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It's a Question of Physics: What is meant by half-life?
The half-life of a radioactive substance is a characteristic constant. It measures the time it takes for a given amount of the substance to become reduced by half as a consequence of decay, and therefore, the emission of radiation.
Archeologists and geologists use half-life to date the age of organic objects in a process known as carbon dating. During beta decay, carbon 14 becomes nitrogen 14. At the time of death organisms stop producing carbon 14. Since half life is a constant, the ratio of carbon 14 to nitrogen 14 provides a measurement of the age of a sample.
In the medical field, the radioactive isotope Cobalt 60 has been used for radiotherapy to shrink tumors that will later be surgically removed, or to destroy cancer cells in inoperable tumors. When it decays to stable nickel, it emits two relatively high-energy gamma rays. Today it is being replaced by electron beam radiation therapy systems.
The half-life of isotopes from some sample elements:
oxygen 16 – infinite
uranium 238 – 4,460,000,000 years
uranium 235 – 713,000,000 years
carbon 14 – 5,730 years
cobalt 60 – 5.27 years
silver 94 - .42 seconds
(click for larger image)
In the illustration above, 50% of the original mother substance decays into a new daughter substance. After two half-lives, the mother substance will decay another 50%, leaving 25% mother and 75% daughter. A third half-life will leave 12.5% of the mother and 87.5% daughter. In reality, daughter substances can also decay, so the proportions of substance involved will vary.
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