State the nuclear equation for this reaction.

Determine the fraction of caesium-137 that will have decayed after 120 years.

Explain, with reference to the biological effects of ionizing radiation, why it is important that humans should be shielded from the radiation emitted by caesium-137.

${}_{56}^{137}{\rm{Ba}}$;
anti-neutrino / $\bar v$;

evidence of use of 4 half-lives;
so 0.938 or 93.8% or $\frac{{15}}{{16}}$ decays;

reference to a short-term effect e.g. skin reddening / burning;
reference to a long-term effect e.g. genetic damage / cancer;
reference to relative penetrative power of beta/ionizing power compared to alpha or gamma;