A detector of radioactivity in a laboratory indicates an average background count rate of 12 counts/min12\text{ counts/min}12 counts/min when no radioactive samples are present. A radioactive sample with a half-life of 3.0 hours3.0\text{ hours}3.0 hours is placed near the detector, which then indicates a total count rate of 268 counts/min268\text{ counts/min}268 counts/min.
Calculate the total count rate indicated by the detector 15.0 hours15.0\text{ hours}15.0 hours later.
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.