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An isotope of an element is radioactive. Explain what is meant by radioactive decay.
At time 
, a sample of a radioactive isotope contains 
nuclei. In a short time 
, the number of nuclei that decay is 
. State expressions, in terms of the symbols , , and for
the number of undecayed nuclei at time (
),
the mean activity of the sample during the time interval ,
the probability of decay of a nucleus during the time interval ,
the decay constant.
-particles, 
-particles, or 
-ray photons.
.
.
.
