A detector placed near a radioactive isotope gives a measured count rate of 700 counts/minute700\text{ counts/minute}700 counts/minute. The half-life of the isotope is four hours.
The background count rate is 60 counts/minute60\text{ counts/minute}60 counts/minute.
What is the count rate on the detector after twelve hours?
80 counts/minute80\text{ counts/minute}80 counts/minute
140 counts/minute140\text{ counts/minute}140 counts/minute
148 counts/minute148\text{ counts/minute}148 counts/minute
220 counts/minute220\text{ counts/minute}220 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.