Deep inside some atoms sits a nucleus that is simply too unstable to hold together. Sooner or later it breaks up, flinging out invisible radiation. This is radioactivity — and it happens whether we watch or not.
Where radiation comes from
Radioactive decay happens in the atom's nucleus — not the electrons, not the shells. An unstable nucleus decays, releasing energy and particles as radiation. You cannot speed it up or slow it down; it is random for any single atom.
The three types
The three types differ hugely in penetrating power — how far they get before something stops them:
- Alpha (α) — the largest, slowest particle. Stopped by a sheet of paper. Least penetrating.
- Beta (β) — smaller and faster. Passes through paper but is stopped by a few mm of aluminium.
- Gamma (γ) — a wave of energy, not a particle. Needs thick lead or concrete to stop it. Most penetrating.
An analogy: alpha is a beach ball (big, easily blocked), beta is a tennis ball (gets a bit further), gamma is a laser beam (slices through almost everything). The bigger and slower the particle, the sooner it is stopped.
| Type | Stopped by | Penetrating power |
|---|---|---|
| Alpha (α) | Paper | Lowest |
| Beta (β) | A few mm of aluminium | Medium |
| Gamma (γ) | Thick lead or concrete | Highest |
Half-life
We cannot predict when one atom will decay, but with billions of them a clear pattern appears. The half-life is the time it takes for half the radioactive nuclei in a sample to decay. After each half-life, the amount remaining halves again: full → a half → a quarter → an eighth…
Worked reasoning
Ask: "how many half-lives fit into 4 days?" 4 ÷ 2 = 2 half-lives.
Start: whole sample. After 1 half-life (2 days): 1/2 left. After 2 half-lives (4 days): half of a half = 1/4 left.
Answer: a quarter (25%). Notice the amount never quite reaches zero — it just keeps halving.
Memory hook: penetration goes Alpha < Beta < Gamma — alphabetical, and each needs a tougher shield: Paper, Aluminium, Lead.