For radiometric dating or tracing of a process with a characteristic timescale, the most useful isotope is one whose half-life is of the same order of magnitude as that timescale: isotopes with much shorter half-lives will have completely decayed away and lost their signal before the process is over, while isotopes with much longer half-lives will show negligible measurable decay over that timescale and thus poor time resolution.
To resolve a process operating on a roughly 1000-year timescale, the ideal isotope's half-life should be comparable to (same order of magnitude as) 1000 years,...
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