A Geiger-Müller tube placed near a radioactive source detects a count rate of 420 counts per minute420\text{ counts per minute}420 counts per minute. The average background radiation count rate in the room is determined to be 25 counts per minute25\text{ counts per minute}25 counts per minute.
How is the corrected count rate of the source determined, and what is its value?
By adding the background rate to the detected rate, giving 445 counts per minute445\text{ counts per minute}445 counts per minute.
By subtracting the background rate from the detected rate, giving 395 counts per minute395\text{ counts per minute}395 counts per minute.
By multiplying the detected rate by the background rate, giving 10 500 counts per minute10\,500\text{ counts per minute}10500 counts per minute.
By dividing the detected rate by the background rate, giving 16.8 counts per minute16.8\text{ counts per minute}16.8 counts per 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.