Chapter 6 — Nuclear Physics: Long Answer Questions
Alpha (α) Radiation
- Nature: Helium nucleus — 2 protons + 2 neutrons (⁴₂He)
- Charge: +2
- Mass: 4 atomic mass units (amu)
- Speed: ~5–7% of speed of light
- Ionising power: Very high (highly ionising — can ionise many atoms per cm of air)
- Penetrating power: Very low — stopped by a sheet of paper or a few centimetres of air
- Effect on nucleus: Atomic number decreases by 2; mass number decreases by 4
Nuclear equation for alpha decay:
Beta (β) Radiation
- Nature: High-speed electron (β⁻) emitted when a neutron converts to a proton
- Charge: −1
- Mass: Negligible (same as electron = 1/1840 amu)
- Speed: Up to 90% of speed of light
- Ionising power: Medium (~100 times less ionising than alpha)
- Penetrating power: Medium — stopped by a few mm of aluminium
- Effect on nucleus: Atomic number increases by 1; mass number unchanged
Nuclear equation for beta decay:
Gamma (γ) Radiation
- Nature: Electromagnetic radiation (high-energy photons) — similar to X-rays but higher energy
- Charge: Zero
- Mass: Zero
- Speed: Speed of light (3 × 10⁸ m/s)
- Ionising power: Very low (least ionising)
- Penetrating power: Very high — several centimetres of lead or metres of concrete needed to stop
- Effect on nucleus: No change in atomic number or mass number (just energy is released)
- Usually emitted along with alpha or beta decay
Comparison Table:
| Property | Alpha | Beta | Gamma |
|---|---|---|---|
| Nature | ⁴₂He nucleus | Electron | EM radiation |
| Charge | +2 | −1 | 0 |
| Mass (amu) | 4 | ~0 | 0 |
| Penetration | Paper | Aluminium | Lead/concrete |
| Ionisation | Highest | Medium | Lowest |
| Deflection by field | Yes | Yes (opposite) | No |
Nuclear Fission
Nuclear fission is the process of splitting a heavy nucleus into two approximately equal fragments (daughter nuclei) when struck by a slow (thermal) neutron, releasing a tremendous amount of energy and 2–3 neutrons.
Equation:
Energy source: The products have slightly less mass than the original nucleus + neutron. This mass defect (Δm) is converted to energy by Einstein's equation:
(c = 3 × 10⁸ m/s) — even a tiny mass produces enormous energy.
Chain Reaction
The 2–3 neutrons released in each fission can each cause further fission in other uranium-235 nuclei → releasing more neutrons → causing more fissions → exponential growth of reactions = chain reaction.
Types:
- Uncontrolled chain reaction: Each fission causes more than one new fission → exponential growth → enormous energy released in a tiny fraction of a second → nuclear bomb (atomic bomb)
- Controlled chain reaction: Exactly one neutron per fission causes the next fission → steady, controlled energy release → nuclear reactor
Nuclear Reactor
A nuclear reactor is a device that maintains a controlled, self-sustaining nuclear fission chain reaction to produce heat energy for electricity generation.
Components:
| Component | Material | Function |
|---|---|---|
| Fuel | Enriched uranium-235 (or Pu-239) | Undergoes fission to release energy |
| Moderator | Heavy water (D₂O), graphite, ordinary water | Slows down fast neutrons to thermal neutrons (slow neutrons cause fission more effectively) |
| Control rods | Boron or cadmium | Absorb excess neutrons to control the rate of reaction; can be inserted/withdrawn to adjust power |
| Coolant | Water, heavy water, CO₂, liquid sodium | Carries heat from reactor core to steam generators |
| Pressure vessel | Thick steel | Contains the reactor core |
| Shielding | Thick concrete + lead | Prevents radiation from escaping |
India's nuclear programme: India has nuclear power plants at Tarapur (Maharashtra), Rawatbhata (Rajasthan), Kalpakkam (Tamil Nadu), Narora (UP), Kakrapar (Gujarat), Kaiga (Karnataka).
Nuclear Fusion
Nuclear fusion is the process of combining two light nuclei to form a heavier nucleus with release of enormous energy.
