Where does half-life collide?

The time required for half the radioactive nuclei in a sample to decay, a fixed constant of a given isotope tied to its decay constant by t½ = ln2 / λ. Because radioactive decay follows a first-order exponential law, this interval does not depend on the amount of material present or on temperature, pressure, or chemistry: it is a property of the nuclide itself. Rutherford and Soddy's 1903 Radioactive change established the exponential law of decay from which the half-life follows. The referent is a nuclear clock set by the physics of an unstable nucleus, not a drug leaving the bloodstream or the ageing of a body of literature.[1]

Unit

A characteristic decay time of an isotope, a constant fixed by the nuclide alone

Recovery means

Not applicable: radioactive decay is irreversible, and the decayed parent nuclei do not return

Measured by

Determined from the decay constant by counting emissions with a detector over time, since t½ = ln2 / λ

The time required for the plasma concentration of a drug to fall by half during elimination, the interval that governs how often a dose must be repeated and how long a drug persists after it is stopped. Unlike the nuclear constant, it is not fixed by any single physical property: it is a derived quantity that depends on both the clearance and the volume of distribution of the drug in a particular patient, so it varies with age, organ function, and disease. Greenblatt's 1985 Elimination half-life of drugs set out both its clinical value and its limits as a summary of drug disposition. The referent is a pharmacokinetic property of a drug in a body, not a property of an unstable nucleus or of a citation record.[2]

Unit

A drug's plasma elimination time, in hours, set by clearance and volume of distribution

Recovery means

A repeated dose restores the plasma concentration, and steady state is reached after roughly four to five half-lives

Measured by

Estimated from the terminal slope of a plasma concentration versus time curve after a dose

The median age of the articles cited by, or citing, a given journal or literature, a measure of how quickly a body of work becomes dated as newer sources displace older ones. Despite the borrowed name, it does not describe any exponential decay of a physical quantity: it is a summary statistic of a citation age distribution, the point at which half the references are older and half younger. Burton and Kebler's 1960 The half-life of some scientific and technical literatures introduced the index and its use in comparing how fast different fields turn over. The referent is a bibliometric feature of a literature, not the decay of nuclei or the clearance of a drug.[3]

Unit

The median age of the cited articles, in years

Recovery means

Not applicable: it is a static summary of a citation age distribution, not a process that recovers

Measured by

Computed as the median age of the references in, or to, a journal's articles in a given year