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The Collaborative International Dictionary of English v.0.48 (gcide)

radioactivity \ra`di*o*ac*tiv"i*ty\
(r[=a]`d[i^]*[-o]*[a^]k*t[i^]v"[i^]*t[y^]), n. (Physics)
a form of instability which is a property of the atomic
nuclei of certain isotopes, which causes a spontaneous change
in the structure of the nucleus, accompanied by emission of
energetic radiation. The radiation emitted is usually
sufficient to cause ionization in matter through which it
passes, and is therefore called {ionizing radiation}. The
radiation emitted by most radioactive substances is one of
three types: {alpha rays}, {beta rays}, or {gamma rays}. Some
chemical elements have no stable isotopes, and these are
referred to as radioactive elements, and the element itself
is said to possess radioactivity.

Note: The changes in radioactive nuclei which cause radiation
in most cases cause the chemical identity of the
nucleus itself to change, as when tritium (an isotope
of hydrogen) emits a beta ray and converts to helium.
The radioactive decay process is a first-order
reaction, and the rate of decay of a particular isotope
can therefore be expressed as the {half life} of the
isotope, which is the time it takes for one half of the
remaining undecayed isotope to decay, and is a constant
independent of the proportion of original material
which has already decayed. The half life of tritium,
for example, is 12.3 years.
[PJC]