Beryllium-7 ($^{7}_{4}\text{Be}$) forms in the upper atmosphere and then falls to the Earth’s surface. Beryllium-7 nuclei decay with a half-life of 53.3 days, producing stable nuclei. A sample of beryllium-7 on a tree leaf has an activity of $39\,\text{mBq}$.
(a)[1]
Demonstrate that the decay constant for beryllium-7 is $1.5 \times 10^{-7}\,\text{s}^{-1}$.
(b)[3]
Find the mass of the beryllium-7 on the leaf.
(c)[2]
The leaf is then covered so that no additional beryllium-7 is supplied from the atmosphere to the current sample. Work out the time that has to pass before the sample activity falls to $2.0\,\text{mBq}$.
Worked solution & mark scheme
This 6-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Application of $\lambda=\ln2/T_{1/2}$” …