A radioactive substance has a half-life of 8 hours8\text{ hours}8 hours. It has an initial rate of emission of 240 counts per second240\text{ counts per second}240 counts per second. How long will it take for this rate of emission to fall to 30 counts per second30\text{ counts per second}30 counts per second?
3 hours3\text{ hours}3 hours
16 hours16\text{ hours}16 hours
24 hours24\text{ hours}24 hours
64 hours64\text{ hours}64 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.