IGCSE Physics / Atoms and radioactivity
Practical: investigating the penetrating power of radiation revision notes
Edexcel International GCSE Physics (4PH1), chapter 22: Atoms and radioactivity, specification point 7.6. Free lesson.
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You can identify radiation by measuring how different absorbers change the count from a radioactive source. Correct for background radiation and keep the method consistent so the results are useful and safe.
How to measure the penetrating power
A Geiger-Müller tube detects radiation and sends a count to a counter. Place an absorber between the source and the tube. Compare the corrected counts to see which types of radiation pass through each material. Measure the background count for 10 minutes with no source nearby. Background radiation is always present. Keep the source about 2 cm from the tube throughout the investigation.
How to identify the radiation
Each absorber gives evidence about what the source emits. A sharp fall in count shows that the absorber has stopped some radiation. Alpha is stopped by paper, beta by aluminium, while lead reduces gamma. If paper sharply reduces the count, alpha is present. If aluminium reduces the count, beta is present. If only lead reduces the count, gamma is present.
What the corrected results show
For this source, paper makes almost no difference, but aluminium reduces the count. Thick aluminium still lets some radiation through, while lead reduces the count further. This shows that the source emits beta and gamma, but not alpha. Corrected count rates, in counts per minute: none 480, paper 476, 1 mm aluminium 290, 3 mm aluminium 70, 5 mm aluminium 42, lead 6. The drop from 480 to 476 with paper is small, so there is no alpha. Aluminium reduces the count, showing beta. Some radiation passes through 5 mm aluminium, so gamma is also present.
How to handle radioactive sources safely
Reduce your exposure by keeping the source away from your skin, increasing your distance from it and limiting the time you handle it. Store it safely when you have finished. Pick it up with long tongs, never your fingers, to keep it away from your skin. Hold it at arm's length and never point it at anyone. Greater distance means less radiation reaches you. Use it for as short a time as possible to receive a smaller dose. Store it in a lead-lined box because lead absorbs radiation.