A student investigates four different radioactive isotopes. The student places a detector near each radioactive material. The background count rate is 24 counts per minute24\text{ counts per minute}24 counts per minute throughout the investigation. The table shows the detector readings at the start and after 6 hours6\text{ hours}6 hours.
| Isotope | Count rate at the start (counts per minute) | Count rate after 6 hours6\text{ hours}6 hours (counts per minute) |
|---|---|---|
| A | 120 | 30 |
| B | 184 | 64 |
| C | 224 | 100 |
| D | 304 | 59 |
Which isotope has a half-life of 3 hours3\text{ hours}3 hours?
A
B
C
D
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.