NEB Class 12 Physics Radioactivity and Nuclear Reactions Notes in PDF Complete Handwritten. Phyaics Notes 2082: All Chapters | New Curriculum | Class 12 Phyaics Notes download.
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NEB Class 12 Physics Radioactivity and Nuclear Reactions Note Handwritten in PDF
What is Radioactivity?
Radioactivity is the spontaneous emission of radiation from unstable atomic nuclei as they disintegrate to reach a more stable state.
- Natural radioactivity: Occurs spontaneously in nature (uranium, thorium, radium)
- Artificial radioactivity: Induced by bombarding stable nuclei with particles in accelerators
Radioactivity is a nuclear phenomenon it is unaffected by temperature, pressure, chemical state, or any external conditions. This distinguishes it from all chemical reactions.
Types of Radiation
Alpha (α) particles:
- Helium nucleus (²₄He): 2 protons + 2 neutrons
- Charge: +2e, Mass: 4 u
- Stopped by a sheet of paper or few cm of air
- Most ionizing (causes most ionization per unit path)
- Low penetrating power
- Dangerous if inhaled or ingested (irradiates internal tissues)
Beta (β) particles:
- β⁻: High-speed electron from nucleus (neutron → proton + electron + antineutrino)
- β⁺: Positron (proton → neutron + positron + neutrino) artificial radioactivity
- Stopped by a few mm of aluminium
- Moderately ionizing and penetrating
Gamma (γ) rays:
- High-energy electromagnetic radiation (photons)
- No charge, no mass
- Requires several cm of lead or metres of concrete to stop
- Least ionizing but most penetrating
- Always emitted after α or β decay (nucleus loses excess energy)
Radioactive Decay Law
dN/dt = −λN
Solving: N = N₀ e^(−λt)
Where:
- N₀ = initial number of atoms
- λ = decay constant (probability of decay per unit time, s⁻¹)
- t = time
Activity (A): A = −dN/dt = λN = A₀ e^(−λt) (measured in Becquerel = disintegrations per second)
Half-Life, Mean Life, and Decay Constant
Half-life (T₁/₂): Time for exactly half the atoms to decay
T₁/₂ = ln2/λ = 0.693/λ
After n half-lives: N = N₀/2ⁿ
Mean life (τ): Average lifetime of a single radioactive atom
τ = 1/λ = T₁/₂/0.693 = 1.44 T₁/₂
At t = τ: N = N₀/e ≈ 0.368 N₀ (about 37% remains)
Relationship summary:
- λ = 0.693/T₁/₂ = 1/τ
- T₁/₂ = 0.693τ
- τ = 1.44 T₁/₂
Geiger-Muller (GM) Tube
Construction:
- Metal cylinder (cathode, outer wall)
- Central wire (anode)
- Filled with inert gas (Argon + small amount of halogen quench gas)
- Thin mica window at one end (to allow α particles to enter)
- High voltage ~400V between anode and cathode
Working:
- Radiation enters through mica window
- Ionizes the gas → creates ion pairs (positive ions + electrons)
- Strong electric field accelerates ions → avalanche ionization (Townsend discharge)
- Produces a current pulse in external circuit
- Electronic counter counts each pulse
Each pulse = one radiation event detected
Can detect α, β, and γ radiation (γ through tube walls, α through mica window).
Carbon Dating
Principle:
- C-14 is produced in atmosphere by cosmic ray neutrons hitting N-14: ¹⁴N + n → ¹⁴C + H
- Living organisms continuously exchange carbon with atmosphere → maintain constant C-14/C-12 ratio
- When organism dies → C-14 exchange stops → C-14 decays with T₁/₂ = 5,730 years
- By measuring remaining C-14/C-12 ratio → calculate time since death
Formula: t = T₁/₂ × log₂(N₀/N) = (T₁/₂/0.693) × ln(N₀/N)
Effective range: Up to ~50,000 years (beyond that, too little C-14 remains to measure accurately)
Uses: Dating ancient wood, charcoal, bone, textiles, seeds famous for dating the Shroud of Turin, Dead Sea Scrolls, and Egyptian mummies.
