Fig. 4.1

Fig. 4.2

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Radioactive substances gradually diminish as time goes by. The atomic nuclei emit radiation and become different substances. The pattern of radioactive decay is an example of a very important pat­tern found in many different situations, a pattern called exponen­tial decay. Figure 4.1 shows the decay graphs for three different radio-isotopes, each with a different rate of decay.

Although the three graphs look different, they all have something in common - their shape. They are curved lines having a special property. If you know what is meant by the half-life of a radio-isotope, then you will understand what is special about the shape of these curves. The half-life t ½ of a radio-isotope is the average (or mean) time taken for half of the nuclei in a sample to decay (Figure 4.2). It takes the same amount of time again for half of the remainder to decay, and a third half-life for half of the new remainder to decay.

In principle, the graph never reaches zero; it just gets closer and closer. (In practice, when only a few undecayed nuclei remain, it will cease to be a smooth curve and will eventually reach zero.) We use the idea of half-life, because we cannot say when a sample will have completely decayed.

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If you are to measure the half-life of a radioactive substance in the laboratory, you need to choose something that will not decay too quickly or too slowly. In practice, the most suitable radio-isotope is protactinium-234, which decays by emitting ir­radiation. This is produced in a bottle containing uranium (Figure 4.3). By shaking the bottle, you can separate the protactinium into the top layer of solvent in the bottle. The Geiger counter allows you to measure the decay of the protactinium.