Public Section Preview
Radioactivity and Nuclear Chemistry
7.1 Types of Radioactive Radiation
Discovered by Henri Becquerel (1896) when he found uranium compounds spontaneously emitted radiation that could expose photographic plates. Marie Curie (Nobel Prize: Physics 1903, Chemistry 1911) discovered polonium and radium.
| Radiation | Nature | Charge | Mass | Penetration | Ionisation |
|---|---|---|---|---|---|
| Alpha (α) | ²He⁴ nucleus | +2 | 4 a.m.u. | Low — stopped by paper or skin | High |
| Beta (β⁻) | Electron | −1 | ~0 | Moderate — stopped by 5 mm aluminium | Moderate |
| Beta (β⁺) | Positron | +1 | ~0 | Moderate | Moderate |
| Gamma (γ) | EM wave (photon) | 0 | 0 | Highest — requires thick lead/concrete | Low |
Half-Life (T½): Time for half the radioactive nuclei in a sample to decay. After n half-lives, remaining fraction = (½)ⁿ.
| Isotope | Half-Life | Medical/Practical Use |
|---|---|---|
| Radon-222 | 3.82 days | Environmental hazard in buildings |
| Iodine-131 | 8.02 days | Thyroid cancer treatment |
| Phosphorus-32 | 14.3 days | Cancer treatment, tracer studies |
| Carbon-14 | 5,730 years | Archaeological dating |
| Uranium-235 | 703 million years | Nuclear fuel |
| Uranium-238 | 4.47 billion years | Geological age dating |
7.2 Nuclear Fission and Fusion
Nuclear Fission
A heavy nucleus (U-235, Pu-239) absorbs a neutron and splits into two medium nuclei, releasing 2–3 neutrons and ~200 MeV energy.
Chain reaction: Released neutrons trigger further fissions → exponential energy release.
- In a nuclear reactor, chain reaction is controlled using control rods (cadmium or boron) that absorb excess neutrons
- In a nuclear bomb, the reaction is uncontrolled
Nuclear Fusion
Light nuclei (deuterium D + tritium T) combine at ~100 million °C to form helium + neutron, releasing ~17.6 MeV per reaction.
- This is the energy source of the Sun and other stars
- ITER (International Thermonuclear Experimental Reactor) in France — India is a member — is building the world's largest fusion experiment (expected first plasma 2025, full power 2035)
- Fusion produces no long-lived radioactive waste and is virtually unlimited fuel (deuterium from seawater)
7.3 Radiation Safety and Nuclear Medicine
ALARA Principle (As Low As Reasonably Achievable): The guiding principle for radiation protection.
Three fundamentals of radiation protection:
- Time: Minimise time of exposure.
- Distance: Increase distance from source (radiation intensity ∝ 1/distance²).
- Shielding: Place absorbing material between source and person.
Nuclear medicine applications:
- PET Scan (Positron Emission Tomography): Uses F-18 (fluorine-18) labelled glucose. Cancer cells consume more glucose — appear as bright spots. Detects cancer, heart disease, brain disorders.
- SPECT (Single Photon Emission CT): Uses Tc-99m (technetium-99m, half-life 6 hours) for bone scans, cardiac imaging.
- Cancer Radiotherapy: Cobalt-60 external beam therapy; Iridium-192 for brachytherapy.
India's Nuclear Programme
India has 22 operational nuclear reactors (capacity ~6,780 MW electric as of 2025) at Tarapur, Rawatbhata, Kalpakkam, Narora, Kakrapar, and Kaiga.
RAPS in Rajasthan (near Kota, on Chambal river) has 6 units with total capacity ~1,180 MWe.