Radioactivity
Background radiation, alpha, beta and gamma emission, decay equations, half-life, safety and uses.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 5.2.1Describe the nature, penetrating power and ionising ability of alpha, beta and gamma radiation.
- 5.2.2Define half-life and use decay curves to determine it.
- 5.2.3Balance nuclear equations for alpha and beta decay.Supplement
- 5.2.4Describe safety precautions when handling radioactive sources and give uses of radioactivity.
9 minute read
Radioactivity and half-life
Background radiation is the low level of radiation always present around us, from natural sources (rocks and soil, cosmic rays, radon gas) and artificial sources (medical procedures, nuclear industry).
Three types of radiation
| Nature | Penetrating power | Ionising ability | |
|---|---|---|---|
| Alpha (α) | 2 protons + 2 neutrons | Stopped by paper or a few cm of air | Strongly ionising |
| Beta (β) | A fast-moving electron | Stopped by a few mm of aluminium | Moderately ionising |
| Gamma (γ) | Electromagnetic wave | Reduced by thick lead, never fully stopped | Weakly ionising |
Notice the trade-off: the more penetrating a radiation type is, the less strongly ionising it is, because it interacts with matter less often.
Nuclear equations
In alpha decay, the nucleon number falls by 4 and the proton number falls by 2 (an alpha particle, ⁴₂He, is emitted). In beta decay, a neutron changes into a proton and an electron is emitted; the nucleon number stays the same but the proton number increases by 1.
Half-life
The half-life is the time taken for half the radioactive nuclei in a sample to decay, or equivalently, for the count rate (corrected for background) to fall to half its value. Radioactive decay is random — you cannot predict when any individual nucleus will decay — but for a large sample, the half-life is constant and predictable.
Safety and uses
Radioactive sources are handled with tongs (never bare hands), stored in lead-lined containers, and kept as far away as practical, since exposure is minimised by distance, shielding and time. Uses include: medical tracers and cancer treatment (gamma), smoke detectors (alpha), thickness control in manufacturing (beta), and sterilising equipment (gamma).
Think of it like this
Half-life is like popcorn popping in a pan: you cannot predict which individual kernel will pop next, but you can reliably say that after a fixed time, about half of whatever is left will have popped — the process is random for one kernel, but statistically predictable for the whole batch.
Worked examples
Method, step by step
A radioactive isotope has a half-life of 8 days. Starting with 640 Bq (corrected for background), calculate the activity after 32 days.
- 132 days ÷ 8 days = 4 half-lives
- 2640 → 320 → 160 → 80 → 40
40 Bq
Common misconceptions
- Thinking half-life means "the time for the sample to completely decay". It is the time for half of what remains to decay — after two half-lives, a quarter remains, not zero.
- Believing you can predict when a specific nucleus will decay. Individual decay is random; only the behaviour of a large sample follows a predictable pattern.
- Assuming more penetrating radiation is more dangerous in every situation. Alpha, though the least penetrating, is the most dangerous if the source is swallowed or inhaled, because all its energy is deposited in a small area of the body.
In the exam
- Alpha, beta and gamma questions are commonly answered as a table in your head: nature, penetration, ionising power — practise reciting all three together, since a question naming one property often expects you to link it to another.
- For half-life graph questions, always subtract the background count rate first before halving, unless the question tells you background has already been accounted for.