AUSTRALIAN FRONTIERS OF SCIENCE, 2008

The Shine Dome, Canberra, 21-22 February

An application of Accelerator Mass Spectrometry (AMS): Cosmogenic exposure dating and the history of Australian arid landforms
by Dr Toshiyuki Fujioka

Toshiyuki Fujioka has a degree in physics and masters in geochemistry, both from Osaka University in Japan, and a PhD from the Research School of Earth Sciences at the Australian National University. His PhD thesis is entitled Development of cosmogenic 21Ne exposure dating and its application to Australian arid landforms combined with 10Be and 26Al, in which he established a method of determining cosmogenic 21Ne in the surface rocks and discussed formation history of stony deserts and dune-fields in Australia. He is currenty a Postdoctoral Fellow in the Department of Nuclear Physics at the ANU, and continues to work in the field of cosmogenic exposure dating and its geological application, exploring the method for a new cosmogenic nuclide, 53Mn, and its application to iron-rich desert rocks in arid part of Australia.

As you have seen from Nanda Dasgupta's talk, the accelerator is very powerful in nuclear physics research. But the accelerator can be used for many other branches of science as well.

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Today I want to talk about AMS, accelerator mass spectrometry, and one of the geological applications: cosmogenic exposure dating and the dating of Australian arid landforms from my PhD research, followed by some other AMS applications.

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First I want to talk about what mass spectrometry is. Most of you probably already know that mass spectrometry is a technique to measure specific isotopes, spatially separating them from other isotopes. Ions are extracted from the sample at the ion source and accelerated through the magnetic field, and the trajectory of the ions bends, depending on the mass of the ions. So by adjusting the strength of the magnetic field you can select isotopes of interest from the other isotopes.

One of the main difficulties of conventional mass spectrometry is molecular interference. Because the ions are selected by mass, a variety of the molecules which have the same mass as the isotope you want will also go to the detector.

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AMS overcomes that fundamental problem in conventional mass spectrometry. The AMS system incorporates an accelerator between the ion source and the detector. In the middle of the accelerator is a high-voltage terminal, which in the case of the ANU system can be charged up to 14 million volts. Negative ions are extracted at the ion source; they are selected by mass and injected into the accelerator. Basically, we use negative ions in the AMS method, and these ions are accelerated up to the centre part of the accelerator.

In the middle part of the accelerator there are carbon foils and a gas stripper. Any molecules going into the accelerator are broken up at this part.

Also, several electrons are stripped off from atoms when they go through this foil or stripper, so basically what is left after this part are multiply-charged positive ions. These positive ions are further accelerated down to the exit of the accelerator, and later selected by the analysing magnet, then led to the detector.

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The advantage of the AMS is that there is no molecular interference, which is often a problem in the conventional methods of mass spectrometry. Also, the large energy given to the ions improves identification of atoms of interest.

Combining these advantages, the AMS can achieve exquisite sensitivity. You can detect one radioisotope atom per 1015 of other atoms, which is equivalent to finding two grains of sugar in a Melbourne Cricket Ground filled to the brim with salt!

This system has a lot of applications across a variety of science, including archaeology, geology, hydrology, pharmacology and medicine. My PhD research is, I think, a good example.

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My PhD topic was the dating of Australian desert landforms, using cosmogenic exposure dating. More than 75 per cent of Australia is classified as having an arid to semi-arid climate, and the continent is covered by various types of desert landforms, such as stony deserts, dune fields and dry lakes. The fundamental question for these landforms is: when and how did they form? Actually, we don't have much knowledge about that, because not many dating methods are available for these types of landscape.

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Cosmogenic exposure dating offers a way to date the landscape. Cosmogenic nuclides are produced by the spallation reaction of cosmic rays with element in the surface rocks where cosmic rays are high energy particles coming from distant space, probably the centre of the galaxy and are striking the Earth.

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Here we have a schematic image of how the cosmogenic nuclides are produced. Cosmic ray particles hit the atoms or molecules in the atmosphere, which produces a cascade of secondary cosmic ray particles such as neutrons and muons. As those particles hit the elements in the rocks at the Earth's surface, they produce several types of nuclides and we call these new nuclides 'cosmogenic nuclides'.

