15a) Explain what is meant by

(i) spontaneous decay

(ii) random decay

15dii) Despite the presence of some yttrium in the sample, the number of zirconium

nuclei is almost equal to the number of strontium nuclei that have decayed. By reference to the half-lives, explain this observation.

16a) Suggest why the determination of Phosphorus-32 by this method requires that the product of the decay be stable.

16aii) Suggest why the student did not need to make an allowance for background radiation.

16c) Explain why, although the count rates are too low for the radiation to cause

immediate symptoms in the student, careful shielding of the source is necessary.