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**"Half life"****Related Terms:**Half-life, Half-life, biological, Radioactive tracer, Effective half-life, Radium, Decay constant, Radioisotope, Biological half-life, Decay, Radioisotopes, Radioactive dating, Parent isotope, Radionuclides, Decay chain, Decay product, Radiotracer, Thorium, Radioactive material, Radiometric dating, Tracer, Parent, Activity, Radionuclide, Radiotoxicity, Radioactive decay, Radioactive contamination, Dirty bomb, Polonium, Curie, Radiopharmaceutical, Decay time, Radiocarbon dating, Disequilibrium, Radon, Radioactive isotope, Cesium, Daughter product, Specific activity, Decay, Radioactive, Decay, radioactive, Americium, Cask, Cobalt 60, Background, Picocurie, Pci/l, Radiopharmaceuticals, Isotope scan

The period of time needed for half of the radioactive isotopes in a sample to decay to daughter atoms.

The period of time required for half of a given quantity of an isotope to undergo radioactive decay.

The time taken for something to fall to half its original value.

a unit of relative time measuring the rate at which a radioactive substance decays, or (more generally) the rate of decrease for any process that decreases exponentially. In the case of radioactivity, the half life is the time required for the activity to be reduced by half. After a second half life, the activity is again reduced by half, so it is then 1/4 the original activity. To reduce the activity to 0.1% of the original amount requires about 9.966 half lives.

the time it takes for half of the nuclei of a radioactive substance to decay or the amount of time required to reduce a drug level to one half of the initial value

The time it takes for one half of the original dose of a medication to leave the body.

The time taken by certain materials to lose half their strength. The time required for half of the atoms of a radioactive element to undergo decay. The time required for the elimination of one half a total dose from the body.

The time required for half of the atoms in a sample to decay or transform. For example: the radionuclide krypton-85 has a half-life of 3,934.4 days and decays to the stable isotope rubidium-85 by emitting a beta particle; after one period of 3,934.4 days, 50 per cent of the initial krypton-85 atoms in a sample will have become rubidium-85; after ten half lives (107 years, 9½ months) the sample will contain just 0.1 per cent krypton-85, and 99.9 per cent rubidium-85.

The half life of a reaction is the time required for the amount of reactant to drop to one half its initial value.

The half-life of a radioisotope is the time taken for half of the atoms in a sample of that radionuclide to decay (emit nuclear radiation). It can also be expressed as the time taken for the activity of the sample to halve. It has the following accepted symbols

The time span necessary for the atoms of a nuclide to disintegrate by one half.

the time taken for half of the parent atoms of a radioactive isotope to decay.

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)

The amount of time it takes for half an initial amount to disintegrate.

The time taken for the activity of a radionuclide to halve its value by decay.

The time taken for one half of a radioisotope to decay or change into another element.

time taken for half of the reactants to be converted into products

the time it takes certain materials, such as persistent pesticides, to become chemically altered List of Glossary Terms

The time it takes for one half of the initial amount of a substance to degrade in the environment.

the time taken for the concentration of a drug in the bloodstream to decrease by one half. Drugs with a shorter half life must be taken more frequently.

The time taken for half the energy of a radioactive substance to be discharged by bursts of nuclear radiation.

The time it takes for a medium storing data to weaken to half of its strength. Magnetic media, including traditional hard drives and floppy disks, have a half-life of five to seven years.

In water treatment, the time it takes to lose of a slug fed product from the system through blowdown, drift and windage when taking into account its continuous concentration by evaporation and dilution by fresh makeup.

the time required for certain materials, such as persistent radioactive compounds, to lose half their original amount through chemically alteration to a new form. The half life of DDT (a banned pesticide in Lebanon) is 15 years, the half-life of radium is 580 years.

The time required for the activity of a given radioactive substance to decrease to half of its initial value due to radioactive decay. The half-life is a characteristic property of each radioactive element and is independent of its amount or condition. See; Radiation.

The rate of the exponential decay of the accumulated penalty is determined by this value.

The period of time required for a radioactive substance to lose half of its active characteristics; used especially in radiological work.

time interval required for half of a quantity of material to be eliminated naturally

THE AMOUNT OF TIME IT TAKES FOR A SAMPLE OF RADIOACTIVE MATERIAL TO LOSE HALF ITS RADIOACTIVITY.

The amount of time required for half of a given substance to be eliminated from the body.

Point in time when the principal on a mortgage backed security (issued or guaranteed by the Government National Mortgage Association, the Federal National Mortgage Association, or the Federal Home Loan Mortgage Association) has been repaid. It is presumed that the security has a half life of 12 years. However, depending on interest rate trends, specific mortgage pools can have longer or shorter half lives. If interest rates rise, homeowners will hold onto their mortgages longer than predicted, and half lives will rise. If interest rates fall, more homeowners will refinance their mortgages. Thus, principal will be paid off more quickly, and half lives will drop. See: Federal National Mortgage Association; Government National Mortgage Association; Principal