Radionuclides occur with half-lives in a vast range of over 37 magnitudes as listed in this site. In question 7584, Lubos Motl explained how Gyr half-lives were determined. This method doesn't appear applicable to evaluate sub-microsecond half-lives. How are extremely short half-lives determined?
Answer
Any kind of fast decay is measured by its line width.
You make a high-precision energy measurements of a statistical number of decays, measure the width of the resulting distribution, subtract off contributions due to Doppler broadening and your instrument's intrinsic resolution and what you are left with goes into
$$ \Delta E \Delta t = \frac{\hbar}{2} $$
This is a staple method of experimental particle physics.
As you can see, very short half-lives translate to very broad lines. Other examples include the decay of the $J/\Psi$ (and indeed most strong decays).
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