|MadSci Network: Physics|
This is done experimentally. There is not magic equation you can plug information into and produce a resulting halflife. The methods are typically observations made through a scientific study of some sort that followed the decay of a nuclide over time. It could also involve the use of mass spectographic analysis of the element(s) in question and/or their progeny. if you look up a nuclides half-life you will find several usually listed for each nuclide as different investigations have yielded slightly different values.
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