Rafter Radiocarbon Laboratory

How is a radiocarbon age measured?

To determine the radiocarbon age of an organic material it is necessary to measure the proportion of radiocarbon (¹⁴C) in the carbon that it contains. There are currently two methods in use for doing this:

• Radiometric counting, in which the rate of radioactive decay of the ¹⁴C in the material is measured;
• Accelerator Mass Spectrometry, in which a particle accelerator is used to directly count the relative numbers of the atoms of the different carbon isotopes present in the material.

Once the ¹⁴C content is known, it is compared to that of a standard material. The difference between the sample material and the standard is attributed to the time that has elapsed since the sample was "alive". For example, the radiocarbon age of a piece of wood gives the time since the wood was growing.

How does radiometric counting work?
Radiometric counting determines the amount of ¹⁴C present in a sample by measuring its radioactivity. This is done by converting the carbon in the material being dated to a gas such as CO₂ or methane, or to benzene, and placing it in a suitable radiation detector. There are two types of counting systems in use:

• Gas counting, in which the sample is converted to methane or CO₂ which is used to fill a proportional counter. The decay of a ¹⁴C atom triggers an electrical discharge in the gas which is electronically detected. The rate at which the decays occur depends on the number of ¹⁴C atoms present in the sample gas. The photo shows the interior of a typical gas counting system, using thick steel plates to shield the counters inside from external radiation.
• Liquid scintillation counting, in which the sample carbon is converted to benzene, mixed with special organic compounds and placed in a transparent container. The solution emits a pulse of light whenever a ¹⁴C atom decays, and the light pulse is detected by sensitive photomultiplier tubes placed close to the container.

Both types of counting system work by measuring the rate of radioactive decay in the sample. The difference is in the technology used to do this.

The radiometric method can detect the presence of a ¹⁴C atom only when the atom undergoes radioactive decay. The radioactive decay law states that the rate at which a collection of radioactive atoms decay depends on the number of atoms present. So by determining the number of ¹⁴C decays that take place in a certain time we can calculate the number of ¹⁴C atoms in the detector, and if we also know the total amount of carbon this is enough to determine the concentration of ¹⁴C, which in turn allows us to calculate the age of the material.

However, the decay of a radioactive atom is a random event, and by measuring the number that occur in a given time we obtain only an estimate of the decay rate: if we repeat the measurement we will almost certainly get a different answer. The degree of uncertainty in the measurement can be calculated from statistical theory, so to attain a certain degree of precision in the measurement we can calculate how many decays we must count. But since the half life of ¹⁴C is 5730 years, this means that in a period of a few hours (or even days) only a minute fraction of the total number of ¹⁴C atoms present will register their presence by radioactive decay. So to reach the precision we require, we must ensure that there are enough ¹⁴C atoms in the detector to give us enough decays in a reasonable time. This typically amounts to several grams of carbon. Since in most cases the carbon is only one component of the material, radiocarbon dating using decay counting may require the consumption of many grams of the sample, and in some cases kilograms.

What is Accelerator Mass Spectrometry?
Accelerator Mass Spectrometry (AMS) is a technique for measuring the concentrations of rare isotopes that cannot be detected with conventional mass spectrometers. The original, and best known, application of AMS is radiocarbon dating, where the technical problem to be solved is the detection of the rare isotope ¹⁴C in the presence of the much more abundant isotopes ¹²C and ¹³C. The natural abundance of 14C is about one ¹⁴C atom per trillion (1012) atoms of ¹²C. Attempts to build a mass spectrometer to detect ¹⁴C were all unsuccessful for the reason that it was not possible to construct an instrument that could distinguish between ¹⁴C and and the common nitrogen isotope ¹⁴N, which comprises 75% of the earth's atmosphere.

