P12: Nuclear Radiation and Hazards
Types of radiation, half-life and hazards
Nuclear Radiation And Hazards
📋 Key Definitions
Alpha particle (α): A particle consisting of 2 protons and 2 neutrons (a helium nucleus). It has a charge of +2 and is emitted from the nucleus during alpha decay.
Beta particle (β): A fast-moving electron emitted from the nucleus when a neutron changes into a proton and an electron. It has a charge of −1.
Gamma ray (γ): An electromagnetic wave emitted from the nucleus. It has no mass and no charge.
Half-life: The time it takes for the number of radioactive nuclei in a sample to halve, or for the activity of the sample to halve.
Contamination: When a radioactive substance gets on or inside the body. The source is inside you and continues to expose you to radiation.
Irradiation: When the body is exposed to radiation from an external source. The source is outside you and exposure stops when you move away.
☢️ Types of Nuclear Radiation
| Property | Alpha (α) | Beta (β) | Gamma (γ) | Neutron |
|---|---|---|---|---|
| What it is | 2 protons + 2 neutrons (helium nucleus) | Fast-moving electron | Electromagnetic wave | Neutron particle |
| Mass | 4 (heavy) | Very small | 0 (no mass) | 1 |
| Charge | +2 | −1 | 0 | 0 |
| Ionising power | High | Moderate | Low | Low |
| Penetrating power | Low — stopped by paper or a few cm of air | Moderate — stopped by aluminium foil (≈5 mm) | High — reduced by thick lead or concrete | High — requires thick concrete or water |
| Speed | Slow (5% speed of light) | Fast (up to 99% speed of light) | Speed of light | Variable |
Ionising power and penetrating power are inversely related: Alpha is the most ionising but the least penetrating. Gamma is the least ionising but the most penetrating. This is because alpha particles are large and heavy (easily knock electrons off atoms) but lose energy quickly.
🧮 Nuclear Equations
In nuclear equations, the total mass number and total atomic number must be the same on both sides.
Worked Example 1: Alpha decay
Uranium-238 (atomic number 92) undergoes alpha decay. Write the nuclear equation.
In alpha decay, the nucleus loses 2 protons and 2 neutrons (an alpha particle, 42He).
23892U → 23490Th + 42He
Check: mass numbers: 238 = 234 + 4 ✓ | Atomic numbers: 92 = 90 + 2 ✓
Worked Example 2: Beta decay
Carbon-14 (atomic number 6) undergoes beta decay. Write the nuclear equation.
In beta decay, a neutron becomes a proton and an electron. The proton stays in the nucleus (atomic number +1), and the electron (beta particle) is emitted.
146C → 147N + 0−1e
Check: mass numbers: 14 = 14 + 0 ✓ | Atomic numbers: 6 = 7 + (−1) ✓
Worked Example 3: Gamma emission
Gamma emission does not change the mass number or atomic number — the nucleus just loses energy.
6027Co* → 6027Co + γ
The * indicates an excited nucleus. Gamma rays often accompany alpha or beta decay.
⏱️ Half-Life
Radioactive decay is a random process — you cannot predict when an individual nucleus will decay. However, for a large sample, the half-life is a predictable time.
Half-life: The time taken for half the radioactive nuclei in a sample to decay. After each half-life, the activity (and number of undecayed nuclei) halves. After 2 half-lives, only a quarter remains. After 3, only an eighth.
Worked Example 4: Calculating remaining activity
A radioactive sample has an initial activity of 800 Bq and a half-life of 6 hours. What is its activity after 18 hours?
Number of half-lives = 18 / 6 = 3
After 1 half-life: 800 / 2 = 400 Bq
After 2 half-lives: 400 / 2 = 200 Bq
After 3 half-lives: 200 / 2 = 100 Bq
Worked Example 5: Finding the half-life
A sample has an activity of 3200 Bq. After 24 days, the activity has fallen to 200 Bq. What is the half-life?
3200 → 1600 → 800 → 400 → 200 = 4 half-lives
4 half-lives = 24 days
Half-life = 24 / 4 = 6 days
Worked Example 6: Fraction remaining
A substance has a half-life of 8 years. What fraction of the original sample remains after 32 years?
Number of half-lives = 32 / 8 = 4
Fraction remaining = (1/2)⁴ = 1/16
⚠️ Contamination vs Irradiation
| Property | Contamination | Irradiation |
|---|---|---|
| Definition | Radioactive substance gets on or inside the body | Body is exposed to radiation from an external source |
| Duration of exposure | Continues until the substance is removed or decays | Stops as soon as you move away from the source |
| Danger level | Generally more dangerous — ongoing exposure | Generally less dangerous — limited exposure time |
| Prevention | Wear protective clothing, use gloves, avoid inhaling or ingesting | Keep distance, use shielding, limit exposure time |
Contamination is usually more dangerous than irradiation because the radioactive source is inside or on you, so it continues to expose you to radiation. Irradiation stops when you leave the area.
🏥 Uses of Radiation
| Use | Radiation type | Why this type? |
|---|---|---|
| Smoke detectors | Alpha | Alpha is easily stopped by smoke, ionises air between electrodes |
| Medical tracers | Beta or Gamma | Pass through body tissues to be detected externally; short half-life reduces patient exposure |
| Radiotherapy (cancer treatment) | Gamma | High penetration reaches tumours deep inside the body; kills cancer cells |
| Sterilisation of medical equipment | Gamma | Penetrates packaging to kill bacteria without heating; no residue left |
| Thickness monitoring of paper/metal | Beta | Beta partially penetrates — changes in thickness change the count rate |
❓ Practice Questions
Q1: Foundation State the penetrating power and ionising power of alpha, beta and gamma radiation.
Q2: Foundation Explain the difference between contamination and irradiation.
