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What has accelerator science ever done for me? |
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Views of Sydney during foggy and clear days. ANSTO scientists are using accelerators to tackle the big problem of atmospheric fine particles
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ANSTO's ANTARES accelerator
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Ask someone for their thoughts on accelerators, and they'd probably talk racing cars or speeding fines. But interrogate an ANSTO accelerator scientist, and they'd wax lyrical about using accelerators to analyse air pollutants in Aussie cities, verify a country's nuclear activities or even pinpoint the age of precious artefacts and archaeological specimens.
The Australian Nuclear Science and Technology Organisation (ANSTO) operates two tandem accelerators. While they might sound like super-charged bikes for lovers, in a tandem accelerator negatively charged ions gain energy by attraction to the very high positive voltage at the centre of the accelerator's pressure vessel. When ions arrive at this central region (called the high voltage terminal), electrons are stripped from them, and become positively charged and are then accelerated away by the high positive voltage. Voltages on these terminals can vary between two and ten megavolts.
These two stages of acceleration - first pulling then pushing the charged particles - give the tandem accelerator its name.
Interesting physics aside, ANSTO's scientists are using accelerators to solve tricky problems all the time. Take Dr David Cohen, who has been looking at the big problem of tiny particles of air pollution.
"Some atmospheric fine particles have diameters smaller than 2.5 microns," David said. (One micron is one thousandth of a millimetre.)
"They are typically produced by man-made combustion sources such as industrial plants and motor vehicles, or by natural sources such as windblown soil and sea spray."
David used sensitive Ion Beam Analysis (IBA) techniques to analyse the lead content in air, as collected on filters twice weekly at two sites in Sydney - one near the CBD and one in a relatively rural setting. The filters were non-destructively analysed using the ANTARES accelerator, allowing David to examine the concentration of up to 35 different elements including lead and manganese.
IBA involves firing a fast moving beam of ions at the sample being studied. When a high energy ion beam hits the sample, it interacts with the sample's atoms. This can cause the sample to emit X-rays or gamma rays, or cause the ions fired at the sample to scattered, showing the location and mass of atoms in the sample.
"The technique allows us to measure and assess the impact of individual sources such as lead and bromine from motor vehicles, aluminium and silicon from windblown soils, hydrogen and sulphur from industry, and sodium and chlorine from sea spray," David said.
With colleagues, David followed the reduction of fine particle lead in Sydney's air, from an annual average value of around 60 nanograms per cubic metre (ng/m³) in 1998 to less than 16 ng/m³ in 2004 (a nanogram is one billionth of a gram). In 1990, lead from petrol in NSW produced around 1 400 tonnes annually, but had declined to less than 100 tonnes by 2001.
"The reduction of lead in Sydney's air occurred because the amount of lead in petrol was reduced, and also because leaded petrol sales fell," David said.
While this is good news, David's team found an increase in manganese in Sydney's air, due to increased use of the lead replacement petrol additive MMT (methylcyclopentadienyl manganese tricarbonyl). At Mascot, annual average levels increased from around 2 ng/m³ to 5 ng/m³ during the greatest use of MMT.
"Australia discontinued the use of MMT in 2004. And though levels were always well below the reference concentration of 50 ng/m3, MMT is toxic when inhaled or swallowed and can cause skin discomfort," said David.
Because fine particles can remain in the atmosphere for weeks and can travel thousands of kilometres from their original source, it's important to understand their regional and even global movement.
"Our research discovered a larger than expected increase of manganese in air given the limited use of MMT, and show that high quality monitoring needs to be undertaken in countries where MMT is used in all petrol," David concluded.
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IBA is a non destructive method for discovering what elements are in a sample, but accelerator mass spectrometry (AMS) would be your tool of choice if you needed to know the ratio of isotopes in a sample. AMS is the only method that allows scientists to determine extremely low concentrations of long-lived radioisotopes, such as carbon-14 or uranium-236, in small environmental samples |
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While carbon dating is a well known application of AMS (see 'Dead Sea Scrolls of Buddhism'), most Aussies would be surprised to learn ANSTO routinely analyses samples on behalf of the International Atomic Energy Agency (IAEA) |
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Nuclear activities, whether legal or clandestine, may produce routine or accidental releases of radionuclides into the environment. So it makes sense to use environmental sampling to verifying a nation's compliance with agreements like the Comprehensive Nuclear Test Ban Treaty |
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The presence of iodine-129 and uranium-236 in environmental samples is a dead giveaway that a country is conducting nuclear activities like reprocessing irradiated nuclear fuel. As a member of the IAEA safeguard network of analytical laboratories, ANSTO scientists regularly use AMS to measure iodine-129 and uranium-236. |
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