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Feature

Counting Atoms One by One by One...

A new, ultra-sensitive means of analysing the atomic makeup of materials is providing interesting information.

by Rodger Sparks

Cosmic rays, particle accelerators, greenhouse gases, reactor emissions, fish earbones and the Shroud of Turin -- all these have been closely examined using the technique of accelerator mass spectrometry.

AMS is a technique that uses a nuclear particle accelerator as the heart of a powerful mass spectrometer capable of making extremely sensitive measurements of "cosmogenic" isotopes in the environment. These are isotopes that are generated by cosmic rays striking the Earth's atmosphere and surface.

The best known cosmogenic isotope is radiocarbon (14C), which forms the basis of radiocarbon dating. The AMS technique was first applied to radiocarbon detection, and it immediately caused a revolution in radiocarbon dating. This was because AMS is many thousands of times more sensitive than the conventional detection method.

To measure radiocarbon conventionally means waiting until the radiocarbon signals its presence by undergoing radioactive decay, and detecting the evidence of this decay with a suitable radiation counter. Since a radiocarbon atom survives, on average, about 8,300 years after it is formed, only a very small fraction of the total present will decay during the time they are in the counter.

AMS's sensitivity comes about because it counts the radiocarbon atoms actually present in the material after they are extracted as a beam of charged particles. As a consequence, AMS makes it possible to perform measurements on very much smaller amounts of material than was previously possible. For example, to date a piece of wood typically requires a piece weighing 10 to 20 grams using the radioactive decay method, whereas AMS can use a splinter weighing 10 milligrams or less.

The most famous application of AMS was the dating of the Shroud of Turin, the claimed burial shroud of Christ. This dating was possible because the sensitivity of AMS meant that an insignificantly small portion of the Shroud could be removed for testing; radioactive decay counting would have required so much material that the Shroud would have been effectively destroyed. We now know that it would not have mattered too much -- the dating revealed the Shroud to have been produced sometime between 1260 and 1384 -- but that is being wise after the event.

At Lower Hutt we use AMS to measure the radiocarbon contained in the atmospheric trace gases methane and carbon monoxide. The measurements give information about the processes governing the formation and destruction of these gases, which have an important role in the greenhouse effect.

Something Fishy

One study has looked at the radiocarbon content of the surface seawater around New Zealand during this century, as recorded in snapper earbones, or otoliths. Otoliths are small carbonate objects found in the heads of many fish species, which allow a fish to orient itself in the water.

Examining the radiocarbon content of tiny otoliths weighing a few milligrams has revealed one effect of the nuclear weapons testing programmes of the 1950s and 60s on the level of environmental radiocarbon. The measurements show the radiocarbon "spike" injected into the atmosphere during the period of the tests, and provide information on the rate at which atmospheric carbon dioxide passes from the atmosphere to the oceans.

The usefulness of AMS is not confined to radiocarbon. There are other isotopes being produced in the atmosphere that can be used as tracers or for dating natural processes. Chief among these are radioisotopes of beryllium, aluminium, chlorine and calcium (10Be, 26Al, 36Cl and 41Ca). They are all radioactive, making them useful as natural clocks, but their half-lives are so long -- 1.5 million years in the case of 10Be -- that the radiation they produce is exceedingly weak, making them even harder to detect than radiocarbon. With AMS this is not a problem, since the radioactivity does not play a part in the detection.

Recycling Beryllium

The isotope 10Be is formed in the atmosphere, and some finds its way into the sea and is eventually deposited in the sediments on the sea floor. In regions where one of the Earth's tectonic plates is drawn under a neighbouring plate -- a process called subduction -- such as occurs under New Zealand and the Aleutian Islands, the sediment may become mixed in with volcanic magma and be "recycled" in volcanic eruptions.

This process can be observed by detecting the 10Be present in the magma, and has been shown to occur in some so-called "island arc" volcanoes located near plate boundaries. The presence or absence of such recycled 10Be can be used to obtain information about the fate of subducted sediments, and is thought to be related to the likelihood of seismic activity in the region. As much as 40% of the 10Be, and hence the ocean floor sediment, that is subducted at the Aleutian Islands may be recycled back to the surface in volcanic activity.