Q3: Higher Radium-226 (atomic number 88) undergoes alpha decay. Write the nuclear equation and identify the daughter element.
Q4: Higher A sample of iodine-131 has an activity of 640 Bq. Its half-life is 8 days. What is its activity after 32 days?
Q5: Foundation Explain why alpha radiation is used in smoke detectors but would not be suitable for medical tracers.
Q6: Higher A radioactive source has a half-life of 10 years. After how many years will only 1/8 of the original sample remain?
✅ Answers
- Alpha: high ionising, low penetrating (stopped by paper). Beta: moderate ionising, moderate penetrating (stopped by aluminium foil). Gamma: low ionising, high penetrating (reduced by thick lead/concrete).
- Contamination is when a radioactive substance gets on or inside the body, exposing you continuously. Irradiation is when you are exposed to radiation from an external source — exposure stops when you move away. Contamination is generally more dangerous.
- 22688Ra → 22286Rn + 42He. The daughter element is radon-222 (atomic number 86).
- Number of half-lives = 32 / 8 = 4. Activity: 640 → 320 → 160 → 80 → 40 Bq. After 32 days the activity is 40 Bq.
- Alpha is used in smoke detectors because it easily ionises air between electrodes and is stopped by smoke (changing the current). It is not suitable for medical tracers because it cannot penetrate out of the body to be detected externally, and its high ionising power would damage tissues.
- 1/8 = (1/2)³, so 3 half-lives are needed. 3 × 10 = 30 years.
🎯 Exam Tips
- Learn the properties table for alpha, beta and gamma — ionising power and penetrating power are the most commonly tested
- In nuclear equations, always check that mass numbers and atomic numbers balance on both sides
- For half-life calculations, first work out the number of half-lives, then halve the activity repeatedly
- Contamination vs irradiation: contamination = source is on/in you, irradiation = source is outside you
- When asked about uses of radiation, always explain why that type is chosen (e.g. gamma for tracers because it penetrates tissue)
- Beta decay increases the atomic number by 1 (neutron → proton + electron). Alpha decay decreases it by 2
- Gamma emission does not change the mass number or atomic number
🔢 Maths Skills
Mathematical Skills
For half-life calculations, work out the number of half-lives first: number of half-lives = total time / half-life. Then halve the activity repeatedly. For nuclear equations, ensure mass numbers and atomic numbers balance on both sides. The fraction remaining after n half-lives is (1/2)ⁿ.
Maths Example
A sample has activity 5120 Bq and half-life 15 minutes. Find the activity after 1 hour. Number of half-lives = 60/15 = 4. Activity = 5120 / 2⁴ = 5120 / 16 = 320 Bq. Fraction remaining = (1/2)⁴ = 1/16.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: After 2 half-lives, all the radioactive material has decayed. Correct: After 2 half-lives, only 3/4 has decayed — 1/4 of the original material still remains.
2. Wrong: Beta decay changes the mass number because a particle is emitted. Correct: Beta decay does NOT change the mass number — a neutron becomes a proton + electron, so mass number stays the same but atomic number increases by 1.
3. Wrong: Irradiation is more dangerous than contamination because the radiation is stronger. Correct: Contamination is generally more dangerous because the radioactive source is inside/on you, exposing you continuously. Irradiation stops when you move away from the source.
✍️ 6-Mark Question
Extended Answer
6 marks: Compare the properties of alpha, beta and gamma radiation. Include ionising power, penetrating power and an appropriate use for each type.
Alpha radiation has the highest ionising power because it is large and heavy, easily knocking electrons off atoms. It has the lowest penetrating power — it is stopped by a few cm of air or a sheet of paper. Alpha is used in smoke detectors because it ionises air and is easily stopped by smoke. Beta radiation has moderate ionising power and moderate penetrating power — it is stopped by about 5 mm of aluminium. Beta is used in thickness monitoring of thin materials like paper or aluminium foil because it partially penetrates, and changes in thickness change the detected count rate. Gamma radiation has the lowest ionising power but the highest penetrating power — it requires thick lead or concrete to reduce it. Gamma is used in medical tracers and radiotherapy because it can pass through body tissue to reach or be detected from deep inside the body.
Mark scheme: 1 mark for alpha — high ionising, low penetrating, stopped by paper; 1 mark for beta — moderate ionising and penetrating, stopped by aluminium; 1 mark for gamma — low ionising, high penetrating, reduced by lead/concrete; 1 mark for correct use of alpha (smoke detector); 1 mark for correct use of beta (thickness monitoring); 1 mark for correct use of gamma (tracers/radiotherapy). (6 marks total)
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A hospital stores a radioactive isotope used as a medical tracer. The isotope has a half-life of 6 hours and an initial activity of 800 MBq. A patient requires a dose of at least 100 MBq for the scan to work effectively.
(a) Calculate how long after delivery the isotope can still be used for scans.
(b) Explain why a short half-life is both an advantage and a disadvantage for a medical tracer.
(c) Another isotope has a half-life of 5 years and is considered for the same purpose. Evaluate why this would be unsuitable.
Answers: (a) 800 → 400 (1 half-life, 6 h) → 200 (2 half-lives, 12 h) → 100 (3 half-lives, 18 h). The isotope can be used for up to 18 hours after delivery. (b) Advantage: the radiation quickly reduces to safe levels in the patient's body, minimising radiation exposure. Disadvantage: the isotope must be used soon after production or it decays too much to be useful, creating logistical problems. (c) A half-life of 5 years means the isotope would remain radioactive in the patient's body for many years, causing prolonged radiation exposure and greatly increasing the risk of tissue damage and cancer. It would also be impractical to store — it does not need frequent replacement but the long-term radiation risk to the patient makes it completely unsuitable as a tracer.
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