Grade 10 physics Waves and Optics – Radioactivity and Stability of Isotopes Notes

• Explain radioactivity terminologies (nuclear stability, radioactivity, half-life, nuclide, radioisotope, background radiation, etc.).
• Identify types and properties of radioactive emissions found in nature (α, β, γ).
• Show how radioisotopes attain stability using nuclear equations (α, β, γ, and uranium fission/decay series).
• Demonstrate detection methods for radioactive emissions (photographic plate, cloud chamber, leaf electroscope, Geiger–Müller counter).
• Determine half-life (t½) of common radioactive elements and model half-life in class (simple demonstration—burette method suggested).
• Appreciate industrial and medical applications and explain safety precautions in real life.

Radioactivity is spontaneous emission of particles or energy from unstable atomic nuclei as they change to more stable forms. The emitting nucleus is a radioisotope (radioactive nuclide).

• Nuclear stability: whether a nucleus is stable or tends to change (unstable = radioactive).
• Radioactivity: spontaneous emission of α, β or γ from an unstable nucleus.
• Half-life (t½): time for half the nuclei in a sample to decay.
• Nuclide / Nucleotide: a species of atom with defined proton and neutron number (nuclide is the correct physics term).
• Radioactive decay: the process of an unstable nucleus losing energy/particles to become more stable.
• Radioisotope: an isotope that is radioactive.
• Background radiation: low-level radiation from natural and artificial sources always present in environment.

Alpha decay example:

Beta minus (β−) decay example:

Gamma (γ) emission example (energy release):

Uranium fission (typical classroom example):

(Also discuss the uranium-238 decay series — first step shown above — and emphasise many steps produce α, β and γ emissions before reaching stable lead isotopes.)

• Photographic emulsion/plate: radiation exposes photographic film; alpha makes large dark spots, beta smaller, gamma broader exposure.
• Cloud chamber: supersaturated alcohol vapour shows tracks — thick short tracks (α), thin longer tracks (β), no track (γ may ionise occasionally producing small tracks).
• Leaf electroscope: ionising radiation frees charges and discharges an electroscope (useful demonstration to show presence of ionising radiation).
• Geiger–Müller (GM) counter: counts ionising events (clicks or counts per minute). Useful for background measurements and comparing sources.

Definition: half the time required for half the nuclei in a radioactive sample to decay.

Class example: If t½ = 2 days and N₀ = 100 nuclei, after 6 days: N = 100 × (1/2)^(6/2) = 100 × (1/2)^3 = 12.5 nuclei.

Classroom demonstration (burette method — modelling exponential decay):

1. Equipment: burette clamped vertically, small outflow tube (thin capillary) so outflow rate ≈ proportional to height, stop-watch, measuring cylinder, notebook.
2. Fill burette with coloured water; open stopcock and start timing. Record volume remaining in burette at equal time intervals (e.g., every 30 s).
3. If outflow rate is roughly proportional to height, volume in burette decreases approximately exponentially. From the recorded volumes find t when volume halves to estimate a model t½.
4. Alternative safer and simpler model: use 64 counters (paper discs). At each step toss a coin for each counter; remove counters showing “tails” (simulate decay). Count remaining and find time (steps) when half remain.

• Medicine: tracers for diagnosis (e.g., iodine-131), radiotherapy for cancer.
• Industry: thickness gauges, radiography to inspect welds, smoke detectors (small amount of americium-241).
• Energy: nuclear reactors for power (fission) — emphasise safety and regulation.
• Agriculture & food: irradiation to preserve food and control pests (regulated use).
• Archaeology & geology: carbon-14 dating and other radiometric dating methods.

• Always follow the rule: time, distance and shielding. Minimise time near sources, increase distance, and use shielding (lead, concrete) where required.
• Handle sources only under teacher/supervisor direction. Use tongs and gloves if required; store in labelled, locked containers.
• Wear dosimeters where required (not normally used in school demos unless trained personnel present).
• Do not bring unknown radioactive materials into school. Use only approved classroom sources or safe analogs (counters, coins) for demonstrations.
• Dispose of radioactive waste according to local regulations (do not pour into drains).

1. Observe tracks in a simple cloud chamber (teacher-prepared) and identify α and β tracks. Discuss track differences.
2. Measure background counts around school using a Geiger counter and compare values (soil, bricks, banana). Record and discuss results.
3. Use the counters/coin simulation or the burette analogue to model half-life. Plot results and find t½ graphically.
4. Write nuclear equations for simple α and β decays and balance mass and charge numbers.
5. Group research: find one Kenyan or local use of radioactivity (e.g., medical imaging at referral hospitals, industrial radiography) and present safety measures used.
6. Teacher demonstration: show photographic plate exposure with a sealed, approved source or using background exposure sample (only under teacher control).

1. Define half-life and calculate remaining nuclei after three half-lives.
2. Write the nuclear equation for 238U α-decay. Label the particles and show conservation of mass and charge.
3. Describe an activity to detect background radiation using a Geiger counter and explain what you would expect.
4. Explain why lead is used to shield γ radiation but not very effective for α.
5. List three safety rules to observe when doing a radioactivity demonstration in school.