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[rey-dee-oh-ak-tiv-i-tee] /ˌreɪ di oʊ ækˈtɪv ɪ ti/
noun, Physics, Chemistry.
the phenomenon, exhibited by and being a property of certain elements, of spontaneously emitting radiation resulting from changes in the nuclei of atoms of the element.
Also called activity.
Origin of radioactivity
First recorded in 1895-1900; radio- + activity Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2017.
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Examples from the Web for radioactivity
Contemporary Examples
Historical Examples
  • It is focused, however, on questions found to be significant in studies of radioactivity in the sea.

    Atoms, Nature, and Man Neal O. Hines
  • The owlies had learned what radioactivity meant, back when they fought the humans.

    Duel on Syrtis Poul William Anderson
  • Second, a neutron source—the type of radioactivity that produced neutrons—to accelerate the reaction.

    Rip Foster in Ride the Gray Planet Harold Leland Goodwin
  • They were unable to venture into his old one because of the radioactivity and micro-organisms.

    Diplomatic Immunity Robert Sheckley
  • The halo has established the extreme rarity of radioactivity as an atomic phenomenon.

  • Vacuum-packed in moisture-proof containers, and free from radioactivity.

    The Return H. Beam Piper and John J. McGuire
  • The amounts and kinds of radioactivity released to the environment.

    Atoms, Nature, and Man Neal O. Hines
  • Red verticals are five-minute intervals, the wiggly black horizontal line is the radioactivity level.

    Planet of the Damned Harry Harrison
  • And yet the facts of radioactivity very positively forbid the past eternity of matter.

British Dictionary definitions for radioactivity


the spontaneous emission of radiation from atomic nuclei. The radiation can consist of alpha, beta, and gamma radiation
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for radioactivity

1899, from French radioactivité, coined 1898 by the Curies; see radioactive.

Online Etymology Dictionary, © 2010 Douglas Harper
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radioactivity in Medicine

radioactivity ra·di·o·ac·tiv·i·ty (rā'dē-ō-āk-tĭv'ĭ-tē)

  1. Spontaneous emission of radiation, either directly from unstable atomic nuclei or as a consequence of a nuclear reaction.

  2. The radiation, including alpha particles, nucleons, electrons, and gamma rays, emitted by a radioactive substance.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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radioactivity in Science
The emission of radiation by unstable atomic nuclei undergoing radioactive decay.

Our Living Language  : In the nuclei of stable atoms, such as those of lead, the force binding the protons and neutrons to each other individually is great enough to hold together each nucleus as a whole. In other atoms, especially heavy ones such as those of uranium, this energy is insufficient, and the nuclei are unstable. An unstable nucleus spontaneously emits particles and energy in a process known as radioactive decay. The term radioactivity refers to the particles emitted. When enough particles and energy have been emitted to create a new, stable nucleus (often the nucleus of an entirely different element), radioactivity ceases. Uranium 238, a very unstable element, goes through 18 stages of decay before becoming a stable isotope of lead, lead 206. Some of the intermediate stages include the heavier elements thorium, radium, radon, and polonium. All known elements with atomic numbers greater than 83 (bismuth) are radioactive, and many isotopes of elements with lower atomic numbers are also radioactive. When the nuclei of isotopes that are not naturally radioactive are bombarded with high-energy particles, the result is artificial radioisotopes that decay in the same manner as natural isotopes. Each element remains radioactive for a characteristic length of time, ranging from mere microseconds to billions of years. An element's rate of decay is called its half-life. This refers to the average length of time it takes for half of its nuclei to decay.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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radioactivity in Culture

radioactivity definition

The emission of elementary particles by some atoms when their unstable nuclei disintegrate (see half-life). Materials composed of such atoms are radioactive. (See alpha radiation, beta radiation, and gamma radiation.)

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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