Dual Nature of Radiation

Activity A = λN, where λ is decay constant and N is number of particles present. Therefore,
N =

A

λ

= Aτ
where τ = 1 / λ is the mean life of the sample. The mass of the sample is
M = mN = mAτ
where m is mass of an atom. Therefore, the amass of the radioactive sample is
M = 10−25×1010×109 = 10−6kg = 1 mg