Past exam of the particle and nuclear physics course of the University of Cambridge / 2021 / Section B / 6 / e / Solution

The number of radioactive atoms is determined by the supplied isotope mass, rather than by the total body mass:The radioactive activity is $\mathcal{A}=\lambda N$, with nuclear decay constant $\lambda =\ln 2/T_{1/2}$. Using $1\mathrm{yr}=365.25\times 86400\mathrm{s}$,ThereforeA becquerel is one decay per second; this is total activity over all decay branches. Multiplying by the argon-producing branching fraction would give that branch's activity, not the total asked for. Activity alone is also distinct from an absorbed radiation dose.