State what radioactive decay means.
Fig. 12.1 shows how the number $N$ of undecayed nuclei in a sample of a radioactive isotope varies with time $t$.
From the gradient of the line in Fig. 12.1, determine the activity, in Bq, of the sample when $t = 4.0\ \text{hours}$. Show your working.
Use your answer in (i) to show that the decay constant $\lambda$ of the isotope is approximately $4 \times 10^{-5}\ \text{s}^{-1}$.
A sample of another radioactive isotope has an initial activity of $4.6 \times 10^{3}$ Bq. It has to be kept in safe storage until its activity falls to $1.0 \times 10^{3}$ Bq. The decay constant for the isotope is $5.5 \times 10^{-7}\ \text{s}^{-1}$. The decay products are not radioactive. Calculate the minimum time, in days, that the sample must be stored.