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Radiation and Radioactivity : Types of Radiation : Decay and Half Lives
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The larger elements, composed of many particles, tend to be unstable -- as if they cannot hold all their bits together. They emit chunks of themselves (as radiation) and slowly evolve into either more stable varieties of themselves or into other, more stable, elements. This is called 'decay' because their radiation slowly dies out. It is possible to measure the time in which radioactive elements decay.
We define these times by the time in which they become half as radioactive as they were at the start. This time is called a half-life and it is measured in anything from seconds to years for different elements. Thus, if one gram of a substance is present to start with, and the half life is X days, only half a gram will still exist after X days; only a quarter gram will exist after 2X days and only an eighth of a gram will exist after 3X days. It is of course the radioactivity that decays, not the mass, so this statement is true only when applied to pure radioactive materials. It is not strictly true when discussing other materials, such as uranium as an ore. Half-life refers specifically to the number of radioactive nuclei present.
Here are some half-lives of different isotopes (elements) to show you how much they vary and how the more active an isotope is, the faster it decays:
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Isotope
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Half-Life
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Activity (Curie/gm.)
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Radium 226
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1599 yr
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1.0
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Radon 222
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3.82 days
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154,000
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Plutonium 238
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86.4 yr
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17.4
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Plutonium 239
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24,390 yr
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0.062
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Plutonium 240
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6,600 yr
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0.23
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Plutonium 241
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13.2 yr
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112.0
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Shorter half-lives indicate species that are more radioactive and longer half-lives indicate those that are less so. Compare, for example, 1 gram of Plutonium-239 with 1 gram of Plutonium-241. Plutonium-239 has the longer half-life that corresponds to a radioactivity level of 0.062 Curies per gram, and Plutonium-241 has the shorter half-life that corresponds to a much higher radioactivity level of 112.0 Curies per gram. Thus Plutonium-239, which stays active for a long time, has only 0.062 curies per gram whereas Plutonium-241 has 112 curies per gram, compensating by decaying rapidly.
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