Radiation and Radioactivity
AdvancedUnstable nuclei emit α, β and γ rays with increasing penetration but decreasing ionizing power; decay is random and nuclei halve each half-life; background radiation is mostly natural, and protection rests on time, distance and shielding.
- Unstable nuclides spontaneously emit radiation and change into other nuclides, unaffected by temperature, pressure or chemical state
- α is a helium nucleus, β a high-speed electron, γ an electromagnetic wave; penetration α < β < γ, and ionizing power exactly the reverse
- In a magnetic field α and β deflect oppositely while γ passes straight; cloud-chamber tracks — α thick and straight, β thin and curved — identify the rays
文體小知識：科學知識說明文——比較三種放射的性質，講解隨機衰變與半衰期計算，並認識背景輻射與防護方法。
§Text Passage
1In 1896 the French scientist Becquerel discovered that uranium salts continuously emit invisible rays that fog a photographic film wrapped in black paper; the Curies later found polonium and radium and named the phenomenon radioactivity. Unstable nuclei spontaneously emit radiation and change into other nuclides. The process is entirely independent of temperature, pressure and chemical state — no physical or chemical means can hasten or hinder it, because it happens deep inside the nucleus, the result of the nuclide's own instability.
2There are three main radiations. Alpha (α) rays are helium nuclei of two protons and two neutrons, positively charged and heaviest; beta (β) rays are high-speed electrons, negatively charged and almost massless; gamma (γ) rays are very short-wavelength electromagnetic waves, uncharged and travelling at the speed of light. Penetrating power and ionizing power are inversely related: α is the least penetrating — a sheet of paper stops it — yet the most ionizing; γ is the most penetrating, needing thick lead or concrete, and the least ionizing.
3Put a source in a magnetic field and the three radiations reveal themselves: α and β deflect in opposite directions because their charges differ, while γ passes straight through undeflected. In a cloud chamber, α tracks are thick and straight, β tracks thin and tortuous, and γ leaves almost no trace. These features let us identify the radiation without touching the sample — a perennial favourite of multiple-choice and experiment questions, so memorise the three radiations as a comparison table.
4When any single nucleus will decay is completely unpredictable — radioactive decay is thoroughly random. But for large numbers of atoms a pattern emerges: in a fixed interval the number of undecayed nuclei halves. That interval is the half-life. The sample's activity — the number of decays per second, in becquerels — falls by the same pattern. Half-life can be read from a decay curve or computed by halving step by step, and it is the most important calculation skill of this topic.
5Radiation is everywhere: cosmic rays, rocks and soil (Hong Kong's granite gives off radon), our food and even our own bodies are slightly radioactive — together called background radiation; medical X-ray examinations are the largest artificial source. Detectors include the photographic film and the Geiger-Müller counter, and dose is measured in sieverts. Protection has three moves: shorten the exposure time, increase the distance, add shielding — sources are stored in lead boxes for exactly this reason, and long-handled tongs are always used to handle them.
1Summary 重點摘要
Unstable nuclei emit α, β and γ rays with increasing penetration but decreasing ionizing power; decay is random and nuclei halve each half-life; background radiation is mostly natural, and protection rests on time, distance and shielding.
2Vocabulary cards Vocabulary
The spontaneous emission of radiation by unstable nuclei as they change into other nuclides, unaffected by temperature, pressure or chemical state.
Radiation of helium nuclei (2 protons + 2 neutrons), positively charged; least penetrating, most ionizing.
Radiation of high-speed electrons, negatively charged; intermediate in both penetration and ionizing power.
Very short-wavelength electromagnetic waves, uncharged; most penetrating, stopped only by thick lead or concrete, and least ionizing.
The ability of radiation to ionize atoms: α strongest, β next, γ weakest — inversely related to penetrating power.
The process by which an unstable nuclide spontaneously emits radiation and changes into another nuclide; when any single atom decays is unpredictable.
The time for the number (or activity) of a radioactive nuclide to halve; each half-life multiplies the remainder by one half.
The number of decays per second in a sample, in becquerels (Bq); activity halves each half-life just like the nucleus count.
The ever-present natural and artificial radiation in the environment, mainly from cosmic rays, rocks and soil, food and medical examinations.
An instrument that detects radiation as a count rate, commonly used to measure a sample's activity and background levels.
3Step by step Steps
- Step 1: Where radioactivity comes fromUnstable nuclei spontaneously emit radiation and transform into other nuclides — radioactivity. It is a nuclear process: completely independent of temperature, pressure and chemical state; heating, compressing or burning cannot change the decay rate. For example, a uranium ore deep underground and the same nuclide refrigerated in a laboratory have exactly the same half-life. Historically, Becquerel stumbled on the phenomenon through fogged photographic plates, and the Curies went on to discover polonium and radium; the discovery opened the door to the nucleus and heralded the nuclear age.
