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Radioactivity is the observed instability of the nuclei of some atoms. Over time, a radioactive substance will change its chemical composition: the radioactive isotopes will disappear, and decay products — usually different elements — will form. The process, called radioactive decay, is often accompanied by the emission of high energy electromagnetic radiation (gamma radiation), electrons (beta radiation) or helium-4 nuclei (alpha radiation). These particles are easy to detect at extremely low levels using fairly simple equipment. Because the number of atoms in a typical laboratory sample is extremely large, methods involving the use of radioactivity are among the most sensitive in all science.
The time it takes for half of a sample of one kind of nucleus (called a nuclide, the time is called a half-life) is normally constant, which allows the dating of materials by looking at how much of a sample has decayed and working out, sometimes with very good accuracy, how long it would have taken for that to happen. This forms the basis of radiometric dating.
It is worthwhile to clarify the definitions of three terms that are often conflated:
- Radiation is the actual, well, radiation. Once it strikes an object or a biological tissue, it imparts its energy, causes its effect, and is gone. The effect of radiation is limited by duration of exposure, distance from the source, and shielding.
- Radioactivity refers to the generic process of energy released by nuclear decay, and also, more specifically, to the rate at which a given radioactive sample releases energy (measured in becquerels, or Curies if you're an atavistic American). For example, a radioactive source for teletherapy may be characterised as containing 6.3 gigabecquerels of Co-60, which means that the source's radioactivity is 6.3 billion decay events per second.
- Radioactive contamination refers to contamination of objects or tissues by radioactive particles. Radioactive contamination is much more dangerous than radiation itself (particularly in biological tissues) because it applies a continuing dose of radiation in the same small volume over an extended period of time. It is still radiation which causes the effects, but the presence of radioactive particles greatly magnifies the applied dose.
These three concepts are often confused in the popular press. Nuclear disasters such as Mayak, Chernobyl, or Fukushima are often described as having released enormous amounts of "radioactivity", or worse yet "radiation", when in fact their most significant (and destructive) effect was the release of enormous amounts of radioactive contaminants that caused large areas to become radioactive, and entered the environment and food chain to contaminate plant and animal life (including humans). If they only emitted radiation, then shielding and a relatively very small exclusion zone would suffice to contain their effects.
 People troubled with radioactivity
There are two main groups of people who have a problem with radioactivity. The first one is biblical literalists, specifically young Earth creationists and some old Earth creationists, because the extent of decay of radioactive elements found on Earth, such as uranium and potassium, suggests that Earth is billions of years old. This obviously contradicts the story of Genesis.
The second group is the anti-nuclear movement, which considers radioactivity that results from the use of nuclear power to be very dangerous and a menace to humans, even though it's actually a tiny drop in the sea of natural radioactivity. Activities of the anti-nuclear movement are partially responsible for the pervasive fear of radiation present in most Western societies.
 See also
- FAQ on radioactivity and nuclear technology
- Radioactive decay
- Radiometric dating
- Evidence against a recent creation
- Second event theory