Contents
1. Nuclear Structure
| Particle | Location | Relative charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Shells around nucleus | −1 | ≈ 0 (1/1840) |
- Atomic number (Z) = number of protons = number of electrons in a neutral atom.
- Mass number (A) = number of protons + number of neutrons.
- Isotopes = atoms of the same element with the same number of protons but different numbers of neutrons.
Carbon-14 is written ¹⁴₆C. Mass number = 14, atomic number = 6.
Number of neutrons = 14 − 6 = 8 neutrons.
2. Types of Radiation
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Composition | 2 protons + 2 neutrons (helium nucleus) | Fast electron | Electromagnetic wave (photon) |
| Charge | +2 | −1 | 0 |
| Mass | 4 (relative) | ≈ 0 | 0 |
| Ionising ability | Strongest | Moderate | Weakest |
| Penetrating ability | Weakest — stopped by paper or few cm of air | Stopped by few mm of aluminium | Reduced by thick lead or concrete |
| Deflection in field | Deflected (positive charge) | Deflected opposite way (negative) | Not deflected |
Nuclear equations
In all nuclear equations, mass numbers and atomic numbers must balance on both sides.
Uranium-238 emits an alpha particle: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
Mass: 238 = 234 + 4 ✓ Atomic number: 92 = 90 + 2 ✓
Carbon-14 emits a beta particle: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e
A neutron turns into a proton + beta particle. Atomic number increases by 1; mass number unchanged.
3. Half-Life
The time taken for the activity (or number of undecayed nuclei) of a radioactive sample to fall to half its initial value.
A source has an initial count rate of 1600 counts/min. Its half-life is 6 hours. Find the count rate after 18 hours.
Number of half-lives = 18 ÷ 6 = 3
Count rate = 1600 → 800 → 400 → 200 counts/min
Half-life does not change with temperature, pressure, chemical state or the amount of substance present. It is a fixed property of each isotope.
4. Background Radiation
Background radiation is low-level radiation present in the environment at all times, from natural and artificial sources.
Sources
- Natural: cosmic rays, radon gas from rocks, radioactive materials in soil and food.
- Artificial: medical X-rays, nuclear weapons testing fallout, nuclear power stations.
In any experiment with a radioactive source, the background count rate must be measured first and subtracted from all readings to find the count rate due to the source alone.
"The count rate fell from 320 to 160 in 4 hours." If background is 20 counts/min, the corrected counts are 300 and 140. The half-life calculation must use corrected values, not raw readings.
5. Uses and Dangers of Radioactivity
| Use | Type used | Reason |
|---|---|---|
| Smoke detector | Alpha | Ionises air in detector; smoke absorbs alpha, reducing ionisation and triggering alarm |
| Thickness gauge (paper/metal) | Beta | Penetrates material; detected amount varies with thickness |
| Cancer treatment (radiotherapy) | Gamma | Penetrates deeply to kill tumour cells |
| Medical tracers | Gamma (short half-life) | Detected outside the body; short half-life reduces radiation dose |
| Sterilising medical equipment | Gamma | Kills bacteria without heating the equipment |
| Carbon dating | Beta (C-14) | Known half-life used to determine age of organic material |
Dangers and safety precautions
- Radiation ionises cells and can damage DNA → cancer, mutation, radiation sickness.
- Safety measures: lead shielding, tongs (never handle with bare hands), keep distance, minimise exposure time, store in lead-lined containers, dosimeter badges for workers.
- Alpha: stopped by paper or 5 cm air. Most ionising. Dangerous if inhaled/ingested (inside body).
- Beta: stopped by 3 mm aluminium. Moderately ionising.
- Gamma: reduced (never fully stopped) by thick lead or concrete. Least ionising. Most penetrating.
- Half-life: after 1 half-life -> 50% remains; 2 -> 25%; 3 -> 12.5%. Use: N = N0 x (0.5)^n
- Nuclear notation: mass number (top-left) = protons + neutrons. Atomic number (bottom-left) = protons only.
- Isotopes: same element (same protons), different number of neutrons. Some isotopes are radioactive.
6. Common Exam Traps
Alpha is the most ionising but least penetrating. Gamma is the least ionising but most penetrating. Students frequently reverse these.
Activity never reaches exactly zero — it halves repeatedly. After 10 half-lives it is less than 0.1% of the original, but never zero. "After 3 half-lives the activity is zero" is always wrong.
In beta decay, a neutron converts to a proton — the atomic number increases by 1. Mass number stays the same. Students often incorrectly change the mass number too.
Gamma emission releases energy only — no particles are emitted. Both mass number and atomic number remain unchanged. It often accompanies alpha or beta decay but is a separate event.
Key Terms — Flashcard Review
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🎯 Practice Quiz — Test Yourself
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.