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'''Radioactivity''' is the emission of high [[energy]] [[particles]] through the natural phenomenon of the decay of unstable [[isotopes]] of [[chemical elements]] into more stable forms, which are called ''daughter products.'' This type of emission is generally called nuclear radiation.

'''Radioactivity''' is the emission of high [[energy]] [[particles]] through the natural phenomenon of the decay of unstable [[isotopes]] of [[chemical elements]] into more stable forms, which are called ''daughter products.'' This type of emission is generally called nuclear radiation.

The most common types of nuclear radiation are alpha and beta radiation, and the processes for each are respectively alpha decay and beta decay.<ref>http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact.html</ref> There can also be gamma radiation associated with a nuclear decay. Alpha particles are [[helium]] nuclei (two protons and two neutrons); beta particles are high-energy [[electron]]s; gamma rays are high-energy [[photon]]s.  Alpha particles can normally be stopped by a sheet of paper or healthy human skin. Beta particles and gamma rays can penetrate one's body to cause great harm. Gamma radiation is also a form of electro-magnetic radiation, like X-rays or visible light.  (Contemporary jargon refers to alpha and beta ''particles'' and gamma ''rays'', though quantum mechanics makes the two actually the same.)

The most common types of nuclear radiation are alpha, beta,  and gamma radiation, and the processes for each are respectively [[alpha decay]], [[beta decay]], and [[gamma decay]].<ref>http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact.html</ref> Alpha particles are [[helium]] nuclei (two protons and two neutrons); beta particles are high-energy [[electron]]s; gamma rays are high-energy [[photon]]s.  Alpha particles can normally be stopped by a sheet of paper or healthy human skin. Beta particles and gamma rays can penetrate one's body to cause great harm. Gamma radiation is also a form of electro-magnetic radiation, like X-rays or visible light.  (Contemporary jargon refers to alpha and beta ''particles'' and gamma ''rays'', though quantum mechanics makes the two actually the same.)

All forms of radioactivity follow the fundamental rules of mass and energy conservation, taking into account their interrelationship due to the formula [[E=mc²]].  In fact, radioactive decay illustrates this formula clearly, while chemical transformations involve a mass change so small as to be imperceptible.

All forms of radioactivity follow the fundamental rules of mass and energy conservation, taking into account their interrelationship due to the formula [[E=mc²]].  In fact, radioactive decay illustrates this formula clearly, while chemical transformations involve a mass change so small as to be imperceptible.

<!-- This section is specifically referenced, under this name, by other articles, such as the "Radiometric dating" page.  Do not change this name without repairing those other pages. -->

<!-- This section is specifically referenced, under this name, by other articles, such as the "Radiometric dating" page.  Do not change this name without repairing those other pages. -->

==Exponential decay and half-life==

==Exponential decay and half-life==

Although the exact mechanisms of alpha, beta, and gamma decay are quite different, all three are dictated by the rules of [[quantum mechanics]].  These rules dictate a fixed probability, per second, that a nucleus will decay.  So a nucleus has some probability, say P, surviving one second without decaying.  For Polonium-210, P=.99999994, for example.  For U-238, P=.999999999999999995.

Although the exact mechanisms of alpha, beta, and gamma decay are quite different, all three are dictated by the rules of [[quantum mechanics]].  These rules dictate a fixed probability, per second, that a nucleus will decay.  So a nucleus has some probability, say P, surviving one second without decaying.  For Polonium-210, P=.99999994, for example.  For Uranium-238, P=.999999999999999995.

After 2 seconds, the fraction of material left is P².  After 10 seconds, it is P¹⁰, and so on.  So the fraction remaining is an exponential function of time.  This is the reason for the characteristic exponential decay of radioactive substances.  The type of decay doesn't matter; the decay is always exponential.  The usual way this is described is the ''half-life'', which is the time for one half of the sample to disintegrate.  The half-life can be seen to be log(1/2) / log(P).  Half-lives can vary from quantities like 10^-21 seconds for Beryllium-13, to 10¹⁸ years (far more than the age of the universe) for Tungsten-182.

After 2 seconds, the fraction of material left is P².  After 10 seconds, it is P¹⁰, and so on.  So the fraction remaining is an exponential function of time.  This is the reason for the characteristic exponential decay of radioactive substances.  The type of decay doesn't matter; the decay is always exponential.  The usual way this is described is the ''half-life'', which is the time for one half of the sample to disintegrate.  The half-life can be seen to be log(1/2) / log(P).  Half-lives can vary from quantities like 10^-21 seconds for Beryllium-13, to 10¹⁸ years (far more than the age of the universe) for Tungsten-182.

While the mechanisms for decay, and hence the half-lives of various isotopes, are well understood on theoretical grounds, the calculations are so complex that half-lives are measured experimentally.  The measured half-lives of the various isotopes of elements are known to be fixed quite accurately.  Careful astronomical observations show that half-lives have not changed significantly in billions of years&mdash;spectral lines from distant galaxies would have shifted perceptibly if the constants involved had changed.  In some cases radioactive decay itself can be observed and measured in distant galaxies when a supernova explodes and ejects radioactive nuclei.

While the mechanisms for decay, and hence the half-lives of various isotopes, are well understood on theoretical grounds, the calculations are so complex that half-lives are measured experimentally.  The measured half-lives of the various isotopes of elements are known to be fixed quite accurately.  Careful astronomical observations show that half-lives have not changed significantly in billions of years&mdash;spectral lines from distant galaxies would have shifted perceptibly if the constants involved had changed.  In some cases radioactive decay itself can be observed and measured in distant galaxies when a supernova explodes and ejects radioactive nuclei.

==Antiquated notions==

==Antiquated notions==

This is not to say that radioactivity isn't dangerous, or that radioactive contamination isn't a tricky problem.

This is not to say that radioactivity isn't dangerous, or that radioactive contamination isn't a tricky problem.

==Notes and references==

==Notes and references==