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Gas counters

In the early 1950s, both proportional (Fig. 2) and Geiger gas counters were employed in 14C work, using carbon dioxide, carbon disulfide, acetylene, methane, or ethane as counting gases. As in the case of the solid carbon system, the center counter containing the sample was surrounded by individual Geiger tubes or an annular or continuous ring guard, all housed within an iron or lead shield assembly. Efforts to reduce the background values in gas detectors have resulted in various types of experimental arrangements, including the location of counters in underground vaults. In such underground facilities, the contribution of the meson flux, the major contributor to the background rate, can be significantly reduced. Because of the 90–95% efficiency in most gas detector systems, the typical maximum age limits were extended to 40,000–60,000 years, depending on the experimental configuration, including the volume of the detectors and the level of the background count rates in specific detectors. Isotopic enrichment of sample gases permits the maximum age attainable to be extended several additional half-lives. In general, sample-size requirements with gas detectors were reduced from that required with the solid carbon method—special systems being designed to permit the measurement of a sample with as little as 0.1 g (3.5 × 10−3 oz) of carbon.  See also: Geiger-Müller counter; Ionization chamber; Meson

Fig. 2  Large-volume proportional counter, which is used for carbon-14 measurements. The outer shield is closed by rolling doors. The sample is introduced into the radiation counter in the form of carbon dioxide gas. (Geochemical Laboratory, Lamont-Doherty Geological Observatory, Columbia University)

 Liquid scintillation systems

 Current liquid scintillation systems involve the conversion of samples to benzene. The addition of a scintillator chemical allows beta-decay events to be monitored by photomultiplier tubes. Earlier liquid scintillation systems used for 14C measurements generally required larger samples. However, currently the amounts required are comparable to gas counting systems. Continuing developments in liquid scintillation technology for low-level measurement have provided the ability to monitor counter performance in much greater detail than typically is possible in gas counting, and have also resulted in reduced background values. In these systems, the maximum ages that can be measured can be extended beyond that possible with typical gas systems.

 Direct detection

 Both of the conventional decay counting methods share a common problem in that they employ an inherently inefficient means of monitoring 14C concentrations in samples. In 1 g of modern carbon, for every decay per minute there are about 4 × 109 atoms of 14C. Counting methods have been developed which employ particle accelerators as very sensitive mass spectrometers, thus counting the 14C atoms directly.

It has long been recognized that if the 14C atoms could be detected directly, rather than by waiting for their decay, smaller samples could be used for dating and older dates could be measured. A simple hypothetical example to illustrate this point is a sample containing only one atom of 14C. To measure the age (that is, the abundance of 14C), the sample can be placed into a mass spectrometer and that atom counted, or the sample can be placed into a Geiger counter and counted, requiring a wait on the average of 8000 years (the mean life of 14C) for the decay. In practice, neither the atoms nor the decays can be counted with 100% efficiency, but the huge advantage for atom counting remains.

Until 1977, attempts at direct atom counting for 14C and other natural radioisotopes failed, because of the extremely low concentration of 14C. Ordinary mass spectrometers could not see the tiny 14C signal in the background of other atoms and molecules in the sample. Even trace amounts of nitrogen would swamp the 14C signal, since 14N forms ions with nearly identical mass and identical charge to that of the 14C atoms.

The technique for detecting 14C atoms [and other natural radioisotopes such as tritium, beryllium-10 (10Be), and chlorine-36 (36Cl)] is a combination of mass spectrometry and accelerator technology, called accelerator mass spectrometry (AMS). The approach was first demonstrated using a cyclotron. This accelerator, which sent particles along a spiral trajectory, was used as an ultrasensitive mass spectrometer to distinguish ionized carbon isotopes by their charge-to-mass ratio. Detecting 14C by this means was possible, but consistent results proved difficult to achieve despite years of effort.  See also: Mass spectrometry

