Uses of radioactive particles

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Uses of radioactive particles

Radioactive nuclides are made use of in many completely different ways. In whatever way they are used, it is always necessary to take precautions so the user is well protected from any radiation.

Generally speaking, specialist companies manufacture radioactive nuclides by placing samples of material in a nuclear reactor for a time. It should be noted, however, that materials are not made radioactive by placing them near a radioactive source. A source kept in school will not cause its lead case to become radioactive, as the nuclei of the lead atoms will not be changed by the absorption of the radiation.

Four main uses of radioactive particles are detailed here: tracers, medical treatment, archaeological dating and scattering experiments.

Use is made in tracer applications of the ability of detectors to measure extremely small concentrations of a radioactive nuclide. For example, leaks can be found in underground pipes by injecting a small amount of a radioactive tracer into the water. A Geiger-Muller tube can be used to detect where the radioactive particles are escaping and hence save digging up the whole pipe.

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Many different tracers are used in medicine to check the functioning of organs within the body. A very small amount of the tracer is used to minimise radiation dose, and the half-life needs to be matched to the process being studied. Sources that emit gamma rays are preferred due to their low ionisation and high penetration properties. Technetium 99m is one of the most useful tracers - the m stands for metastable meaning that the nuclei are slightly more energetic than usual. A sample of technetium 99m has a half-life of 6 hours, emits gamma rays and returns to a stable state with a half-life of 216000 years emitting very little further radiation.

When radiation is passed through living cells it may do no harm at all. On the other hand, it can kill living cells or cause them to undergo mutation. Immature cells and cells that are growing or dividing rapidly are most sensitive to radiation. This is made use of in the treatment of cancer. Cancer cells often grow rapidly and therefore are more likely to be killed by a high dose of gamma radiation than normal cells.

The atmosphere contains a small amount of the radioactive nuclide carbon-14. During a plant's life carbon dioxide is absorbed, so it too becomes slightly radioactive and the level of radioactivity of plants can be measured. When a tree is cut down radioactive decay commences - the longer the wood has been around, the lower the concentration of carbon-14 remaining in the future. This method can used to date events to an accuracy of better than 100 years.

Radioactive particles can be used to probe matter. High-energy particles are directed at a target under investigation. Arrays of detectors are positioned around to detect how the particles are scattered. Such experiments reveal information about the structure and scale of the target object.

The most famous scattering experiment was carried out under the direction of Rutherford in 1909. Alpha particles were directed at the gold nuclei in an attempt to probe the structure of the atom. The repulsion between the positive alpha particles and positive gold nuclei caused the particles to scatter in a way that proved the existence of a small, positive, central nucleus.

More recently, electrons (which are not affected by nuclear forces within the nucleus like alpha particles) have been used in experiments to probe the structure of the nucleus itself. Very high-energy electrons have a wavelength similar to the dimensions of the nucleus and hence their scattering produces a characteristic diffraction pattern. The diffraction pattern can be used in the usual way to find the dimensions of the object that caused the pattern in the first place.

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