Chapter 24: Radioactivity and Nuclear Reaction

24.1 Alpha, Beta & Gamma Particles

• α-particle → Helium nucleus (2p + 2n), +2 charge, heavy, low penetration (stopped by paper).
• β-particle → Electron (β⁻) or positron (β⁺), light, medium penetration (stopped by Al sheet).
• γ-ray → High-energy EM wave, no mass/charge, high penetration (needs lead block).

24.2 Laws of Radioactive Disintegration

24.3 Half-life, Mean-life & Decay Constant

24.4 Geiger-Muller Tube

• Detects α, β, γ radiation.
• Filled with gas → ionized by radiation → produces pulse → counted electronically.

24.5 Carbon Dating

• Based on radioactive isotope C-14 (half-life = 5730 years).
• Living organisms maintain constant C-14/C-12 ratio.
• After death, C-14 decays → measure remaining C-14 to estimate age.

24.6 Medical Use & Hazards

• Uses: Cancer treatment (radiotherapy), tracers in diagnosis, sterilization of instruments.
• Hazards: Tissue damage, mutation, cancer risk, radiation sickness if exposed too much.

Short Questions and Answers in Short

Numerical Examples

✅ Formula Sheet (Quick Revision)

Q1. What is an α-particle?
Ans: Helium nucleus (2 protons + 2 neutrons).

Q2. Which radiation has highest penetrating power?
Ans: Gamma rays.

Q3. Write the law of radioactive decay.
Ans: Rate of disintegration ∝ number of nuclei present.

Q4. Define half-life.
Ans: Time taken for half of the radioactive nuclei to decay.

Q5. Relation between half-life & decay constant?
Ans: T1/2 = 0.693/ λ

Q6. What is carbon dating used for?
Ans: To find the age of fossils/archaeological samples.

Q7. Mention one medical use of radiation.
Ans: Radiotherapy in cancer treatment.

Q8. What is the working principle of G-M tube?
Ans: Ionization of gas molecules by radiation produces pulses.

Q9. Define mean life.
Ans: Average lifetime of a radioactive nucleus = = 1/λ

Q10. Why is γ-ray more dangerous than α?
Ans: Due to high penetration power.

Numerical Examples

✅ Formula Sheet (Quick Revision)

Comparison of α, β, and γ Radiations