- radioactivity
- radioactivity
a`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]
The Collaborative International Dictionary of English. 2000.