7 · IGCSE Physics
Penetrating power of radiation
Fire alpha, beta or gamma radiation through paper, aluminium and lead, watch where it is stopped and how strongly it ionises the air, and compare the count rate after each absorber.
count rate = counts ÷ time
corrected rate = measured − background
γ in lead: R = R₀ × (½)^(x ÷ x½), x½ ≈ 10 mm
Expected at detector: –
Measured (last 10 s, incl. background): –
Total counts: 0
Ionising ability: –
Try this
- Choose alpha and add only the paper. Does the measured count rate fall to zero, or to the background level?
- Choose beta with aluminium in place and increase the thickness from 0.5 mm to 3 mm. At what thickness is the beta completely absorbed?
- Choose gamma with lead only. Set 10, 20 and 30 mm and check that the expected count rate halves for each extra 10 mm.
Common error: the count rate never drops to exactly zero because of background radiation, so subtract the background before deciding whether an absorber has stopped the radiation.