Example — Hydrogen fusion (in the Sun):
Or the simpler D-T (deuterium-tritium) fusion:
Conditions for fusion:
- Extremely high temperature (~10⁷ to 10⁸ K) — to overcome electrostatic repulsion between nuclei
- Extremely high pressure and density
- These conditions exist in the cores of stars
Why fusion releases more energy per kilogram than fission:
- The binding energy per nucleon increases more sharply when light nuclei fuse than when heavy nuclei split
Comparison: Fission vs Fusion
| Feature | Nuclear Fission | Nuclear Fusion |
|---|---|---|
| Process | Heavy nucleus split into two | Two light nuclei combine into one |
| Fuel | Uranium-235, Plutonium-239 | Deuterium (²H), Tritium (³H) |
| Energy per kg of fuel | High | Very high (several times fission) |
| Conditions | Slow neutrons; room temperature | Extreme temperatures (10⁷–10⁸ K) |
| Waste | Radioactive waste (long-lived) | Very little; tritium is mild |
| Chain reaction | Yes | No |
| Practical use | Nuclear reactors, atomic bombs | Sun and stars; hydrogen bomb |
| Controlled use | Yes (nuclear reactors) | Not yet achieved sustainably on Earth |
Fusion energy dream: If controlled fusion is achieved (ITER project, France), it would provide virtually unlimited, clean energy from seawater (deuterium source) — the ultimate energy solution.
Half-Life (T₁/₂)
Definition: The half-life of a radioactive substance is the time required for half the radioactive atoms (nuclei) in a sample to decay.
It is a constant property of each radioactive isotope — independent of temperature, pressure, chemical form, or initial amount.
Where:
- N = number of atoms remaining after n half-lives
- N₀ = initial number of atoms
- n = number of half-lives elapsed (n = t/T₁/₂)
Worked example: A radioactive sample has a half-life of 10 years. Starting with 800 g, how much remains after 40 years?
n = 40/10 = 4 half-lives N = 800 × (1/2)⁴ = 800 × 1/16 = 50 g
Half-Lives of Common Isotopes
| Isotope | Half-life | Use |
|---|---|---|
| Carbon-14 (¹⁴C) | 5730 years | Radiocarbon dating of ancient organic materials |
| Uranium-238 (²³⁸U) | 4.5 billion years | Dating of rocks and Earth's age |
| Radium-226 (²²⁶Ra) | 1600 years | — |
| Iodine-131 (¹³¹I) | 8 days | Thyroid cancer treatment |
| Cobalt-60 (⁶⁰Co) | 5.27 years | Cancer radiotherapy |
| Polonium-214 (²¹⁴Po) | 0.000164 seconds | — |
Applications of Half-life: Radiocarbon Dating
Living organisms continuously absorb carbon from the environment, maintaining a constant ratio of ¹⁴C to ¹²C. When an organism dies, it stops absorbing carbon, and the ¹⁴C decays (half-life = 5730 years) while ¹²C remains constant.
By measuring the remaining ¹⁴C/¹²C ratio in a fossil or artefact, scientists can calculate when the organism died (up to ~50,000 years).
Example: If a fossil has half the ¹⁴C ratio of a living organism, it died approximately 5730 years ago.
Harmful Effects of Radiation
Radiation damages living cells by ionising molecules (breaking chemical bonds), disrupting DNA, and killing or mutating cells.
Effects on the human body:
| Level of exposure | Effects |
|---|---|
| Low dose (over time) | Increased risk of cancer; genetic mutations in future generations |
| Moderate dose | Radiation sickness: nausea, hair loss, fatigue, anaemia, increased infection risk |
| High dose (acute) | Severe radiation burns, organ failure, death within weeks |
| Extremely high dose | Death within hours to days (acute radiation syndrome) |
Specific effects:
- Genetic mutations: Radiation causes mutations in germ cells → inherited by future generations
- Cancer: Particularly leukaemia (blood cancer), thyroid cancer, lung cancer
- Cataracts: Radiation damages the eye lens
- Growth retardation: In foetuses exposed during pregnancy
- Bone marrow damage: Suppresses blood cell production → anaemia, lowered immunity
Historical examples:
- Hiroshima and Nagasaki (1945) — atomic bombs
- Chernobyl nuclear disaster (1986) — Ukraine
- Fukushima nuclear disaster (2011) — Japan
Safety Measures
For radiation workers:
- Personal dosimeters — badge worn at all times to monitor total radiation exposure
- Lead-lined aprons and gloves — worn during procedures involving radiation
- Distance: Stay as far as possible from radiation source (intensity decreases with square of distance)
- Time: Minimise time of exposure
- Shielding: Thick lead or concrete walls between worker and source
- Regular health check-ups — blood counts, eye examinations
For general public near nuclear facilities:
- Emergency evacuation plans
- Distribution of iodine tablets (to protect thyroid from radioactive iodine)
- Monitoring of food, water, and air quality
For nuclear waste disposal:
- High-level waste stored in deep underground geological repositories
- Encased in multiple barriers (glass, steel, concrete, clay)
- Sites chosen for geological stability for thousands of years
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