IGCSE Physics / Atoms and radioactivity
Detecting radiation and background radiation
Edexcel International GCSE Physics (4PH1), chapter 22: Atoms and radioactivity, specification point 7.9, 7.10. Part of the full course.
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Radiation is detected using photographic film or a Geiger-Müller tube. Background radiation is always present, so you subtract its count from a measurement to find the count from a radioactive source alone.