I was inspired by
https://en.wikipedia.org/wiki/Curie. All isotopes have a fixed half-life so all have a fixed decay constant lambda (a fixed probability of decay). Activity A = Aoexp(-lambda.t) and activity A = dN/dt. Hence the activity is dependent on the number of nuclei and thus the mass. The link for Americium-241 is given as 3.4 curies per gram. 1 microcurie then gives 2.9 x 10^-7 gram. That's a tiny amount.