- Step 2: Alpha rays — heavy, strongly ionizingAlpha rays are helium nuclei: two protons plus two neutrons, positively charged, the most massive and slowest of the three radiations. Their large mass and charge make them collide frequently with atoms — the strongest ionizing power — but their energy is spent quickly, so they are the least penetrating: a sheet of paper or a few centimetres of air stops them. For example, a smoke detector holds a weak α source: normally the alpha particles keep the air slightly conducting; smoke blocks them, the current changes and the alarm sounds. For exam answers, write 'α is a helium nucleus, least penetrating, most ionizing' — three points for the marks.
- Step 3: Beta and gamma — penetration climbingBeta rays are high-speed electrons, negatively charged and almost massless; penetration is moderate — a few millimetres of aluminium stops them — and so is their ionizing power. Gamma rays are very short-wavelength electromagnetic waves, uncharged and travelling at the speed of light; they are the most penetrating, needing several centimetres of lead or thick concrete to be strongly attenuated, and are the least ionizing. For example, factories use a β source to gauge paper thickness: thicker paper absorbs more, the count rate falls and rollers adjust automatically; hospitals use cobalt-60 machines to send γ rays deep into the body to kill cancer cells. The comparison table of the three radiations is essential memorisation.
- Step 4: Magnetic fields and cloud chambers — identifying raysCharged particles deflect in a magnetic field: α and β, with opposite charges, bend in opposite directions, and β — far lighter — deflects more; γ is uncharged and passes straight through. In a cloud chamber, radiation ionizes the gas along its path and supersaturated vapour condenses into visible tracks: α tracks are thick and straight (strongly ionizing, easily stopped), β tracks thin and wandering, γ leaves almost nothing. For example, an exam question may show three tracks in a field: the straight centre line is γ, and the two splitting sideways are α and β — identify them from charge and mass to judge the deflections.
- Step 5: Random decay and half-lifeWhen any single nucleus will decay is unpredictable — decay is thoroughly random — yet the statistical pattern over many atoms is rock-steady: each half-life, the number of undecayed nuclei halves, and so does the activity. Half-life is a fingerprint of the nuclide, from seconds to billions of years. For example, carbon-14 has a half-life of about 5730 years; after death an organism stops exchanging carbon, the carbon-14 fraction in its remains declines, and archaeologists date artefacts from it — radiocarbon dating. Standard method: first count the half-lives n = total time ÷ half-life, remaining fraction = (12)ⁿ; halving step by step is the safest route.
- Step 6: Detection and activityTwo instruments dominate detection. The photographic film records accumulated dose by fogging — the principle of the film badge worn by radiation workers. The Geiger-Müller counter converts each ray into an electrical pulse and reads a count rate in counts per second. For example, point a GM counter at a sample and read 500 counts per second; remove the sample and the meter still shows 20 — that 20 is background radiation, which must be subtracted before quoting the sample's activity. Cloud chambers and spark counters appear occasionally in experiment questions; the key is understanding what each instrument actually measures.
- Step 7: Background radiation and safetyBackground radiation comes mainly from natural sources: cosmic rays, radioactive materials in rocks and soil (radon levels are higher over Hong Kong's granite), food and air, even potassium-40 inside our bodies; among artificial sources, medical X-rays and nuclear medicine dominate, while nuclear tests and accidents contribute only a tiny fraction. Dose is measured in sieverts; excessive exposure damages cells. The three principles of protection: shorten exposure time, increase distance, add shielding. For example, radiographers work behind lead walls, sources are sealed in lead boxes and handled with long tongs — all applications of the three principles. Long questions often ask you to list background sources and protective measures: answer in points.
4Diagrams Diagrams
5Worked example Example
Example: a radioactive sample has a half-life of 8 days and an initial activity of 800 Bq (800 decays per second). Find its activity after 24 days, stating the number of half-lives used.
- Number of half-lives: n = 24 ÷ 8 = 3
- Halve the activity for each half-life: 800 → 400 → 200 → 100 Bq
- In formula form: activity = 800 × (12)³ = 800 ÷ 8 = 100 Bq
- Check: after 3 half-lives the remaining fraction is 18, i.e. 12.5% of the original activity
- Answer: 24 days = 3 half-lives, so the activity = 800 × (12)³ = 100 Bq. Decay is a random process and half-life is a statistical law over many atoms; count the half-lives first, then halve step by step, and state the unit Bq.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the natures of α, β and γ radiations?
Q2Which radiation is the most penetrating, and what stops it?
Q3A sample has a half-life of 5 hours. What fraction remains after 15 hours?
Q4A sample has an initial activity of 640 Bq and a half-life of 2 days. What is its activity after 6 days?
Q5Why is radioactive decay said to be random, and does this contradict the half-life?
8Recap Recap
- Unstable nuclides spontaneously emit radiation and change into other nuclides, unaffected by temperature, pressure or chemical state
- α is a helium nucleus, β a high-speed electron, γ an electromagnetic wave; penetration α < β < γ, and ionizing power exactly the reverse
- In a magnetic field α and β deflect oppositely while γ passes straight; cloud-chamber tracks — α thick and straight, β thin and curved — identify the rays
- Half-life is the time for the nucleus count and activity to halve; count n half-lives first, remaining fraction = (12)ⁿ
- Background radiation comes mainly from cosmic rays, rocks and soil, and food; protection: shorten time, increase distance, add shielding