Radiocarbon dating with accelerator mass spectrometry (AMS)

Radiocarbon (¹⁴C) is one of the radioisotopes produced in the atmosphere by cosmic rays coming into earth’s atmosphere. The ¹⁴C atoms produced exist mainly as carbon dioxide molecules in abundant levels to the stable carbon isotope 12C, i.e. ¹⁴C/12C ratio of 1.2 x 10-12. As carbon atoms in the atmosphere are incorporated in living material by photosynthesis, ¹⁴C atoms also move into the living material in the similar isotope ratio ¹⁴C/12C of 1.2 x 10-12. When the living material dies, the ¹⁴C atoms contained in the living material start to decrease by the radioactive decay process. The half-life of ¹⁴C is 5730 years, and the ratio in the present abundance of the ¹⁴C atoms in the sample material after decreasing by the decay process to its initial abundance is translated as the age that passed since the time of death of the living material. This is the principle of ¹⁴C dating.

 

¹⁴C dating can provide an objective method for determining the time ranges within which a sample taken from a cultural relic or antiquity may be placed. However it can only date the material itself which might not necessarily be the date of the process by which the object was made (i.e. the carving of wood, or the weaving of a textile) as the time interval between the formation of the material and its death and the actual production of the object could be quite different.

 

Accelerator Mass Spectrometry (AMS)

AMS is an ultra-sensitive technique for measuring the concentrations of rare radioisotopes that are as low as 10-12 to 10-16 in ratio to their stable isotopes. In the case of ¹⁴C measurements with AMS, a carbon amount less than 1 mg is enough for measurement with the statistical uncertainty of about 0.5%, where as more than 1 g of carbon is required for the conventional decay counting method. In ¹⁴C measurements with AMS, a tandem electrostatic accelerator is normally used. (The necessity of a tandem accelerator will be described below.) The AMS system consists of five parts; (1) cesium sputter negative ion source, (2) negative ion beam injector to accelerator, (3) a tandem accelerator, (4) a high energy ion beam separator consisting of a high-sensitive magnetic mass analyzer and an electrostatic analyzer, and (5) a heavy ion detector. Each of the five parts will be described briefly in the following.

 

Cesium sputter negative ion source

In ¹⁴C measurement with AMS, special care is paid to eliminate 14N, the main abundant isotope of nitrogen, an isobar of ¹⁴C. Since ¹⁴C and 14N possess identical mass numbers, it is difficult to separate 14N from ¹⁴C atoms by a magnetic mass analyzer. It is well known that the negative ion of 14N is unstable, and there is no chance for 14N atoms to mix into negative carbon ions. For this reason, a tandem accelerator is used for the AMS system, with negative-ion injection into the accelerator and positive-ion output from the accelerator end. In the cesium sputter negative ion source, a graphite target (the production process is described below) is used and negative carbon ions are produced. Though negative ions of 14N are not produced, negative carbon molecular ions 13CH- and 12CH2- with the same mass number as ¹⁴C are produced in a large amount, about 106 times more than ¹⁴C- ions. These molecular ions may disturb the ¹⁴C detection and counting. However, these ions can be eliminated from carbon atomic ions in the tandem accelerator as described later.

 

Negative ion beam injector to tandem accelerator

In the cesium sputter negative ion source, in addition to carbon negative atoms 12C-, 13C-, ¹⁴C-, several other ions with different mass number are produced simultaneously. Among them, mass numbers 12, 13, 14 are selected by a mass analyzing magnet, and ions with such mass numbers are sequentially injected into the tandem accelerator by using a bouncer system, to quantify the respective amounts of 12C, 13C and ¹⁴C atoms.

 

Tandem accelerator

A tandem accelerator consists essentially of two linear accelerator tubes joined with the terminal section. A high positive voltage (0.2 - 5 million volts) is applied to the terminal. The negative ions are injected at the low voltage end of the accelerator tube and electrostatically accelerated toward the terminal of positive high voltage. At the terminal, they pass through either a very thin carbon film or a tiny tube filled with argon gas at a low pressure (called as stripper). Collisions of negative ions with carbon film or argon gas in the stripper takes place and a few electrons are removed from the negative carbon ions, and the ions are changed of their polarity from negative to positive. The produced positive ions are then accelerated through the second acceleration tube, reaching kinetic energies of the order of 0.4 to 30 million electron volts.

During the charge exchange process, 13CH- and 12CH2- atoms that may prevent ¹⁴C detection and counting also react with carbon film or argon gas and dissociate into their component atoms. The kinetic energy of the molecules is distributed among the dissociated individual atoms, none of which has the same kinetic energy as a single ¹⁴C ion. It is easy to separate the 12C, 13C and ¹⁴C atoms from the "background" ions caused by the molecule dissociation, through the selection of the carbon ions with genuine kinetic energy.

 

Measurement of 12C and 13C beam intensities with high-energy ion beam separator

For the precise measurement of 12C, 13C and ¹⁴C contents of the carbonaceous sample, those of the NIST (National Institute of Standards and Technology) standard carbon whose isotope ratio is known are quantified as well, and ¹⁴C/12C ratios for sample ((¹⁴C/12C)spl) and those for the NIST standard ((¹⁴C/12C)NIST; known value) are compared with each other, to evaluate the value of (¹⁴C/12C)spl and its experimental error. The contents of 12C and 13C atoms for the relevant graphite target are quantified in the high-energy ion beam separator. The intensities of 12C and 13C positive-ion beams are measured by respective Faraday cup after mass analysis with a high precision bending magnet.

 

Heavy ion detector

At the kinetic energy of the ion beam typically used in an AMS system, an ion detector is available to identify and count the individual ¹⁴C ions. The detector may be a solid-state detector or a rather complicated device based on the gridded ionization detector. The latter type can measure both the total energy of the incoming ion and also the energy-loss rate (proportional to atomic number squared of the incoming ion) when the ion passes through the initial part of the detector. The two sets of information are sufficient to identify perfectly the ¹⁴C ion.

 

The characteristics of ¹⁴C AMS measurement in contrast to the radiometric method

The main advantage of ¹⁴C AMS is that it requires much smaller sample sizes for measurement, and it is less probable to produce any significant damage to the sampled object. The traditional radiometric counting method detects and counts the disintegration process of ¹⁴C atoms and the process is difficult for carbon samples of limited amounts, and inevitably a larger sample is required. The AMS system, on the other hand, identifies and directly counts the ¹⁴C atoms according to the techniques of well-established nuclear physics. In addition, the system can determine the amount of the stable isotopes 12C and 13C at the same time. As a consequence the elaborate radiometric measurement that takes 24 hours and requires several grams of carbon sample has been replaced by the simple ¹⁴C AMS measurement that takes only 30 minutes and requires only one hundred milligrams of carbon sample.

As a new trend, a compact tandem accelerator with terminal voltage less than 0.5 million volts is used to make up an AMS system for ¹⁴C measurement. It has been demonstrated that the AMS system with a small accelerator can achieve high-precision ¹⁴C measurement, which has recently been more commonly used in the world.