A water-treatment team adds a radioactive tracer to a channel to follow the movement of water. The tracer is released 2 hours after it is prepared, and monitoring ends 10 hours after release. For the monitoring result to be usable, the activity then must be at least 30 MBq30\ \mathrm{MBq}30 MBq.
An environmental check takes place 30 hours after preparation. The maximum permitted activity at this check is 4.0 MBq4.0\ \mathrm{MBq}4.0 MBq.
The tracer has a half-life of 6 hours. Its activity changes only by radioactive decay.
The activity when the tracer is prepared is called its initial activity.
Calculate the range of initial activities that would meet both requirements.
97 exam-style questions on Edexcel GCSE Physics 6.5 Uses and dangers of radioactivity, covering 6.5.1 Uses of radioactivity, 6.5.2 Dangers of ionising radiation and precautions, 6.5.3 Half-life and radiation dangers, 6.5.4 Safety precautions for radiation exposure, and 6.5.5 Contamination and irradiation. Each one has a worked solution and a mark scheme showing where the marks go.