Nuclear Physics
Natural radioactivity, half-life, nuclear reactions (fission & fusion), and radiation hazards.
A. Natural Radioactivity
What is Radioactivity?
Radioactivity is the spontaneous emission of radiation from an unstable nucleus. Discovered by Henri Becquerel (1896). Pioneered by Marie Curie.
α Alpha
⁴₂He
Charge: +2
Mass: 4 amu
Ionisation: Highest
Penetration: Lowest
Stopped by a sheet of paper. Dangerous if inhaled/ingested.
β Beta
e⁻ or e⁺
Charge: ±1
Mass: ~0
Ionisation: Medium
Penetration: Medium
Stopped by aluminium sheet. Can penetrate skin.
γ Gamma
γ (photon)
Charge: 0
Mass: 0
Ionisation: Lowest
Penetration: Highest
Needs thick lead or concrete. Most dangerous externally.
| Radiation | Symbol | Charge | Mass | Penetration | Ionisation |
|---|---|---|---|---|---|
| Alpha (α) | ⁴₂He | +2 (2p, 2n) | 4 amu | Lowest (stopped by paper) | Highest |
| Beta (β) | e⁻ or e⁺ | −1 or +1 | ~0 | Medium (stopped by aluminium) | Medium |
| Gamma (γ) | γ | 0 | 0 | Highest (needs lead/concrete) | Lowest |
Alpha (α)
Symbol⁴₂He
Charge+2 (2p, 2n)
Mass4 amu
PenetrationLowest (stopped by paper)
IonisationHighest
Beta (β)
Symbole⁻ or e⁺
Charge−1 or +1
Mass~0
PenetrationMedium (stopped by aluminium)
IonisationMedium
Gamma (γ)
Symbolγ
Charge0
Mass0
PenetrationHighest (needs lead/concrete)
IonisationLowest
B. Half-Life
Half-Life Formula & Decay Table
Time for half the radioactive nuclei in a sample to decay
Radioactive Decay
N = N₀ × (½)ⁿ
N₀ = initial number · n = number of half-lives elapsed · n = t / t½
0
100%
1
50%
2
25%
3
12.5%
4
6.25%
C. Nuclear Reactions
Nuclear Fission
- A heavy nucleus splits into two smaller nuclei + neutrons + energy
- Example: ²³⁵U + n → Ba + Kr + 3n + energy
- Used in nuclear power plants and atomic bombs
- Chain reaction: neutrons released cause further fissions
Nuclear Fusion
- Two light nuclei combine to form a heavier nucleus + energy
- Example: 2 Hydrogen (deuterium) nuclei → Helium + energy
- Powers the Sun and other stars
- Releases MORE energy than fission — future energy source
⚡ MCQ Tip
Alpha = stopped by paper, highest ionisation. Beta = stopped by aluminium.
Gamma = needs lead, lowest ionisation.
Half-life: after each half-life, quantity halves. Fission = splitting. Fusion = joining.
D. Radiation Hazards & Safety
Hazards, Effects & Safety Measures
| Hazard | Effect | Safety Measure |
|---|---|---|
| Ionising radiation | Damages DNA, causes cancer and mutations | Lead shielding, distance, minimise exposure time |
| Alpha particles | Dangerous if inhaled or ingested | Avoid inhalation; not a skin hazard externally |
| Beta particles | Can penetrate skin; damages tissues | Aluminium shielding; protective clothing |
| Gamma rays | Deep tissue penetration; most dangerous externally | Thick lead or concrete walls |
| Nuclear waste | Long-term radioactive contamination | Deep geological storage; sealed containers |
Live Animation: Alpha Decay & Half-Life
Radioactive Decay — Alpha Particle Emission
Watch nuclei spontaneously emit alpha particles. Half-life counter updates in real time.
Remaining nuclei: 40
Decayed: 0
Elapsed time: 0.0 s
Half-lives passed: 0
% remaining: 100%
Quick MCQ Revision
| Formula / Fact | Meaning |
|---|---|
| N = N₀ × (½)ⁿ | Radioactive decay — n = number of half-lives |
| n = t / t½ | Number of half-lives = total time ÷ half-life |
| Alpha (α) | ⁴₂He · charge +2 · stopped by paper · highest ionisation |
| Beta (β) | e⁻ · charge ±1 · stopped by aluminium · medium ionisation |
| Gamma (γ) | EM wave · no charge/mass · needs lead · lowest ionisation |
| Fission | Heavy nucleus SPLITS → energy + neutrons (nuclear power) |
| Fusion | Light nuclei JOIN → energy (powers the Sun) |
| Half-life | After 1 half-life → 50% remains. After 2 → 25%. After 3 → 12.5% |
| Becquerel | Discovered radioactivity (1896) |