A student investigates how the radiation from a radioactive source changes over time. The half-life of the source is known to be exactly 3.0 minutes.
The table shows the results recorded by the detector:
| Time / minutes | Count rate / counts per minute |
|---|---|
| 0 | 495 |
| 3.0 | 255 |
| 6.0 | 135 |
| 9.0 | 75 |
| 12.0 | 45 |
The experiment is repeated many times, and the detector consistently measures a count rate that is always greater than half of the previous value after each 3.0-minute interval.
Why is the measured count rate always greater than half of the previous value?
Radioactive emissions occur randomly with time.
There is background radiation present.
The detector is positioned very close to the source.
The radioactive source is decaying into a stable isotope.
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.