Radioactivity and half-life
Physics · GCSE · The School
Three radiations
ALPHA is a helium nucleus: highly ionising, stopped by paper or a few centimetres of air. BETA is a fast electron from the nucleus: moderately ionising, stopped by a few millimetres of aluminium. GAMMA is an electromagnetic wave: weakly ionising, reduced but never fully stopped by thick lead. Penetration and ionisation run in opposite directions, and that is the pattern worth memorising.
Half-life
Half-life is the time for half the unstable nuclei in a sample to decay, or equivalently for the count rate to halve. Decay is random — you cannot say which nucleus goes next — but with vast numbers the average is highly predictable, which is how a random process yields a reliable number. To find what is left, halve repeatedly and count how many halvings the time allows.
Contamination and irradiation
IRRADIATION is being exposed to radiation from a source outside you; move away or shield and it stops, and you do not become radioactive. CONTAMINATION is getting the radioactive material itself onto or into you; it keeps irradiating you until it is removed or decays. Contamination is the more serious of the two for that reason, and exam questions test the distinction directly.
AFTER 6 HOURS, one half-life: 400. AFTER 12 HOURS, two: 200. AFTER 18 HOURS, three: 100. AFTER 24 HOURS, four: 50. Notice it never reaches zero — that is the misconception above, shown as numbers. NOW BACKWARDS, which is how the harder question is asked: a source reads 25 counts per minute and started at 800. How long has passed? Halve from 800 until you reach 25 — 400, 200, 100, 50, 25 — that is five halvings, so 5 × 6 = 30 hours. And one refinement worth a mark: real readings include BACKGROUND radiation, so subtract the background count before doing any of this, or every number above is slightly too high.