[haf-lahyf, hahf-] Pronunciation Key
[-lahyvz] Pronunciation Key. | 1. | Physics. the time required for one half the atoms of a given amount of a radioactive substance to disintegrate. |
| 2. | Pharmacology. the time required for the activity of a substance taken into the body to lose one half its initial effectiveness. |
| 3. | Informal. a brief period during which something flourishes before dying out. |
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2006.
| half-life (hāf'līf', häf'-)
n.
|
The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2006 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
half-life
| half-life | |
noun | |
| the time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate) [syn: half life] |
| half-life
(hāf'līf') Pronunciation Key
The average time needed for half the nuclei in a sample of a radioactive substance to undergo radioactive decay. The half-life of a substance does not equal half of its full duration of radioactivity. For example, if one starts with 100 grams of radium 229, whose half-life is 4 minutes, then after 4 minutes only 50 grams of radium will be left in the sample, after 8 minutes 25 grams will be left, after 12 minutes 12.5 grams will be left, and so on.
|
Copyright © 2002 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
half-life
In physics, a fixed time required for half the radioactive nuclei in a substance to decay. Half-lives of radioactive substances can range from fractions of a second to billions of years, and they are always the same for a given nucleus, regardless of temperature or other conditions. If an object contains a pound of a radioactive substance with a half-life of fifty years, at the end of that time there will be half a pound of the radioactive substance left undecayed in the object. After another fifty years, a quarter-pound will be left undecayed, and so on.
Note: Scientists can estimate the age of an object, such as a rock, by carefully measuring the amounts of decayed and undecayed nuclei in the object. Comparing that to the half-life of the nuclei tells when they started to decay and, therefore, how old the object is. (See radioactive dating.)
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
half-life n.
- The time required for half the nuclei of a specific radionuclide or radioactive substance to undergo radioactive decay. Also called physical half-life.
- The time required for half the quantity of a drug or other substance deposited in a living organism to be metabolized or eliminated by normal biological processes. Also called biological half-life.
- The time required for the radioactivity of material taken in by or administered to an organism to be reduced to half its initial value by a combination of biological elimination processes and radioactive decay. Also called effective half-life.
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
half-life
- The length of time before half the principal on a debt is expected to be repaid through amortization or sinking fund payments. For example, a 25-year bond issue may require the issuer to retire 5% of the beginning principal commencing 5 years after the issue date. Thus, the bond issue has a half-life of 5 years plus the number of years required to retire half the issue, or 15 years. Mortgage-backed securities often have a relatively short half-life because many homeowners pay off or refinance their mortgages early. Also called average life.
Copyright © 2003 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
Half Way, MO Zip code(s): 65663
Half Moon, NC (CDP, FIPS 28900) Location: 34.82701 N, 77.46649 W
Population (1990): 6306 (2106 housing units)
Area: 16.3 sq km (land), 0.0 sq km (water)
Country Life Acres, MO (village, FIPS 16876) Location: 38.62475 N, 90.45593 W
Population (1990): 101 (34 housing units)
Area: 0.3 sq km (land), 0.0 sq km (water)
Half Moon Bay, CA (city, FIPS 31708) Location: 37.47012 N, 122.43690 W
Population (1990): 8886 (3402 housing units)
Area: 16.8 sq km (land), 0.1 sq km (water)
Zip code(s): 94019
Covenant Life, AK (CDP, FIPS 17670) Location: 59.41723 N, 136.02513 W
Population (1990): 47 (16 housing units)
Area: 39.7 sq km (land), 1.0 sq km (water)
Copyright © 2008, Dictionary.com, LLC. All rights reserved.