A detector placed near a radioactive isotope gives a measured count rate of 1000 counts/minute1000\text{ counts/minute}1000 counts/minute. The half-life of the isotope is five hours.
The background count rate is 40 counts/minute40\text{ counts/minute}40 counts/minute.
What is the count rate on the detector after fifteen hours?
120 counts/minute120\text{ counts/minute}120 counts/minute
125 counts/minute125\text{ counts/minute}125 counts/minute
160 counts/minute160\text{ counts/minute}160 counts/minute
165 counts/minute165\text{ counts/minute}165 counts/minute
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.