Definitions

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. A naturally-occurring radioactive isotope of potassium, 4019K, having nineteen protons and twenty-one neutrons, and a half-life of 1.3 billion years; it is the chief source of natural radioactivity in living tissue

Etymologies

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Examples

  • (Presumably, the need for a license is a matter of degree, given that our own bodies contain radioactive potassium-40, and given that the walls of most buildings contain the radioactive isotopes from the natural uranium-238 and thorium-232 decay chains).

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  • That 4 million tonnes of coal contains 5.2 tonnes of uranium, 12.8 tonnes of radioactive thorium, as well as 0.22 tonnes of radioactive potassium-40!

    Obama announces $8.3B for nuke plants

  • Presumably the individual particles of silt or sand that go to make sedimentary rocks contain potassium-40 and other radioactive isotopes, and therefore could be said to contain radioactive clocks; but unfortunately these clocks are no use to us because they are not properly zeroed, or are zeroed at different times from each other.

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  • If you start with some quantity of potassium-40, after 1.26 billion years half of the potassium-40 will have decayed to argon-40.

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  • Igneous rocks typically contain many different radioactive isotopes, not just potassium-40.

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  • So geologists, by measuring the ratio between potassium-40 and argon-40 in a piece of igneous rock that they pick up today, can tell how long ago the rock first crystallized out of its molten state.

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  • After another 1.26 billion years, half of the remaining potassium-40 will have been converted to argon-40, and so on.

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  • When the clock was zeroed, the ratio was 100 per cent in favour of potassium-40.

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  • But, for the reason I have just explained, this quantity is meaningful only if expressed as the ratio of potassium-40 to argon-40.

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  • If, before that, the half-life of potassium-40 was radically different, only a few centuries, say, rather than 1.26 billion years?

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