Chemistry Reference
In-Depth Information
with dates measured during the early days of the radiocarbon
method, the fifth decade of the twentieth century. The discrepancy is
taken into account and corrected, together with others caused by
natural variations in radiocarbon.
3.
The relative amount of radiocarbon in the earth is assumed to have
been the same during geologic times.
4.
The relative concentration of radiocarbon in the samples dated is cor-
rected for isotope fractionation, based on the ratio C-13/C-12 (see text
below).
Practical considerations limit the use of the radiocarbon dating technique to
a range of 200-50,000 years; samples less than 200 years old cannot be called
(a)
(b1)
(b2)
FIGURE 63
Dating with radiocarbon.
Radiocarbon dating is the most widely used
method for determining the age of archaeological materials containing carbon. Accu-
rate radiocarbon dating requires great scientific expertise and advanced equipment.
Elaborate chemical instrumentation
(a)
is required, for example, when using the radio-
carbon decay counting technique, just to isolate the carbon in the material before deter-
mining the relative amount of the radiocarbon isotope. Acceleration mass spectrometry
(AMS) involves the use of large devices, as illustrated by the tank
(b1)
; in the large
tank, the atoms of carbon in the sample are first ionized (converted into ion) by remov-
ing a negatively charged electron, and then accelerated to precise velocities and sep-
arated according to their mass. From the measured amount of carbon 14 relative to
that of the other stable isotopes of the element, the age of a sample can be deter-
mined. All the AMS operations are accurately controlled in a complex, computerized
control panel
(b2)
.
