of carbon.
4 Carbon-14
53
number of neutrons per cm
2 per second corresponds to
pre-1950 estimates (modified according to 10):
k
14 C
12 C
f g
¼
Q
12 C
The predicted activity should have been between 1 and 10
decays per minute per gram of carbon, given the uncertainties, in all living matter.
14 C activities of 10.5 disintegrations
per minute per gram were measured from isotopically enriched samples of biomethane in 1947, in good agreement with
the prediction. Libby and collaborators measured then the
specific activities of natural tree samples from different
continents by reducing background counting with lead and
iron shielding. They found worldwide homogeneous
14 C
activities at around 12.5 disintegrations/min/g of carbon
1
(Libby 1981). A precise measurement of the
14 C half-life,
and the specific activity of samples of known ages were
further undertaken to validate the
14 C dating method.
Estimation of the Half-Life and the First
14
C
Dating
The half-life of
14 C was determined through many experiments (Engelkemeir et al. 1949; Olsson et al. 1962). One of
them consisted of measuring, by mass spectrometry, the
isotopic ratio
14 C/
12 C of highly enriched barium carbonates
[BaCO 3 ] (up to 6%) produced by submitting to neutrons
beams, solutions of ammonium nitrate in a cyclotron
(Engelkemeir et al. 1949). Proportional counters were then
filled with the CO 2 released by BaCO 3 hydrolysis. By measuring the number of disintegrations per minute and per gram
of carbon (dN/dt) and knowing the number of atoms of
14 C
(N) in the samples, the half-life was estimated at
5720 ± 47 years. Averaging all the published estimates,
Libby estimated the half-life at 5568 years. Redeterminations
have led to a value of 5730 ± 40 years (Godwin 1962). The
latter value has recently been debated (Chiu et al. 2007).
Libby and collaborators then undertook the dating of
samples of known ages, mostly from the tombs of the
Egyptian kingdoms, and published them as the Curve of
Knowns (Libby 1964).
Principle of the Method
Carbon-14 is formed in the upper atmosphere, where it is
rapidly oxidized to form
14
CO 2 molecules. All living matter
contains carbon, and, thus a very small proportion of
14 C.
The
14 C abundance is of some 1.2 Â 10
−10 % (or
1.2 Â 10
−12 g of
14 C per g of carbon), while those of the
isotopes
13 C and
12 C are respectively 1.108% and 98.892%.
The
14 C exchanges between the living material and their
environment cease at the death of the animals or plants. The
time (t) since the death can be measured by comparing the
residual specific activity in dead organisms to that of the
atmosphere.
Fig. 4.1 Diagram of the
14
C
formation and mixing in the
different terrestrial reservoirs
(Libby 1964)
1
The specific activity is now determined at 13.56 ± 0.07
disintegrations/min/g of carbon.
4 Carbon-14
53