A detector of ionising radiation gives a background reading of 15 counts/second15\text{ counts/second}15 counts/second.
A radioactive isotope with a half-life of 4.0 hours4.0\text{ hours}4.0 hours is brought near to the detector. The reading on the detector increases to 135 counts/second135\text{ counts/second}135 counts/second.
How long does it take for the reading on the detector to decrease to 30 counts/second30\text{ counts/second}30 counts/second?
4.0 hours4.0\text{ hours}4.0 hours
8.0 hours8.0\text{ hours}8.0 hours
12 hours12\text{ hours}12 hours
16 hours16\text{ hours}16 hours
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.