Medical Uses and Health Hazards
Medical uses:
- Diagnosis: Radioactive tracers (¹³¹I for thyroid, ⁹⁹ᵐTc for bone scans) injected into body → gamma camera detects → creates image
- Cancer treatment (Radiotherapy): High-dose γ radiation (from Cobalt-60) destroys cancer cells
- PET scans: Positron emitting isotopes → detect cancer, brain activity
- Sterilization: Medical instruments sterilized by gamma radiation
Health hazards:
- DNA damage (mutations → cancer, genetic disorders)
- Radiation sickness at high doses (weakness, hair loss, immune failure)
- Burns and tissue destruction
- Long-term cancer risk
Protection:
- Distance (inverse square law double distance, quarter intensity)
- Shielding (lead for γ, aluminium for β, paper for α)
- Time (minimize exposure time)
Class 12 Science Faculty All Subject Notes
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Frequently Asked Questions
What is radioactive decay law in Class 12 Physics?
Radioactive decay law: N = N₀ e^(−λt). Number of undecayed atoms decreases exponentially with time. λ is the decay constant (probability of decay per unit time). Activity A = λN = A₀e^(−λt) measured in Becquerel (1 Bq = 1 disintegration per second). This law applies equally to all radioactive elements regardless of their half-life.
What is the half-life formula in Class 12 Physics?
Half-life T₁/₂ = 0.693/λ = 0.693τ. It is the time for exactly half the radioactive atoms to decay. After n half-lives: N = N₀/2ⁿ. Half-lives range from microseconds (radon-215) to billions of years (uranium-238). For NEB numericals: first find λ from T₁/₂ = 0.693/λ, then substitute into N = N₀e^(−λt).
What is the difference between half-life and mean life?
Half-life T₁/₂ = 0.693/λ is when exactly half the atoms have decayed. Mean life τ = 1/λ = 1.44 T₁/₂ is the average time a single atom survives before decaying. Mean life is always longer than half-life. After one mean life, N = N₀/e ≈ 36.8% of original remains. Relationship: T₁/₂ = 0.693 × τ always.
What are properties of alpha beta gamma radiation Class 12?
Alpha (²₄He nucleus): charge +2, mass 4u, stopped by paper, most ionising, least penetrating, deflected by fields. Beta (fast electron): charge −1, mass negligible, stopped by aluminium, moderately ionising. Gamma (electromagnetic photon): no charge or mass, stopped by thick lead, least ionising, most penetrating, not deflected by fields.
How does Geiger-Muller tube work Class 12?
GM tube has metal cylinder (cathode) and central wire (anode) filled with argon gas at ~400V. Radiation enters through thin mica window, ionises gas creating ion pairs. Strong electric field accelerates ions causing avalanche ionisation. This produces a current pulse counted electronically. Each pulse equals one detected radiation event.
What is carbon dating and how does it work Class 12?
C-14 (half-life 5730 years) is produced in atmosphere by cosmic rays and maintained at constant ratio in living organisms. When organism dies, C-14 stops being replenished and decays. By measuring remaining C-14/C-12 ratio and using N = N₀e^(−λt), the time since death is calculated. Effective for dating up to approximately 50,000 years.
What is the difference between natural and artificial radioactivity?
Natural radioactivity: spontaneous emission from naturally occurring unstable nuclei like uranium-238, thorium-232, and radium-226 discovered by Henri Becquerel in 1896. Artificial radioactivity: induced by bombarding stable nuclei with particles in nuclear reactors or accelerators discovered by Irène and Frédéric Joliot-Curie in 1934.
Which radioactivity topics are most important for Exam?
Most important: radioactive decay law N=N₀e^(−λt) derivation (4 marks), half-life formula and numericals (4 marks), properties of alpha beta gamma radiation comparison (4 marks), GM tube working (2 marks), carbon dating principle (2 marks). Radioactive decay numericals finding remaining amount or age appear in every NEB board paper.