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The principle of the exposure dating is that cosmogenic nuclides are produced and accumulate, in the first order approximation, at a constant rate in exposed rocks, so if we can measure the amount of these nuclides in the surface rocks we can calculate how long the rock has been exposed. The production rate of these nuclides is very small, one to 100 atoms per gram of rock per year, which means that even if the rock is exposed for 10,000 or 1,000,000 years, we still need the sensitivity to detect one atom in 1013 or 1015 of other atoms. No mass spectrometry can do this measurement except AMS.

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I did some fieldwork in the centre of Australia. This is a picture from near Coober Pedy, a very typical landscape in central Australia. There is almost nothing, just some rock cover all over the surface. We collected stony rock samples from the top of the tableland and the underlying fan, and measured the cosmogenic nuclides. And by combining the nuclide data with some geomorphological modelling we deduced the exposure age of the fan and the tableland as a couple of million years.

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I also took some samples from the dune field in the west Simpson Desert, again combining measurement of the cosmogenic nuclides in sand dunes with the geomorphological model. We concluded that the age of the dunes in the west Simpson Desert dates to one million years.

My PhD thesis goes into some discussion of the history of aridification in Australia and the relationship with the global climate of the last couple of million years. But I will not go into that in this talk because it is slightly off the topic.

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In the following couple of slides I am going to show some other AMS applications.

The first, very famous application is radiocarbon dating. I will give you a bit of detail about radiocarbon dating. Radiocarbon dating was the first application for which the AMS method was developed. Carbon has three isotopes, carbon-12, carbon-13 and carbon-14. Only carbon-14 is radioactive.

The ratio between carbon-14 and carbon-12 in the atmosphere is equilibrated. That can be incorporated into organic materials through the respiration of plants or animals; when they die respiration stops. Then the carbon-14 starts decaying. If you can measure the residual amount of carbon-14 in the fossil of a plant or animal you can estimate the age of the fossil. That is the basic idea of carbon-14 dating.

Conventionally the carbon-14 has been measured by decay counting. This method has been limited by the rate of radioactive decay. For example, to achieve 0.5 per cent precision, you need 48 hours of decay counting with one gram of the sample. But, in contrast, if you use AMS you need only 10 minutes of counting the carbon-14 directly, and then the sample requirement is only one milligram. So because radiocarbon dating is in high demand from archaeologists, and their samples are usually very rare, AMS has a very significant advantage in archaeological studies.

Also, the high sensitivity of AMS offers the dating of organic products up to 57,000 years, which is almost double the ability of decay counting.

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Homo Floresiensis from Indonesia was dated by radiocarbon. This skull was measured by the AMS group at ANU.

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There are other types of AMS applications, such as 'tracing'. ANU is the first to develop a plutonium method for environmental tracing studies. Plutonium was emitted at the atomic bomb tests during the 1950s. It became mixed in the atmosphere, and was then distributed all over the globe. The plutonium fell to the ground in rain and was absorbed into the soil, where it readily attached to the soil particles.

From the measurement of the plutonium concentration in soil or sediment, you can evaluate erosion rate or source of sediment in a catchment area. The plutonium concentration in the soil is very small, like one atom in 1012 of the other atoms, so again only AMS can detect such small amounts of atoms.

The AMS group at ANU is now working with the water and energy company ActewAGL to investigate the significance of soil erosion in the Cotter area west of Canberra after the 2003 bushfires in Canberra.

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Finally, the tracing technique using radioisotopes also has a high demand from biomedical research. The role of AMS in this field has been increasing recently. AMS so far has the major advantage of using very much smaller doses of the radioactive isotope for the tracings than are required by the traditional liquid-scintillation counting, which reduces the expense of preparing the labelling compound and also reduces the problem of disposal. It also allows more detailed study of the human body.

Biological systems are very complicated, so for a meaningful conclusion from these kinds of studies you need both a large number of samples and also very good control of them. So if AMS is to play an important role in the future in this field, perhaps emphasis should be placed on the simplification and measurement of sample preparation, and also low-cost operation of the AMS system.

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In summary, AMS can achieve exquisite sensitivity. It can measure ultra-low concentrations of atoms from the 10-12 to the 10-15 level.

AMS has diverse applications in fields such as archaeology, geology, palaeoclimate, environment, medicine as I have shown in my talk and also hydrology for ground water tracing, mining exploration for the identification of ore deposits, and nuclear safeguards like monitoring nuclear waste.

The development of the detection system and techniques for using new isotopes is ongoing, and we are really enthusiastic about exploring new applications in the various disciplines of science.