Radiocarbon decay

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. 

The conventional radiocarbon decay counting technique generally provides reliable results, but it has some limitations the following are worth mentioning ... 

Dating with Radiocarbon. The important information held in a sample to be dated by radiocarbon is its present radiocarbon concentration comparing this concentration to that of radiocarbon in the atmosphere, which is considered to be constant (however, see discussion below), yields the conventional radiocarbon date of the sample. All that is required to establish the age of a sample, therefore, is to determine the present-day relative amount of radiocarbon in the sample. Once this has been determined by either the conventional radiocarbon decay counting or by the AMS method (see Fig. 63), a number of internationally established conventions and assumptions are used to calculate the age of a material or object ... 

The activity of 0X1 changes through time as 14C in the standard decays (i.e., Aon measured in 2007 is less than if it were measured in 1950). For dating purposes, both the sample and AON decrease at the same rate (the radiocarbon decay constant). In other words, F14C is constant with time. However, when considering an open and dynamic system, such as soil, the need arises for a standard that represents a constant value. Stuiver and Polach (1977) thus proposed an absolute international standard activity (Aabs) that would incorporate a yearly correction for the decay in the 0X1 standard ... 

Some of these radiocarbon atoms find their way into iiving trees and other piants in the form of carbon dioxide, as a resuit of photosynthesis. When the tree is cut down photosynthesis stops and the ratio of radiocarbon atoms to stabie carbon atoms begins to faii as the radiocarbon decays. The ratio in the specimen can be measured and enables the time that has elapsed since the tree was cut down to be calculated. The method has been shown to give consistent results for specimens up to some 40 000 years old, though its accuracy depends upon assumptions concerning the past intensity of the cosmic radiation. The technique was developed by Willard F. Libby (1908-80) and his CO workers in 1946-47. 

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