Radiocarbon decay counting

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 ... 

For both decay counting and AMS, it is critical to prepare standard materials of known 14C/12C content. These include the 0X1 standard described below and materials relatable to it, as well as materials that are radiocarbon-free. Standards allow assessment of the overall accuracy and the effects of sample pretreatment procedures, and radiocarbon-free samples provide a blank to determine the radiocarbon introduced to the sample during processing. 

Table II. Sunnyvale Skeleton Radiocarbon Measurements by Carbon Dioxide Gas Proportional Decay Counting and...

Quantitation by decay counting is inefficient for isotopes with lives as long as radiocarbon, for which only 1 in 4.4 billion atoms decay per minute. 

Example To illustrate the capabihties of AMS, we compare carbon-14 determination by radioactive counting to AMS. A sanple of 1 g of environmental carbon contains 6 x 10 atoms of (and 1.2 x 10 times more atoms). Due to the 5730 years half-life of C, only 13 atoms will decay per minute. For a statistical precision of 0.5% as normally required in radiocarbon dating, decays from 1 g of carbon need to be counted for more than 48 h. AMS does not have to wait for the decays, it is more efficient because it uses the whole sanple. A sample of 1 mg carbon, only one thousandth of the material needed for decay counting, is completely sputtered in the ion source within 1-2 h and delivers about 6 x 10 atoms, which is 1% of the total content, to the AMS detector system Conventional mass spectrometers can not be used here, because the ions are superimposed by atomic and molecular isobars that are by orders of magnitude more abundant. These are and small fragments such as and Li2. Ab-... 

Gove, H. (1992). The history of AMS, its advantages over decay counting applications and prospects. In Radiocarbon After Four Decades An Interdisciplinary Perspective (R. E. Taylor, R. Kra, and A. Long, A., eds.). Springer-Verlag, New York. 

The usual procedure for radiocarbon dating is to bum a tiny sample of the object to be dated, collect the CO2 that is produced, and compare its rate of radioactive decay with that of a fresh CO2 sample. The ratio of counts gives Nq jN, which can then be substituted into Equation to calculate t. Mass spectroscopic isotope analysis can also be used to obtain the Nq jN value, as Example illustrates. 

Soon after this discovery the harnessing of the technique to the measurement of all the U isotopes and all the Th isotopes with great precision immediately opened up the entire field of uranium and thorium decay chain studies. This area of study was formerly the poaching ground for radioactive measurements alone but now became part of the wonderful world of mass spectrometric measurements. (The same transformation took place for radiocarbon from the various radioactive counting schemes to accelerator mass spectrometry.)... 

Expression and Interpretation of Results. Archaeological interpretation of a radiocarbon age may depend critically on the error associated with that age. Errors are commonly expressed as a variance range attached to the central number (e.g., 2250 80 years). The 80 years in this example may correspond to the random error for a single analytical step. Both decay and direct-atom counting are statistical in nature, and lead to errors that vary as the square root of the number of counts. The error may also be expressed as the overall random experimental error (the sum of individual errors.) Overall random error can be determined only by analyzing replicate samples. 

A living plant contains about the same fraction of carbon-14 as atmospheric carbon dioxide. The observed rate of decay of carbon-14 from a living plant is 15.3 counts per minute per gram of carbon. How many counts per minute per gram of carbon will be measured from a 15,000-yr-old sample Will radiocarbon dating work well for small samples of 10 mg or less ... 

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