A student is investigating how the radiation from a radioactive source changes with time. The table below shows the results from a radiation detector at intervals equal to the half-life of the source, which is exactly 5 minutes5\text{ minutes}5 minutes:
| Time (min\text{min}min) | Measured count-rate (counts per minute) |
|---|---|
| 0 | 615 |
| 5 | 315 |
| 10 | 165 |
| 15 | 90 |
Determine the constant count-rate due to background radiation, in counts per minute.
484 exam-style questions on CIE IGCSE Physics 5.2 Radioactivity, covering 5.2.1 Background radiation and measuring count rate, 5.2.2a Alpha, beta and gamma emission, 5.2.2b Deflection and ionising effects of nuclear radiation, 5.2.3a Radioactive decay, 5.2.3b Effects of decay on the nucleus and decay equations, 5.2.4a Half-life, 5.2.4b Choosing isotopes for applications, and 5.2.5 Effects of ionising radiation and safety precautions. Each one has a worked solution and a mark scheme showing where the marks go.