A student uses a Geiger-Müller (GM) tube and counter to measure the radioactivity of a sample. Before placing the sample near the GM tube, the student records a reading of 25 counts per minute25\text{ counts per minute}25 counts per minute.
What does this reading measure?
the visible light entering the detector from room lighting
the infra-red radiation emitted by the student and the detector
the ionising radiation present in the environment when the sample is not nearby
the ionising radiation that has leaked from the sample's protective container
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.