Science notes, chapter 8
Radioactivity
Textbook Science Form 3 (KSSM, Dual Language Programme), pages 230–255
What this chapter covers
- Describe how radioactivity was discovered and who contributed (8.1)
- Define radioactivity and radioactive decay; give examples of radioactive substances (8.1)
- State the units of radioactivity and explain half-life, including calculations and graphs (8.1)
- Explain the formation of positive ions and negative ions (8.2)
- Differentiate ionising and non-ionising radiation, and compare alpha, beta and gamma radiation (8.3)
- Give natural and man-made sources of ionising radiation and explain how to keep the dose safe (8.3)
- Describe the uses of radioactive radiation and the safe handling of radioactive substances and waste (8.4)
8.1Discovery of radioactivity
| Year | Scientist | Discovery |
|---|---|---|
| 1895 | Wilhelm Roentgen (German) | The X-ray; first Nobel Prize in Physics (1901) |
| 1896 | Henri Becquerel (French) | Radioactivity: a uranium compound blackened a photographic plate even in the dark (Nobel Prize 1903) |
| 1897 | Marie and Pierre Curie | Detected radioactive radiation through its ionising power; extracted polonium and radium from pitchblende (uranium ore) |
Radioactivity is the random and spontaneous decay of an unstable nucleus by emitting radioactive radiation: alpha particles (α), beta particles (β) and gamma rays (γ).
The nucleus keeps emitting radiation until it becomes more stable. Examples of radioactive substances: carbon-14, radon-222, thorium-234 and uranium-238.
Alpha decay
U-238 (92 p, 146 n) emits a helium nucleus (2 p, 2 n) and becomes Th-234 (90 p, 144 n).
U-238 (92 p, 146 n) emits a helium nucleus (2 p, 2 n) and becomes Th-234 (90 p, 144 n).
Beta decay
Th-234 (90 p, 144 n) emits an electron and becomes Pa-234 (91 p, 143 n).
Th-234 (90 p, 144 n) emits an electron and becomes Pa-234 (91 p, 143 n).
Gamma decay
An unstable Co-60 nucleus (27 p, 33 n) emits a gamma ray and stays Co-60, now more stable.
An unstable Co-60 nucleus (27 p, 33 n) emits a gamma ray and stays Co-60, now more stable.
Marie Curie is the only woman to receive two Nobel Prizes (Physics 1903, Chemistry 1911). She and Pierre had no idea of the risks of prolonged exposure to gamma rays.
Radioactive decay is random: you cannot tell when one particular nucleus will decay. It is spontaneous: heat, pressure or chemicals do not start it.
Units of radioactivity
Radioactivity measures the rate of decay (decays per second).
| Unit | Value |
|---|---|
| Becquerel (Bq), the S.I. unit | 1 Bq = 1 decay per second |
| Curie (Ci), the first unit | 1 Ci = 3.7 × 1010 decays per second = 3.7 × 1010 Bq |
Half-life
Half-life, T½, is the time taken for the number of undecayed nuclei (or the activity) to fall to half of its original value. Its S.I. unit is the second.
Example: remaining mass (textbook p. 235)
Pa-234 has a half-life of 5.2 hours. Original mass 80 g. Find the mass after 20.8 hours.Number of half-lives = 20.8 ÷ 5.2 = 4.
80 g → 40 g → 20 g → 10 g → 5 g. Remaining mass = 5 g.
To read a half-life from a graph, take the starting activity, halve it, then read the time on the horizontal axis. Always start from the original value at time 0.
A sample has an activity of 240 Bq. Its half-life is 10 minutes. What is its activity after 30 minutes?
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30 ÷ 10 = 3 half-lives. 240 → 120 → 60 → 30. Activity = 30 Bq.
8.2Atom and nucleus
Dalton's Atomic Theory said the atom is the smallest particle and cannot be divided. Science later found smaller particles: protons (positive) and neutrons (neutral) in the nucleus, and electrons (negative) around it. An atom is neutral when protons = electrons.
An ion is a charged particle formed when an atom loses or gains electrons. The number of protons never changes.
| Ion | How it forms | Example |
|---|---|---|
| Positive ion (cation) | Atom loses electrons | Na (11 p, 11 e) loses 1 e → Na+ (11 p, 10 e), charge +1 |
| Negative ion (anion) | Atom gains electrons | Cl (17 p, 17 e) gains 1 e → Cl− (17 p, 18 e), charge −1 |
Charge = number of protons − number of electrons. Mg with 12 p and 10 e: 12 − 10 = +2, so Mg2+.
A particle has 9 protons and 10 electrons. Is it a positive or negative ion? What is its charge?
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Negative ion, because there is one more electron than protons. Charge = 9 − 10 = −1 (fluoride ion, F−).
