Radioactivity: alpha, beta and gamma rays
Unstable heavy nuclei break up on their own, giving out alpha particles, beta particles and gamma rays. Alpha and beta emission change one element into another.
What is radioactivity?
In 1896, Henri Becquerel found that uranium salts fogged photographic plates wrapped in black paper. A new, penetrating radiation was coming from the uranium. Soon Marie and Pierre Curie found two more such elements, polonium and radium. This spontaneous (self-started) emission of radiation by unstable nuclei is called radioactivity. Heat, pressure or chemical change cannot speed it up or stop it. In heavy nuclei, the repulsion between the many protons is too strong for stability. So they break up by giving out α (alpha) or β (beta) particles, often with γ (gamma) rays.
Alpha, beta and gamma compared
| Property | Alpha (α) | Beta (β⁻) | Gamma (γ) |
|---|---|---|---|
| What it is | Helium nucleus ⁴₂He (2 protons, 2 neutrons) | Fast electron from the nucleus | Electromagnetic wave (high-energy photon) |
| Charge | +2e | −e | None |
| In electric and magnetic fields | Deflected (slightly, being heavy) | Deflected more, the opposite way | Not deflected |
| Speed | About 1.4 to 2 × 10⁷ m/s | Up to about 0.99 c | Speed of light, c |
| Ionising power | Very high | Medium | Low |
| Penetrating power | Low: stopped by paper or 0.02 mm of aluminium | Medium: a few mm of aluminium | High: several cm of lead or iron |
Decay rules: what happens to Z and A
In every decay, the total charge (Z) and the total mass number (A) are the same before and after.
- 1Alpha decay: the nucleus loses ⁴₂He. Z falls by 2 and A by 4. Example: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He.
- 2Beta decay: a neutron in the nucleus becomes a proton, and an electron (⁰₋₁e) leaves. Z rises by 1; A stays the same. Example: ²³⁴₉₀Th → ²³⁴₉₁Pa + ⁰₋₁e.Some nuclei give out a positive beta particle (a positron) instead; then a proton becomes a neutron and Z falls by 1.
- 3Gamma decay: an excited nucleus (marked *) gives out a γ-ray photon. Z and A do not change. Example: (⁶⁰₂₈Ni)* → ⁶⁰₂₈Ni + γ.
- 4★Alpha or beta decay makes a new element (the daughter nucleus). Gamma decay does not.
★ key step
Worked example: a chain of decays
²³⁸₉₂U decays to ²⁰⁶₈₂Pb through a series of alpha and beta decays. How many alpha and how many beta particles are given out?
- Only alpha decay changes A, by 4 each time. Fall in A = 238 − 206 = 32.
- Number of α decays = 32 ÷ 4 = 8.
- Eight α decays lower Z by 8 × 2 = 16: from 92 to 76.
- But lead has Z = 82. Each β⁻ decay raises Z by 1, so the number of β decays = 82 − 76 = 6.
8 alpha particles and 6 beta particles
Worked example: completing decay equations
Find X and Y: (a) ²²⁶₈₈Ra → X + ⁴₂He; (b) ³²₁₅P → Y + ⁰₋₁e.
- (a) A: 226 = A(X) + 4, so A(X) = 222. Z: 88 = Z(X) + 2, so Z(X) = 86.
- X is ²²²₈₆Rn (radon).
- (b) A: 32 = A(Y) + 0, so A(Y) = 32. Z: 15 = Z(Y) − 1, so Z(Y) = 16.
- Y is ³²₁₆S (sulphur).
X = ²²²₈₆Rn; Y = ³²₁₆S
How to draw α, β and γ rays in a magnetic field
- Draw a lead block with a narrow hole and label it radioactive substance in lead block.
- Draw crosses (×) all over the space above it to show a magnetic field into the page.
- Draw three paths starting from the hole and going up.
- Bend one path slightly to the left and label it α (positive, heavy).
- Bend one path sharply to the right and label it β (negative, light).
- Draw the third path straight up and label it γ (no charge).
Many smoke detectors used in offices hold a tiny alpha source. Smoke entering the detector absorbs the alpha particles, the small current they produce drops, and the alarm sounds.
2 marks: “Compare the ionising and penetrating powers of α, β and γ radiations.” Ionising power: α > β > γ. Penetrating power: α < β < γ (α is stopped by paper, β by a few mm of aluminium, γ needs several cm of lead).
2 marks: “Complete the equations: (a) ²¹⁰₈₄Po → ? + ⁴₂He (b) ¹⁴₆C → ? + ⁰₋₁e” (a) ²⁰⁶₈₂Pb; (b) ¹⁴₇N. A and Z balance on both sides.
1 marks: “Which radiation is not deflected by a magnetic field?” Gamma rays.
Alpha: A down 4, Z down 2. Beta: Z up 1. Gamma: nothing changes. Ionising α > β > γ; penetrating γ > β > α.