Physics. the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.
Also called biological half-life. Pharmacology. the time required for the activity of a substance taken into the body to lose one half its initial effectiveness.
Informal. a brief period during which something flourishes before dying out.
the time taken for half of the atoms in a radioactive material to undergo decay τ
the time required for half of a quantity of radioactive material absorbed by a living tissue or organism to be naturally eliminated (biological half-life) or removed by both elimination and decay (effective half-life)
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.
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.)
- Biological marker
noun a substance, physiological characteristic, gene, etc that indicates, or may indicate, the presence of disease, a physiological abnormality or a psychological condition Also called biomarker
- Biological mother
a parent who has conceived (biological mother) or sired (biological father) rather than adopted a child and whose genes are therefore transmitted to the child. Contemporary Examples Brooklyn’s Real-Life Norman Bates Accused of Cross-Dressing Fraud Michael Daly April 28, 2012 How One Young Woman Escaped Childhood Abuse and a Forced Marriage Moral Courage October 7, […]
- Biological shield
noun a protective shield impervious to radiation, esp the thick concrete wall surrounding the core of a nuclear reactor
- Biological vector