8.3Ionising and non-ionising radiation
Ionising radiation produces positive and negative ions when it passes through air (or matter). Non-ionising radiation does not.
| Non-ionising | Ionising |
|---|---|
| Radio waves, microwaves, infrared, visible light, very low frequency waves | Ultraviolet, X-rays, gamma rays (and alpha and beta particles) |
Alpha, beta and gamma radiation
| Alpha, α | Beta, β | Gamma, γ | |
|---|---|---|---|
| Nature | Helium nucleus | High-speed electron | Electromagnetic wave |
| Charge | Positive | Negative | Neutral |
| Ionising power | High | Moderate | Low |
| Penetration power | Low (stopped by paper) | Moderate (stopped by about 3 mm aluminium) | High (needs about 10 cm lead) |
| Electric field | Bends towards the negative plate | Bends towards the positive plate | Not deflected |
| Magnetic field | Deflected one way (upwards in the textbook figure) | Deflected the opposite way (downwards) | Not deflected (goes straight) |
Textbook Table 8.4, p. 241
The two powers go in opposite orders: the more ionising the radiation, the less it penetrates. Alpha: most ionising, least penetrating. Gamma: least ionising, most penetrating.
Sources of ionising radiation
| Natural | Man-made |
|---|---|
| Cosmic rays (high-energy radiation from outside the Solar System); background radiation | Nuclear accidents; nuclear tests; radioisotopes used in medicine; background radiation |
Background radiation and dose
Background radiation is the ionising radiation in the environment. Absorbed radiation damages body cells, so its effect is measured as a dose. 1 sievert (Sv) means 1 joule of radiation energy absorbed by 1 kg of living tissue. The dose rate is measured with a Geiger counter in microsieverts per hour (μSv/h).
Safe background dose rate: below 0.2 μSv/h (below 1.752 mSv per year).
| Source of dose | Safety measure |
|---|---|
| Background radiation (UV) | Anti-UV spectacles and umbrellas |
| Medical X-ray | Only with a doctor's prescription |
| Television | Sit at least 2 m away |
| Contaminated food | Do not eat food from contaminated areas |
| Cosmic rays | Limit the flying hours of pilots and aircrew |
| Source (textbook Figure 8.11) | Estimated dose |
|---|---|
| TV or computer | 0.01 mSv/h |
| Flight (cosmic rays) | 0.003 mSv/h |
| Environment (background radiation) | 0.4 to 1.0 mSv/year |
| Medical X-ray | 5.5 mSv per medical test |
Example: dose from television (textbook p. 246)
Dose rate from a TV = 0.01 mSv/h. A student watches 2 h a day for 5 days.Total time = 2 × 5 = 10 h. Dose = 0.01 × 10 = 0.1 mSv.
Cosmic ray dose rises with altitude because the atmosphere absorbs most of it. That is why airline crew, not ground staff, can exceed the safe level.
8.4Uses of radioactive radiation
| Field | Use |
|---|---|
| Archeology and geochronology | Carbon-14 dating: after death, C-14 (half-life 5 700 years, emits beta) is no longer replaced, so its activity shows the age of fossils and artifacts |
| Industry | Beta radiation checks the thickness of metal sheets; too much beta detected means the sheet is too thin |
| Agriculture | P-32 shows how fast plants absorb phosphate fertiliser; radiation kills beetles and sterilises pests |
| Medicine | Gamma rays from Co-60 or Cs-137 kill cancer cells; Na-24 locates blood clots; Tc-99 treats brain tumours; I-131 treats the thyroid |
| Food preservation | Gamma rays kill bacteria in food. The Radura logo labels such food |
| Defence | Nuclear weapons: radiation destroys living things and its effect lasts for generations |
Example: carbon-14 dating
A fossil has one quarter of the C-14 activity of a living organism. Quarter = 2 half-lives (1 → ½ → ¼).Age = 2 × 5 700 = 11 400 years.
Safe handling of radioactive substances and waste
- Store sources and waste in containers with thick lead walls.
- Shield substances with thick slabs of lead.
- Handle them with robotic hands, and wear protective clothing.
- Wear radiation badges to detect the dose absorbed.
- Dispose of waste safely and properly.
Lead is used because gamma rays penetrate most materials. Its disadvantage is that it is heavy and toxic, so lead clothing is tiring to wear.
Irene Joliot-Curie made the first artificial radioactive element, phosphorus-30, in 1934. Radioactive radiation brings great benefits, so we should be grateful and use it responsibly.
Summary
| Remember | Key fact |
|---|---|
| Radioactivity | Random, spontaneous decay of an unstable nucleus emitting α, β or γ |
| Units | Bq (1 decay/s) and Ci (3.7 × 1010 Bq) |
| Half-life | Time for activity or undecayed nuclei to halve |
| Ion | Lose e → positive; gain e → negative |
| Ionising power | α > β > γ; penetration γ > β > α |
| Safe dose rate | Below 0.2 μSv/h |