half-life

During radioactive decay, the time in which the activity of a radioactive source decays to half its original value (the time taken for half the atoms to decay). In theory, the decay process is never complete and there is always some residual radioactivity. For this reason, the half-life of a radioactive isotope is measured, rather than the total decay time. It may vary from millionths of a second for some radioisotopes to billions of years for others, but each radioisotope has a definite half-life.

To determine a short half-life a Geiger–Müller tube can be used to count the number of particles emitted by a sample. To determine a longer half-life a mass spectrometer is used. Some examples are: sodium-24, 15 hours; carbon-14, 5,730 years; plutonium-239, 24,000 years; and uranium-238, 4,500 million years.

half-life

In medicine, the time taken for the peak plasma concentration of a drug to decline by half. This is due to redistribution of the drug from the plasma to the tissues and to metabolism and excretion of the drug. The half-life of the drug is one of the factors that determines the frequency of administration required to achieve optimal therapeutic effects. ...KEYB: cathyt