A student is investigating how the radiation from a radioactive sample changes with time. The table below shows the recorded count-rates at regular intervals:
| Time / min | Count-rate / counts per minute |
|---|---|
| 0 | 340 |
| 3 | 180 |
| 6 | 100 |
| 9 | 60 |
| 12 | 40 |
The half-life of the radioactive isotope is known to be exactly 3 minutes.
Why is the measured count-rate always higher than half of the previously recorded value?
The emissions from the radioactive source occur randomly over time.
The detector is placed too close to the radioactive source.
There is background radiation present in the environment.
The isotope in the source is undergoing active radioactive decay.
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.