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An iodine radionuclide with approximately eight-day half-life, emitting beta particles and gamma rays. Its uptake and retention depend strongly on the carrier and tissue physiology.

Radiometric dating estimates elapsed time from changes in radionuclide populations, using a known nuclear decay constant and assumptions about the initial composition and subsequent closure to matter exchange.

A pharmaceutical containing a radionuclide, with uptake and clearance chosen for imaging or treatment. Biological targeting and radioactive decay jointly determine its useful signal and tissue dose.