What is radioactivity?
Every atom has a tiny centre called the nucleus. It holds protons (positive) and neutrons (no charge). Together they are called nucleons.
- Atomic number (Z) = number of protons. It decides the element.
- Mass number (A) = protons + neutrons.
- Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 and carbon-14 are isotopes.
Some nuclei have too many neutrons, too many protons, or are just too big. They are unstable. An unstable nucleus gives out radiation to become more stable. This is radioactive decay. It happens by itself. Heat, pressure or chemicals cannot change it. Henri Becquerel found it in 1896, and Marie and Pierre Curie studied it further.
Alpha, beta and gamma radiation
- Alpha (α): 2 protons + 2 neutrons, the same as a helium nucleus. Charge +2. Heavy and slow.
- Beta (β⁻): a fast electron made when a neutron turns into a proton inside the nucleus. Charge −1.
- Gamma (γ): an electromagnetic wave with very high energy. No mass, no charge. It often comes out after alpha or beta decay.
Nuclear equations
In a nuclear equation, the top numbers (A) add up on both sides and the bottom numbers (Z) add up on both sides.
- Alpha: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He (A goes down by 4, Z by 2)
- Beta: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e (A same, Z up by 1)
- Gamma: A and Z stay the same; only energy leaves.
Some nuclei can also give out a neutron, or a positron (β⁺), which you may meet in higher classes.
Penetration and ionisation
Ionisation means knocking electrons off atoms. This is how radiation harms living cells.
| Radiation | Stopped by | Range in air | Ionising power |
|---|---|---|---|
| Alpha | A sheet of paper or skin | A few cm | Very high |
| Beta | A few mm of aluminium | About 1 m | Medium |
| Gamma | Reduced by thick lead or concrete | Very far | Low |
Alpha and beta are bent by electric and magnetic fields in opposite directions, because their charges are opposite. Gamma is not bent.
Half-life and random decay
We can never say which nucleus will decay next. It is random, like tossing coins. But with millions of nuclei, the pattern is steady.
Activity is the number of decays each second. Its unit is the becquerel (Bq): 1 Bq = 1 decay per second. A Geiger–Müller (GM) tube counts the radiation; the count rate is shown in counts per second or per minute.
Half-life (T½) is the time for half of the unstable nuclei (or half of the activity) to decay. After n half-lives, the amount left = starting amount ÷ 2ⁿ.
Half-lives differ a lot: technetium-99m about 6 hours, iodine-131 about 8 days, carbon-14 about 5,700 years, uranium-238 about 4.5 billion years.
Background radiation, uses and safety
Background radiation is around us all the time. It comes from rocks and soil (radon gas), space (cosmic rays), food, and medical X-rays. Always measure background first and subtract it from your readings.
Uses
- Medicine: gamma rays kill cancer cells; tracers like technetium-99m show how organs work (medical imaging); gamma sterilises equipment.
- Dating: carbon-14 for things that were alive; uranium for rocks.
- Industry: beta sources check paper thickness; smoke alarms use alpha.
Contamination and irradiation
Irradiation means being exposed to radiation from outside; you do not become radioactive. Contamination means radioactive material gets on or inside you, so it keeps giving radiation. Keep sources far away, use tongs, shield them, and limit time near them.
Try it: coin half-life
Take 32 coins (or dice). Throw them all. Remove every coin that shows heads: those nuclei have "decayed". Count what is left and throw again. Record each round. You should get about 32, 16, 8, 4, 2. Each throw is one half-life. Then check it in the 3D free play with the slider.
Key formulas and definitions
- Mass number A = protons + neutrons; atomic number Z = protons
- Alpha: A − 4, Z − 2 · Beta: A same, Z + 1 · Gamma: no change
- Amount left after n half-lives = N₀ ÷ 2ⁿ
- Activity: 1 becquerel (Bq) = 1 decay per second
Worked examples
1. Polonium-210 (Z = 84) gives out an alpha particle. What are A and Z of the new nucleus?
Alpha takes away 4 from A and 2 from Z. New A = 210 − 4 = 206. New Z = 84 − 2 = 82 (this is lead).
2. Carbon-14 (Z = 6) gives out a beta particle. What is formed?
Beta: A stays 14, Z goes up by 1 to 7. Z = 7 is nitrogen, so nitrogen-14 is formed.
3. A sample has 80 g of a radioactive isotope with a half-life of 5 days. How much is left after 15 days?
15 ÷ 5 = 3 half-lives. 80 → 40 → 20 → 10 g. So 10 g is left.
4. The count rate of a source falls from 1,200 to 150 counts per minute in 6 hours. Find the half-life.
1,200 → 600 → 300 → 150 is 3 halvings. 6 hours ÷ 3 = 2 hours.
5. A GM tube reads 45 counts per minute near a source. Background is 15 counts per minute. What is the count from the source?
Subtract background: 45 − 15 = 30 counts per minute.
6. Why is an alpha source safe outside the body but very dangerous if swallowed?
Outside, alpha is stopped by dead skin. Inside, nothing stops it from reaching cells, and because it ionises strongly it does a lot of damage.
Common mistakes
- Thinking heating or cooling changes the rate of decay. It does not; decay is random and cannot be controlled.
- Forgetting that in beta decay the atomic number goes UP by 1, not down.
- Saying lead fully stops gamma. Lead only reduces gamma a lot.
- Thinking half-life means the whole sample is gone after two half-lives. After two half-lives a quarter is still left.