As the AMS technique is refined and becomes even more sensitive, attention is turning to the study of cosmogenic isotopes formed directly in the surface layers of the earth, as distinct from those formed in the atmosphere and deposited on the surface. Cosmic rays reaching the surface can form isotopes such as 10Be and 26Al directly in the rocks. The isotopes decay at different rates, so by measuring the ratio of the two quantities it is possible to infer the exposure history of the surface, enabling, for example, rates of erosion over long time scales to be deduced.

It is also possible to study these isotopes formed in glacial ice, providing estimates of glacial advance and retreat rates over long periods, and this in turn gives important information for climate models. The study of in situ production of cosmogenic isotopes is likely to be one of the more exciting developments coming from the AMS technique, and it could not have been done in any other way.

Another area where AMS is set to have a major impact is in biomedical-related research. Radiocarbon has long been used as a tracer to study reaction kinetics and mechanisms in biological and biochemical processes, but large quantities of the isotope have been required because of the need to use radioactive decay techniques to follow its progress. This can lead to a number of problems, including the possible side effects on the organism being studied due to the radiation doses being administered, and the need for special safety precautions by those performing the experiments.

AMS, on the other hand, allows the tracer to be used at near-natural levels. Even at radiocarbon levels several thousand times those found in nature, the radiation produced is insignificant and neither affects the experiment nor poses a hazard to the researchers. The sensitivity of AMS detection makes it possible to conduct experiments in which specific locations of complex compounds can be labelled with radiocarbon and the compounds administered at very low concentrations.

Already it looks possible that, for example, conclusions drawn about the behaviour of some carcinogens as a result of high concentration experiments may need to be revised in the light of recent low-level experiments where AMS is used to detect the tracer.

Ancient Mice

At the Lawrence Livermore National Laboratory in California they are pushing the sensitivity of the method even further by breeding a strain of laboratory mice that are depleted in natural radiocarbon, so lowering the natural background that limits the sensitivity. Generations of the mice are being reared on special foodstock based on petrochemical derivatives that contain little or no radiocarbon. They now have obtained mice that, if radiocarbon dated, would appear to be over 11,000 years old.

So great is the potential seen for the biomedical applications of AMS that special, dedicated accelerator systems designed specifically for biomedical research are being discussed.

AMS provides a classic example of a technology whose time had come. From its beginnings in 1977, physicists in accelerator laboratories around the world, including New Zealand, sat up and took notice, as it offered to revitalise smaller particle accelerators that were being left behind as the frontiers of nuclear physics moved to higher and higher energies. The southern hemisphere's first dedicated accelerator mass spectrometry facility was installed in Lower Hutt in 1987, and is now operated by the Nuclear Sciences Group of the Institute of Geological and Nuclear Sciences.

It was immediately apparent that this was not a technique developed for technology's sake. AMS was the solution to a problem that had long frustrated isotope-dating laboratories everywhere -- how to detect a rare radioisotope without having to wait for it to decay. That the solution would lie in some kind of mass spectrometer was easy to see, but the technical problems of building an instrument with the required sensitivity had thwarted all previous attempts. Nuclear physics succeeded where other approaches had failed, and since then AMS has been a spectacular success.

The one sure thing that can be said about it is that its full potential has not yet been explored.

About Mass Spectrometry

The principles behind a mass spectrometer are fairly simple. When an electrically-charged atom moves through an electric field, its path curves. How much it curves depends on how fast it's travelling, its electric charge and how heavy it is -- light, highly-charged or fast-moving atoms curve the most.

With a bit of technical wizardry to compensate for the effects of speed differences, it's possible to arrange things so that the path of an atom leaving the field is dependant -- and very precisely so -- on its mass and charge. Set up a target to show what paths the atoms take, and you can work out what atoms are in the beam and in what proportions, right down to differentiating between different isotopes of the same element.

Rodger Sparks is Section Leader of the Rafter Radiocarbon Laboratory, Institute of Geological & Nuclear Sciences.