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Topic 9 Radiation in medicine and industry
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In Topic 8 Radiation for research, we saw how nuclear technologies can be used to see inside many different materials and systems. These techniques are also very useful in medicine for diagnosing illnesses, as well as to control and improve industrial processes. But 'seeing' is not the only useful application of nuclear science: 'touching' is also possible. Radiation can be used to alter materials or living cells, sometimes on a molecular scale.
Radioactive substances began to be used in all kinds of ways almost as soon as they were discovered. Early in the 20th century, before the biologically damaging effects of nuclear radiation were understood, some of these applications were very dangerous. Highly radioactive radium was used to treat arthritic joints, sometimes with disastrous results. Water containing highly radioactive radium was sold as a health tonic. In the 1920s, brightly glowing radioactive paint became popular. When many women working in the paint factories began to die of cancer, people realised this technology needed to be used with greater care. However, it was some time before the hazard was fully recognised and radioactive paint was still being used up to the 1960s.
Nuclear medicine
Today, radioactive substances (for example radiopharmaceuticals) are very useful in medicine, as long as the dose is carefully controlled. In Australia, about 550,000 people benefit from nuclear medical procedures every year, and most people will make use of nuclear medicine at some stage in their lives. Two of the major tools in nuclear medicine are radioisotopes, and energy and particle beams.
Radiopharmaceuticals can be used as tracers to diagnose medical problems, or to treat certain illnesses. The tracers used in medicine are gamma emitters – they give off gamma radiation. Gamma radiation is less biologically damaging than alpha or beta, and is strong enough to escape from within the body and reach the measuring instruments outside. The gamma emitter used in about 80% of nuclear medicine diagnoses is technetium-99m. It is a very unusual but readily available radioisotope. It is formed from molybdenum-99 in a research reactor and supplied to hospitals in a lead pot enclosing a glass tube containing the radioisotope. The molybdenum-99 has a half-life of 66 hours and decays to technetium-99m, which can be extracted from the pot when it is needed for medical use for a period of up to 2 weeks. In Australia the source of molybdenum-99 is ANSTO's research reactor.
Medical tracers such as radioactive forms of atoms can be attached to molecules that target specific tissues in the body, such as cancerous tumours or organs such as the liver, lungs, heart or kidneys that are not functioning normally. The radioisotope then allows the location of the tumour or the characteristics of the other tissue to be revealed. The process is one of the forms of imaging that can be used in medicine. It is called positron emission tomography (PET). This technique can even be used to show brain activity in real time – a movie of your thought processes.
Many PET radioisotopes are short lived (their half-life is usually only a matter of minutes), which means that most PET facilities have to be close to the cyclotrons that generate the radioisotopes. PET isotopes with a slightly longer half-life, such as fluorine-18 (with a half-life of 110 minutes), are also commonly used and this allows hospitals within 1–2 hours of the cyclotron to provide PET. The Royal Adelaide Hospital currently obtains its fluorine-18 from Melbourne on a daily basis.
The other major technique in nuclear medicine is irradiation. Gamma rays are focused onto cancers located in the body in places that scalpels can't reach or where cutting is best avoided. Only the tissue that receives the concentrated dose is destroyed. This is called radiotherapy.
Other methods of medical diagnosis include low doses of X-rays can be 'shone through' the body to identify broken bones. Because bones block X-rays, they cast a strong 'shadow' on the film, producing an image that we can see. A Computed Axial Tomography (CAT) scan uses several X-ray machines at different angles to produce a three-dimensional image of structures within the body. During these procedures other parts of the body must be shielded with a heavy lead apron, to minimise unnecessary exposure.
Figure 9.1 During radiotherapy, gamma rays from multiple sources are focused inside an inaccessible tumour to kill the cancer while leaving surrounding the tissue intact.
Radiation and radioactivity in industry
Radiation and radioactive substances are used in industry to control and improve industrial processes, and to produce materials with highly advanced properties. In industry radiation is sometimes used to kill harmful living organisms such as bacteria and viruses. For example, irradiation may be used to sterilise medical instruments. Food can be irradiated to kill any harmful bacteria, thus extending its shelf life. Fruit flies are only reproductively sterilised – not killed – in a strategy to control damage to commercial fruit orchards. Large numbers of fruit flies are raised in captivity, sterilised and then released in fruit-growing regions. Wild non-sterile fruit flies that mate with sterile partners will have their efforts wasted – no fly larvae to eat the fruit! In this way, scientists can keep fly populations under control without having to use environmentally damaging pesticides.
Irradiation is also used to change the molecular or atomic properties of some industrial materials. For example, silicon is irradiated to give it better conductive properties for use in electronics. Irradiation of pure silicon changes some of the atoms. This is called doping and it is a controlled way of introducing impurities at just the right concentration to improve the electrical conductivity of the silicon. The new replacement reactor has the facility to meet the global computer industry's growing (and lucrative) demand for irradiated silicon. Similar methods can be used to alter the molecular structure of many other materials for use in nanotechnology, or to produce nano-composites. Nature makes fabulous nano-composites, which we are only just beginning to copy. The abalone shell, for example, is made of alternating layers of calcium carbonate and a rubbery biopolymer. It is twice as hard and a thousand times tougher than its components!
 Figure 9.2 Neutron beams are used to detect microscopic cracks in metal welds of aeroplane wings, oil pipes and other critical steel structures.
Radiation is also used to control industrial processes very precisely, such as the micro-structure of artificial hip joints, to measure the thickness of rubber sheeting, to determine the shape of margarine tubs and to monitor the mass of chips in your chip packet.
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