The nuclei of some isotopes are unstable: they spontaneously and randomly lose energy and/or particles, usually changing their identity in the process. This is called radioactivity. Radioactivity can be natural (discovered
by Becquerel in 1896) or induced by particle bombardment (first seen by Rutherford
in 1919.)
There are six basic kinds of radioactivity:
Alpha decay: the ejection of an alpha particle (consisting of two protons and two neutrons, i.e.
a Helium-4 nucleus). The remaining nucleus has an atomic number
reduced by 2, and mass reduced by 4. The resulting nuclide is called the daughter nucleus.
The original nuclide is called the parent.
Beta decay: the ejection of an electron from the nucleus. An electron? From the nucleus?! How did that get in there? Beta decay can be visualized as the transformation of a neutron into a proton and an electron. The electron doesn't belong in the nucleus, however, and is ejected. The proton remains
in the nucleus. Thus the atomic mass does not change, but the atomic number increases
by one.
There is another kind of beta decay, where a proton is transformed into a neutron and a so-called positron. The positron is a particle just like an electron (same mass and magnetic properties) but with a positive charge. The positron
is ejected, and the neutron remains part of the nucleus, which now has one unit
less charge than before.
The positron is sometimes called an anti-electron, and is an example of antimatter. All particles have antimatter versions; for example, there is the anti-proton, in all ways identical to the proton except its charge is negative. When matter and antimatter particles meet, they destroy each other in a blaze of gamma rays. Fortunately, there isn't much antimatter in the universe. Why the atoms of the universe are matter and not a more even mix of matter and antimatter is a mystery.
Gamma emission: the emission of a photon, usually of very high energy. The identity of the nuclide doesn't change.
Neutron emission: the ejection of a neutron from the nucleus. The atomic number doesn't change, but the mass number decreases by one.
Proton emission: the ejection of a proton from the nucleus. The atomic number and mass number both decrease by one.
Spontaneous Fission (SF). Some nuclei don't eject small particles such as
neutrons or alpha particles, but instead split into two chunks, more or less in
half.
In each of these reactions, we have charge conservation and nucleon
conservation.
The breakup of a nucleus is a random process — there is no way
to predict when a particular nucleus will fragment. The decay of any one nucleus is totally
independent of what the others are doing. With a large
number of nuclei, however, we can measure the average lifetime. This is usually denoted
as the halflife. The halflife is the time it takes for half the number
of nuclei present to disintegrate. After two halflives, only one-quarter of them will remain. After three halflives, only one-eighth remain.
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Activities & Practice
to do as you read
1. Mercury-206 decays by Beta decay, with a half-life of 450 seconds. If you had a kilogram of this stuff, how long would you have to wait for 7/8 of it to decay?
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