A radioactive isotope has a half-life of 90 minutes90\text{ minutes}90 minutes. It emits radiation at a rate of 160 particles per second160\text{ particles per second}160 particles per second. How long does it take for the rate of emission to fall to 20 particles per second20\text{ particles per second}20 particles per second?
90 minutes90\text{ minutes}90 minutes
180 minutes180\text{ minutes}180 minutes
270 minutes270\text{ minutes}270 minutes
360 minutes360\text{ minutes}360 minutes
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.