radioactivity

[rey-dee-oh-ak-tiv-i-tee]
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:
1895–1900; radio- + activity

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World English Dictionary
radioactivity (ˌreɪdɪəʊækˈtɪvɪtɪ)
 
n
the spontaneous emission of radiation from atomic nuclei. The radiation can consist of alpha, beta, and gamma radiation

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American Heritage
Medical Dictionary

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

  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.

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American Heritage
Science Dictionary
radioactivity   (rā'dē-ō-āk-tĭv'ĭ-tē)  Pronunciation Key 
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.
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Cultural Dictionary

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.)

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Example sentences
Entering the disaster center was a time-consuming process because of the
  radioactivity precautions.
Back then, radioactivity was new, poorly understood and a hot marketing
  property.
Seafloor reactor sites and surrounding waters are periodically tested for
  radioactivity levels.
Radioactivity makes things both simpler and more complex.
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