Calculate the activity of isotopes from the half life

mass of the isotope: g/mol (e.g. 39.9639987 for 40-K)
quantity of the isotope: g (how about 0.00012)
half life: (e.g. 1.277e9 a for 40-K)
clear form

A=NA / half life * ln(2) * mass / atomic mass

A year is assumed to be 365.25 days of 86400 seconds.

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Bernhard Kuemel