A radioactive source has a half-life of 20 minutes20\text{ minutes}20 minutes. A detector near the source shows a reading of 8000 counts per second8000\text{ counts per second}8000 counts per second. Background radiation can be ignored. What is the reading on the detector 80 minutes80\text{ minutes}80 minutes later?
500 counts per second500\text{ counts per second}500 counts per second
1000 counts per second1000\text{ counts per second}1000 counts per second
2000 counts per second2000\text{ counts per second}2000 counts per second
4000 counts per second4000\text{ counts per second}4000 counts per second
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.