Definitions
from Wiktionary, Creative Commons Attribution/Share-Alike License.
- noun physics A naturally-occurring
radioactive isotope ofpotassium , 4019K, having nineteenprotons and twenty-oneneutrons , and ahalf-life of 1.3 billion years; it is the chief source of natural radioactivity in living tissue
Etymologies
Sorry, no etymologies found.
Support
Help support Wordnik (and make this page ad-free) by adopting the word potassium-40.
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).
Archive 2009-04-01 Gordon McCabe 2009
-
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!
-
But, for the reason I have just explained, this quantity is meaningful only if expressed as the ratio of potassium-40 to argon-40.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009
-
Igneous rocks typically contain many different radioactive isotopes, not just potassium-40.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009
-
If, before that, the half-life of potassium-40 was radically different, only a few centuries, say, rather than 1.26 billion years?
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009
-
When the clock was zeroed, the ratio was 100 per cent in favour of potassium-40.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009
-
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.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009
-
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.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009
-
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.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009
-
After another 1.26 billion years, half of the remaining potassium-40 will have been converted to argon-40, and so on.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009
Comments
Log in or sign up to get involved in the conversation. It's quick and easy.