Accelerator Mass Spectrometry

Watch a Video: Albert Zondervan explains the workings of the modified compact AMS particle accelerator at the National Isotope Centre, GNS Science, New Zealand.

What is AMS?

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 you are trying to detect 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.

How does AMS work?

A nuclear particle accelerator 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).

A nuclear particle accelerator 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).

1. Injecting negatively charged carbon ions from the material being analysed into a nuclear particle accelerator based on the electrostatic tandem accelerator principle.
2. 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 remove several electrons from the carbon ions, changing their polarity from negative to positive.
3. 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. This has the effect of eliminating the 14N ions (that are extremely common in the Earth’s atmosphere) which would otherwise swamp the 14C ions.
4. 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 –if three or more electrons are removed from the molecular ions the molecules dissociate into their component atoms.
5. The kinetic energy that 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.

ams sampling

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 take several days and require grams of sample using decay counting may take only 30 minutes and consume a milligram 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. For example:

  • 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 or single seeds 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 of AMS?

AMS tends to be more expensive than decay counting because purchasing and maintaining a particle accelerator and its associated components is expensive.