Another type of AMS technology uses a tandem electrostatic accelerator (Fig. 3). The device employs two stages. First, a negative ion beam is accelerated and passed through a stripper, which removes the electrons, converting the beam to positive ions. Then the particles are further accelerated. The stripping process breaks up molecules of mass 14, which would otherwise interfere with the detection of 14C, and the negative ion beam eliminates 14N since there are no known stable negative nitrogen ions. Almost all AMS 14C applications currently use tandem accelerators to accomplish routine measurements.  See also: Particle accelerator

  Fig. 3  Accelerator mass spectrometry system for the direct detection of 14C atoms. (Center for Mass Spectrometry, Lawrence Livermore National Laboratory)

 The advent of AMS technology in the late 1970s brought about an enormous boost in detection efficiency that promised three important advantages for 14C dating. First the amount of carbon required was reduced from grams to milligrams. Second, counting times were reduced from days, weeks, or even months to minutes. Finally, it was initially thought that the detection sensitivity would increase so that the maximum age datable with 14C might be extended to 100,000 years. However, sensitivity is limited by very small amounts of contamination introduced during sample preparation. Much of this contamination stems from the requirement in most laboratories that samples be converted to graphitic carbon for measurement. In routine operation, current AMS technology can measure between 40,000 and 50,000 years, and rarely, 60,000 years.

The ability to use milligram (rather than gram) samples is very important for dating. Certain irreplaceable objects (for example, parchments, cloth, and chips of wood) would have had to be destroyed in order to extract the gram of carbon required for a date; for the accelerator method, only a small piece of the artifact is required. In addition, the ability to date by using only milligrams of carbon allows careful selection of the sample used. Any part of the object which may have been contaminated by modern carbon can be ignored; small seeds trapped in the object, or even specific amino acid compounds which are less likely to come from modern carbon contamination, can be selected.

Despite the sensitivity of the accelerator technique, decay dating will probably continue to be used for 14C dating when gram amounts of carbon are available. However, the technology of atomic mass spectrometry continues to be developed and will increasingly be used for routine 14C analysis. The accuracy of 14C values based on atomic mass spectrometry has become essentially comparable to that obtained with decay counting.

 Accuracy of Radiocarbon Determinations

 A measurement of the 14C content of an organic sample will provide an accurate determination of the sample's age if it is assumed that (1) the production of 14C by cosmic rays has remained essentially constant long enough to establish a steady state in the 14C/12C ratio in the atmosphere, (2) there has been a complete and rapid mixing of 14C throughout the various carbon reservoirs, (3) the carbon isotope ratio in the sample has not been altered except by 14C decay, and (4) the total amount of carbon in any reservoir has not been altered. In addition, the half-life of 14C must be known with sufficient accuracy, and it must be possible to measure natural levels of 14C to appropriate levels of accuracy and precision. Studies have shown that the primary assumptions on which the method rests have been violated both systematically and to varying degrees for particular sample types. Several approaches have been developed to provide calibration and corrections of conventional 14C values. The basis of the calibration and correction procedures will be discussed in the context of a brief review of the assumptions of the method.

 Constancy in radiocarbon production rates

Carbon-14 determinations on known-age samples have revealed systematic discrepancies in the 14C time scale. The first hint of such anomalies came from early 14C measurements on Egyptian archeological materials. Samples which, on historical grounds, should have dated to the early part of the third millennium B.C. yielded 14C values some 700–800 years too young. Carbon-14 determinations carried out on dendrochronologically dated wood, periglacial varves, and lake sediments confirmed the fact that there have been systematic variations in 14C values over time. The data which first contributed most directly to the study of these anomalies was the dendrochronological time scale provided by the bristlecone pine (Pinus longaeva), from the White Mountains of east-central California, developed by C. W. Fergusson. His data provide an unbroken tree-ring series back to almost 6700 B.C. An independently developed bristlecone tree-ring chronology from a different locality in the southern portion of the White Mountains, developed by V. C. La Marche and T. P. Harlan, supports the accuracy of the Fergusson chronology at least as far back as about 3500 B.C. Carbon-14 determinations on bristlecone pine as well as tree-ring-dated samples from the sequoia (Sequoia gigantea) and European oaks (Quercus spp.) have been undertaken by a number of laboratories, and 14C determinations on these samples provide data over the last 10,000 years.  See also: Dendrochronology; Varve