Short Answer Questions — Chapter 6: Nuclear Physics
Radioactivity is the spontaneous disintegration of unstable atomic nuclei with the emission of radiation (alpha, beta, or gamma rays) to attain a more stable configuration.
Key features:
- It is a spontaneous process — not triggered by any external factor
- It results in the transformation of one element into another
- The process continues until a stable nucleus is formed
- Radioactivity is unaffected by temperature, pressure, or chemical state
Discovery: Radioactivity was discovered by Henri Becquerel in 1896 when he accidentally found that uranium compounds emitted invisible radiation that could expose photographic plates wrapped in black paper.
Marie Curie and Pierre Curie further studied radioactivity and discovered the radioactive elements polonium and radium. Marie Curie coined the term "radioactivity."
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Particle (nucleus) | Particle (electron) | Electromagnetic wave |
| Composition | 2 protons + 2 neutrons | Fast electron | High-energy photon |
| Symbol | ⁴₂He | ⁰₋₁e | γ |
| Charge | +2 | −1 | 0 (neutral) |
| Mass | 4 u | Very small (~0) | 0 |
| Speed | ~1/10 of c | ~9/10 of c | c (speed of light) |
| Penetrating power | Least (paper stops it) | Medium (aluminium stops it) | Highest (lead/concrete needed) |
| Ionising power | Highest | Medium | Least |
| Deflection by fields | Yes (toward −ve plate) | Yes (toward +ve plate) | No |
Nuclear fission is the process in which a heavy atomic nucleus is split into two or more lighter nuclei when bombarded by a neutron, accompanied by the release of a tremendous amount of energy and 2–3 neutrons.
Example: Fission of Uranium-235
²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n + Energy (200 MeV)
Key features of fission:
- Energy released: ~200 MeV per fission (1 MeV = 1.6 × 10⁻¹³ J)
- The 2–3 neutrons released can trigger more fissions → chain reaction
- In a nuclear reactor, the chain reaction is controlled
- In an atomic bomb, the chain reaction is uncontrolled (explosive)
Applications:
- Nuclear power plants (generate electricity)
- Atomic bombs (nuclear weapons)
Nuclear fusion is the process in which two light atomic nuclei combine to form a single heavier nucleus, releasing enormous amounts of energy.
Example:
²₁H + ³₁H → ⁴₂He + ¹₀n + Energy (17.6 MeV) (Deuterium + Tritium → Helium + neutron + energy)
Why nuclear fusion is the Sun's energy source: The Sun fuses hydrogen nuclei (protons) into helium under the extreme temperature (15 million °C) and pressure at its core.
Advantages of fusion:
- Fuel (hydrogen isotopes) is abundant in seawater
- No long-lived radioactive waste (unlike fission)
- Releases more energy per unit mass than fission
- Much safer — reaction stops if anything goes wrong
Why not commercially available yet: 1. Requires extremely high temperatures (~10–100 million °C) to overcome electrostatic repulsion between nuclei 2. Very difficult to confine plasma at such temperatures (tokamak devices, laser confinement) 3. Has not yet achieved net energy gain (energy produced > energy input) 4. Enormous engineering challenges
Research in fusion continues internationally (ITER project in France).
Half-life (t₁/₂) is the time required for half the radioactive atoms in a sample to decay.
Key properties:
- Each radioactive isotope has a unique, constant half-life
- Unaffected by temperature, pressure, or chemical state
- After n half-lives, amount remaining = N₀ × (1/2)ⁿ
Example:
| Element | Half-life |
|---|---|
| Uranium-238 | 4.5 billion years |
| Carbon-14 | 5,730 years |
| Iodine-131 | 8 days |
| Radon-222 | 3.8 days |
| Polonium-214 | 0.00016 seconds |
Calculation example: If 100 g of a substance has a half-life of 10 years:
- After 10 years: 50 g remain
- After 20 years: 25 g remain
- After 30 years: 12.5 g remain
Applications:
- Carbon dating (radiocarbon dating): Uses the half-life of C-14 (5730 years) to date ancient organic materials (fossils, artifacts).
- Nuclear medicine: Short half-life isotopes used in diagnosis and treatment.
Beneficial uses of radioactivity:
| Field | Use |
|---|---|
| Medicine | Cancer treatment (radiotherapy), diagnostic imaging (PET scans), sterilising medical equipment |
| Agriculture | Radiation-induced mutations to develop disease-resistant crops; food preservation (irradiation) |
| Industry | Thickness gauges, detecting cracks in pipes (γ-radiography), smoke detectors (Am-241) |
| Dating | Carbon-14 dating of archaeological samples; uranium dating of rocks |
| Energy | Nuclear power plants provide low-carbon electricity |
Harmful effects of radiation:
1. Biological damage: Radiation ionises atoms in cells, breaking DNA strands → mutations, cell death 2. Radiation sickness: High doses cause nausea, hair loss, organ failure, and death 3. Cancer: Long-term low-dose exposure increases risk of leukemia and other cancers 4. Genetic mutations: Radiation can damage reproductive cells, causing birth defects in offspring 5. Environmental contamination: Nuclear accidents (Chernobyl 1986, Fukushima 2011) contaminate large areas for years
Protective measures:
- Use lead/concrete shielding
- Maintain safe distance (inverse square law)
- Limit exposure time
- Monitor with dosimeters
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