Chemistry Reference
In-Depth Information
Two basically different technical procedures are at present in use to
measure the amount of radiocarbon in matter:
1.
An older and long-established technique,
radiocarbon decay counting
,
also known as the “conventional” method of radiocarbon dating, is
based on detecting and counting the amount of beta radiation emitted
in unit time by radiocarbon atoms in a sample of known weight.
2.
A more recently developed technique, known as the
accelerator mass
spectrometry
(AMS) radiocarbon dating technique, based on counting,
in a
mass spectrometer
, the relative amount of radiocarbon to stable
carbon isotopes in a sample (see Textbox 10).
The conventional radiocarbon decay counting technique generally provides
reliable results, but it has some limitations; the following are worth
mentioning:
•
A relatively large sample (several grams but often more) is required to
obtain reliable data.
•
A long time (usually several hours) is required for counting the beta
radiation emitted by the disintegrating radiocarbon.
•
Since the counting of radioactive disintegration events is subject to sta-
tistical considerations, the dates obtained with this method are inher-
ently subject to some degree of uncertainty.
The AMS technique, on the other hand, requires only very small samples
(usually below one gram in weight), and the time necessary to measure the
amount of radiocarbon in each sample is only a few minutes, much less than
that required for counting disintegration events in the conventional tech-
nique. AMS demands, however, a large initial financial investment for build-
ing a dating facility, which greatly increases the cost of measurements as
compared with those derived by the radiocarbon decay counting technique.
Although the size of a sample required for radiocarbon dating is deter-
mined by the actual method used (see text above), it is also dependent on
the actual composition of the material to be dated; Table 68 lists the amounts
of various materials usually required for either of the radiocarbon dating
techniques. The importance, particularly in archaeology, of using small
samples (weighing only a few milligrams, all that is required for AMS)
cannot be overemphasized: removing only small samples makes it possible
to sample valuable and irreplaceable objects (fractional remains of parch-
ment or textiles for example) that might otherwise be destroyed to provide
a sample of sufficient size to be assayed by the radiocarbon decay counting
