half life period

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.)

half-life 

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.

Main Entry: half–life
Pronunciation: -"lIf
Function: noun
: the time required for half of something to undergo a process: as a : thetime required for half of the atoms of a radioactive substance to become disintegrated b : the time required for half the amount of a substance (as a drug or radioactive tracer) in orintroduced into a living system or ecosystem to be eliminated or disintegrated by natural processes half–life is about 2 days —Journal of the American MedicalAssociation>

half-life n.

1. The time required for half the nuclei of a specific radionuclide or radioactive substance to undergo radioactive decay. Also called physical half-life.

2. 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.

3. 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.

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.