Topic 8 Radiation for research

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Soon after radioactive substances were discovered in the late 19th century, they began to be used in research, medicine and industry. Today, radioactive substances are used every day in many parts of society. At ANSTO, the nuclear research reactor is used to generate fresh radioisotopes daily, as well as for irradiation and to produce neutron beams for research. Some medical radioisotopes can be produced in a cyclotron, such as ANSTO's cyclotron in Sydney, or by other small cyclotrons in some hospitals around Australia. Other ANSTO facilities, including particle accelerators and other advanced radiation facilities, do not use or produce any radioactive materials, but rather make use of high intensity radiation.

Radioisotopes as tracers

One of the most important and useful nuclear technologies today is the use of radioisotopes as tracers. A tracer is a substance that can be followed through a system, without affecting the performance of the system and its organs, in order to understand how that system works. Many different kinds of systems can be studied. They can be used to follow the development of a fertilised egg into a fully grown mouse (for example) and to see when and where different genes are switched on, to discover the sources and destinations of underground rivers, or track the migration of whole beaches of sand during powerful rip tides. It is also possible to produce an image of the blood supply to a brain tumour, or even a person's brain functions while they are thinking! How does such a versatile tool work?

A radioactive tracer can label a small particle, just like a colourful uniform labels a sportsperson in a game. In each case, the label makes the individual easy to identify and follow. Instead of labelling a whole person, radioisotopes enable individual atoms or molecules to be labelled and tracked! The 'visible' signal is the nuclear radiation given off by the radioisotope as it decays: a combination of alpha, beta and gamma radiation, depending on the isotope. This signal is detected on photographic film, or sometimes by using more sophisticated equipment such as a scintillation counter. Because a radioisotope has exactly the same chemical properties as a non-radioactive isotope of the same element, it can be incorporated into any of the molecules normally present in the system, and then 'fed' into the system where and when the researcher wants. Sometimes the labelled particles can be seen moving within the system. At other times the researcher must wait and measure when and where they exit, and make deductions about what is going on inside the system.

This tracer technique has been used by ANSTO to investigate the long-term sustainability of irrigation practices in NSW. By using trace amounts of 'heavy water' (water labelled with radioactive tritium instead of normal hydrogen), ANSTO scientists determined the sources and ages of the bore water used in cropping areas. By understanding subterranean water flows between the Macquarie River and the water bores, scientists can advise where, when and how much water can be sustainably used for agriculture, natural habitat, recreation and other human uses.

Figure 8.1 This geological transect shows underground water flow patterns (yellow) over bedrock, as detected using radioactive tracers or by borehole data.

Different radioisotopes are used for different purposes. Some have very short half-lives, and must be supplied fresh every week, or even daily. ANSTO produces these radioisotopes in its cyclotron and its nuclear research reactor, from where they are transported to hospitals, research institutions and industries around the country. Some are even exported to Asia, US and the UK. Radioisotopes with very short half-lives don't remain radioactive for long enough to require ongoing management. However, others become low-level radioactive waste after use, and require careful management.

Energy and particle beams

High-intensity radiation beams can be shone onto and through different kinds of materials to reveal their internal molecular or atomic structure. There are many different kinds of beams such as neutron, proton and electron beams and X-rays. Some are generated from a nuclear research reactor, some from other advanced equipment.

Beams of neutrons are very useful for studying the nanostructure and function of advanced materials such as superconductors, computer disk read-writer heads, heat-resistant ceramics and nano-composites - materials made of nano-sized particles. (One nanometre is one billionth of a metre). Because neutrons are small and uncharged, they can penetrate very deeply - even into dense materials such as lead. The neutrons pass easily through the large spaces within all atoms, but when they collide with a dense nucleus they are deflected in characteristic ways that can be measured and interpreted by scientists. This technique is called neutron scattering. It can reveal the atomic structure of materials, and even the precise orientation of atoms. The neutron flux from the replacement research reactor will power eight neutron beam instruments. Just a few of the questions scientists could try and answer include: What materials are most suitable for constructing buildings? How do clumps of material form together, move about or break up? What materials would make a safer, more durable car? Why do golf clubs bend but not break when swung quickly?

Figure 8.2 Gaps in the 3 m thick shielding (made of lead, boron, steel and concrete) of the High Flux Australian Reactor core allow neutrons to stream into ANSTO's neutron beam instruments.

Other beams have different properties and so are used for different purposes. A synchrotron can produce X-rays in intensely focused beams, which are used to study very tiny samples. Purified and crystallised proteins can be examined, and their molecular structure worked out – for example in the design of new medicines, or to understand the structures inside plant and animal cells. This technique is called X-ray crystallography.

Other techniques use beams of electrons or protons. Unlike neutrons and X-rays, these beams do not easily penetrate deeply, because their electric charge causes them to interact with the atoms on the surface of the target sample. In doing so, they can cause these atoms to emit their own radiation, and so reveal secrets about the material's surface composition and microstructure. Despite their charge, the beams can be used to examine a material's deeper structure. For this, a particle accelerator is used to accelerate protons to such high speeds that they are forced all the way through a sample and out the other side. This causes atoms within the sample to emit a radiation signature, thus revealing their secrets to science.

Figure 8.3 This helium refrigerator prepares crystal samples for particle beam research, by cooling them to -270°C!

Recently, ANSTO used a combination of these techniques to unlock the history of the heavy armour and helmet worn by a member of bushranger Ned Kelly's gang, Joe Byrne. They concluded the armour had probably been made from iron ploughshares, in a low temperature bushfire rather than a blacksmith's forge. Apparently Kelly didn't have any blacksmith mates – not even at gunpoint! Similar techniques can be used in environmental research, for example to identify the sources of particles of pollution in the atmosphere. These sources can than be cleaned up or shut down.

Figure 8.4 The Australian National Tandem Accelerator for Applied Research is used to help build a picture of the world's past climate, to precisely date precious artefacts and archaeological specimens, to analyse samples from nuclear sites, and in many other research programs.

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