AMS works by injecting negatively charged carbon ions from the material being analysed into a nuclear particle accelerator based on the electrostatic tandem accelerator principle. This device consists essentially of two linear accelerators joined end-to-end, with the join section (called the terminal) charged to a very high positive potential (3 million volts or higher). The negative ions are accelerated towards the positive potential. At the terminal they pass through either a very thin carbon film or a tube filled with gas at low pressure (the stripper), depending on the particular accelerator. Collisions with carbon or gas atoms in the stripper removes several electrons from the carbon ions, changing their polarity from negative to positive.

The positive ions are then accelerated through the second stage of the accelerator, reaching kinetic energies of the order of 10 to 30 million electron volts. The significance of this process for ¹⁴C measurements is that nitrogen negative ions are very unstable and do not survive long enough to reach the accelerator terminal. This has the effect of eliminating the ¹⁴N ions which would otherwise swamp the ¹⁴C ions. However, ¹⁴N is not the only problem. The instrument that produces the negative ions, the ion source, also inevitably produces negatively charged molecules that can mimic ¹⁴C, viz. ¹³CH^- and ¹²CH₂^-. These ions are stable, and while of relatively low abundance, are still intense enough to overwhelm the ¹⁴C ions. This problem is solved in the tandem accelerator at the stripper. Provided three or more electrons are removed from the molecular ions the molecules dissociate into their component atoms at the stripping stage. The kinetic energy thay had accumulated up to now is distributed among the separate atoms, none of which has the same energy as a single ¹⁴C ion. It is thus easy to distinguish the ¹⁴C from the more intense "background" caused by the dissociated molecules on the basis of their kinetic energy.

Accelerating the ions to high energy has one more advantage. At the kinetic energies typically used in an AMS system it is possible to use well-established nuclear physics techniques to detect the individual ¹⁴C ions as they arrive at a suitable particle detector. This may be a solid-state detector or a device based on the gridded ionisation chamber. The latter type of detector can measure both the total energy of the incoming ion, and also the rate at which it slows down as it passes through the gas-filled detector. These two pieces of information are sufficient to completely identify the ion as ¹⁴C.

Why use AMS?
The main advantage is the much smaller sample size that is needed to make a measurement. Radiometric counting can only detect ¹⁴C atoms at the rate at which they decay. This requires sufficient atoms to be present to provide a large enough decay rate, as described above. AMS, on the other hand, does not rely on radioactive decay to detect the ¹⁴C. The AMS technique literally extracts and counts the ¹⁴C atoms in the sample, and at the same time determines the amount of the stable isotopes ¹³C and ¹²C. As a consequence, a measurement that may last 12 hours and require several grams of sample using decay counting may take only 30 minutes and consume a few milligrams using AMS.

Are there any other advantages of AMS?
A small sample size may or may not be a decisive advantage in a particular case, depending on the task and the nature of the sample material. The real advantages of AMS lie in the possibilities it offers for doing completely new kinds of measurements and using new kinds of sample materials. Measuring the ¹⁴C content of seawater is much simpler when the sample size needed is about 100 cm3 than when hundreds of litres of water are required. Monitoring the ¹⁴C in rare atmospheric gases such as methane and carbon monoxide is virtually impossible using decay counting but quite feasible with AMS. AMS allows more accurate dating of sediment core sequences if pollen grains can be extracted and dated, something which is not possible with decay counting. Radiocarbon dating by AMS is now used by a number of museums and dealers in antiquities to authenticate the age of objects, such as wood carvings and textiles. The small samples required for AMS mean that it is possible to remove a sample for dating without significantly damaging the object.

A novel application of AMS is the measurement of ¹⁴C tracer used at near natural levels in biomedical and pharmaceutical research. While ¹⁴C has long been used as a tracer for chemical processes and pathways, the amount of tracer required using decay counting can be hazardous to the researchers, pose contamination problems or, in some cases, itself influence the process being studied. AMS allows very low levels of tracer to be used, completely avoiding these problems.

Are there any disadvantages?
AMS tends to be significantly more expensive than decay counting. Purchasing and maintaining a particle accelerator and its associated componentry is very capital intensive, and if this cost has to be recovered in the prices charged for dating it makes those prices comparatively high.