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ScienceWise - Summer 2013

Nuclear solution

Article Illustration
Some members of the AMS team with the accelerator to scale
Article Illustration
Inside the 14UD accelerator. When operating, the massive tank is filled with an insulating gas that prevents the 15 million Volt potential at the central terminal arcing across to the walls

Applying accelerator technology to some very Australian problems

The Australian National University hosts Australia’s largest and most powerful ion accelerator, the 14UD. Inside its massive concrete and steel tower, charging chains raise the potential of a central electrode to +15 million Volts. Negative ions created at the top of the tower are strongly attracted to this and accelerate towards it through an evacuated pipe in the accelerator’s core. At the peak potential the ions hit a microscopically thin carbon foil which strips off some of their electrons changing their charge from negative to positive. The now positively charged ions are repulsed by the positive central voltage and so are further accelerated as they leave.

At the bottom of the tower a huge magnet bends the ion beam and directs it into any one of several target lines at the end of which are a variety of nuclear physics experiments. Two of these belong to Professor Keith Fifield’s Accelerator Mass Spectrometry group.

Conventional, low-energy mass spectrometry exploits the fact that ions of different mass are bent at different angles in a magnetic field rather like the colours of light are split by a prism. In its crudest form, mass spectrometry simply enables elements of different masses such as say iron and cobalt to be separated. However the addition of the massive 14UD accelerator, coupled with techniques derived from fundamental nuclear physics research for identifying ions of the same mass but from different elements, makes the ANU system so sensitive it takes the concept to a whole new level.

Not only can this Accelerator Mass Spectrometry (AMS) system separate different isotopes of the same element and different elements with the same mass, but it also has the sensitivity to make reliable measurements on samples containing incredibly low concentrations of those isotopes. This makes it one of the most powerful tools for nuclear forensics anywhere in the world, attracting scientists from many countries to come here and use the facility.

But other than enhancing our credibility in international science, how does this kind of technology directly benefit Australia?

One of the cornerstones of our economic prosperity is agriculture and in a dry country like ours, one of the greatest threats is erosion. Many of Australia’s agricultural soils are very old, and are being replenished at very low rates by natural processes. Modern agricultural practice aims to conserve this valuable soil, but the effectiveness is difficult to assess without a method to measure the rates of soil loss and deposition. AMS can provide just such a method with a little help from what seems like a very unlikely source; the nuclear weapons tests of the cold war era.

During the 1950s and 60s there were literally hundreds of atmospheric tests of nuclear weapons. These introduced plutonium into the Earth’s atmosphere in small quantities. Dispersed across the entire surface of the Earth by stratospheric winds, this plutonium eventually fell to earth where due to its chemical properties, it bonded strongly with soil particles.

It is nowhere near as horrendous as it sounds because away from the immediate vicinity of the test sites, the concentrations are so incredibly small that the resulting radioactivity makes essentially no difference at all to the Earth’s natural radioactive background. The Plutonium’s presence is however, measurable using a super sensitive technique like AMS.

“We have a fairly clear picture of the isotopes of plutonium that were created by nuclear testing and the way they were distributed,” Professor Fifield says, “So when we find less plutonium than expected in a soil sample we can say with some confidence that the area has experienced significant erosion since the 1950s.”

The ANU team has successfully applied the technique in the prospective Daly Basin agricultural area in the Northern Territory, and in Canberra’s water catchment following the 2003 bush fires and subsequent torrential rains. But it’s not just the study of Australia’s recent history that can benefit from AMS. The technique also has applications over a far longer timescale.

Ever since the solar system formed, the Earth’s atmosphere and surface have been constantly bombarded by cosmic rays. These energetic particles from space occasionally score a direct hit on an iron nucleus within a rock.

This can transmute the iron into an isotope of manganese, 53Mn which has a half-life of around 3 million years. In an environment with no erosion at all, the 53Mn builds up in the surface rock until the production and decay rate are equal. However, if the rock surface is being slowly worn away, 53Mn is being lost to erosion and the concentration of the isotope is lower by an amount that is proportional to the erosion rate. Alternatively, if the surface has only been exposed for less than a few million years, then the concentration of the 53Mn isotope is a measure of the length of time since first exposure.

Although the long half-life is a bonus when it comes to using 53Mn as a geological marker, it does mean that the rate of radioactivity associated with its decay is very low. This would make it impossible to detect using methods that rely on detecting the emission of decay products. However since AMS directly measures the presence of the isotope by counting 53Mn atoms, this presents no problem at all. “We’re able to measure reliably concentrations corresponding to one atom of 53Mn in 100 million million atoms of iron!” Professor Fifield says, “This is equivalent to finding 20 grains of sugar in the Melbourne Cricket Ground filled to the brim with salt.”

The technique is well suited to Australia, because many of our landscapes are richly endowed with iron bearing rocks at the surface. Such 53Mn measurements complement the measurement of other isotopes by the group in their studies of soil erosion and exposure dating of landscape surface features. Taken together the work provides a valuable source of data to test climate change models.  For instance, it can be used to date the formation of desert features, the advance and retreat of glaciers, and the effects of changes in rainfall.

It’s yet another example of how a country built on primary production can benefit by also having world-class scientific research program.

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