Radioactivity
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先生の方へ： Radioactivity（Physics、CIE）向けのすぐ使えるレッスンスライド, 復習ノート, 図解 — レッスンで使うか、生徒がライブゲームとして遊ぶインタラクティブなクラス活動としてトピックを実施できます。
レッスンノート
Background Radiation
- Background radiation is the radiation that exists around us all the time.
- Natural sources: radon gas (in air), rocks and buildings, food and drink, cosmic rays.
- Man-made sources: medical X-rays, nuclear waste, nuclear fallout, nuclear accidents.
- The count rate (decays per second) is measured using a Geiger-Müller tube.
- To correct for background radiation, measure count rate with no source and subtract from readings with source.
Types of Radiation
- α (α): helium nucleus (2 protons, 2 neutrons), charge +2, range a few cm in air, stopped by paper, highly ionising.
- β (β): high-speed electron, charge -1, range tens of cm in air, stopped by a few mm of aluminium, moderately ionising.
- Gamma (γ): electromagnetic wave, no charge, infinite range in air, reduced by thick lead, weakly ionising.
- Radioactive decay is spontaneous and random – cannot predict which nucleus will decay next.
Penetrating power of alpha, beta, and gamma radiation
Ionising Power & Deflection
- Ionisation is the removal of an electron from an atom, creating an ion.
- α is most ionising (charge +2, large mass); β is moderate (charge -1); gamma is least (no charge).
- In an electric field: α deflects towards negative plate, β towards positive plate, γ undeflected.
- In a magnetic field: α and β deflect in opposite directions (due to opposite charges); γ undeflected.
- β deflects more than α because β has much smaller mass.
Ionisation of an atom by radiation
Radioactive Decay
- α decay: nucleus emits α-particle (₂⁴He). Mass number decreases by 4, atomic number decreases by 2.
- β decay: neutron changes into proton and electron; electron (β⁻) is emitted. Atomic number increases by 1, mass number unchanged.
- Gamma decay: nucleus emits γ-ray; no change in mass or atomic number, only energy reduces.
- Decay equations must balance mass and atomic numbers on both sides.
- Example: ₈₄²¹²Po → ₈₂²⁰⁸Pb + ₂⁴α
Beta decay process showing a neutron transforming into a proton and emitting an electron
Half-Life
- Half-life is the time taken for half the nuclei in a sample to decay.
- It is constant for a given isotope and unaffected by conditions.
- Half-life can be found from an activity–time graph: read time for activity to halve.
- Correct for background radiation before calculating half-life.
- After n half-lives, fraction remaining = (½)ⁿ.
A half-life graph showing activity falling from A0 to A0/2 to A0/4
Uses of Radiation
- α: smoke detectors – α ionises air, smoke absorbs α, triggers alarm.
- β: measuring thickness of thin materials (e.g., paper, aluminium foil) – absorption varies with thickness.
- Gamma: sterilising medical equipment (penetrating, kills microbes), irradiating food, radiotherapy for cancer.
- Tracers: short half-life gamma emitters used in PET scans to diagnose cancer.
How a smoke detector works using alpha radiation
Dangers of Radiation
- Ionising radiation can damage DNA, causing cell death, mutations, or cancer.
- High doses cause skin burns and reduce white blood cells (weakened immune system).
- Safe handling: store in lead-lined boxes, use tongs, minimise time, maximise distance.
- Shielding (lead, concrete, water) absorbs radiation; radiologists stand behind barriers.
- Radioactive waste with long half-life is buried deep underground.
Ionising radiation damaging DNA strands and causing a mutation
スライド
練習問題
無料プレビュー — 54問中8問。すべて見るには登録を。
1.Which of the following is a natural source of background radiation?
Easy- ARadon gas
- BMedical X-rays
- CNuclear waste
- DNuclear fallout
2.What is the main reason radioactive sources are stored in lead-lined containers?
Easy- ATo reduce the amount of radiation emitted, protecting people nearby
- BTo increase the rate of radioactive decay
- CTo prevent the source from getting cold
- DTo stop the source from moving
3.Which type of radiation is the most ionising?
Easy- Aα particles
- Bβ particles
- CGamma rays
- DNeutrons
4.Which statement about radioactive decay is correct?
Medium- AIt is a random process and cannot be predicted when a particular nucleus will decay
- BThe rate of decay increases if the substance is heated
- CPlacing a radioactive substance in a lead box prevents it from decaying
- DThe decay always produces poisonous gases
5.A radioactive source has a half-life of 0.5 hours. A detector positioned near the source reads 6000 counts per second. Background radiation can be ignored. What will the detector read 1.5 hours later?
Medium- A750 counts per second
- B1500 counts per second
- C2000 counts per second
- D3000 counts per second
6.A student measures 210 counts in 3 minutes from a radioactive source. The background count rate is 20 counts per minute. What is the corrected count rate for the source?
Medium- A50 counts per minute
- B70 counts per minute
- C190 counts per minute
- D270 counts per minute
7.A nucleus of radium-226 (⁸⁸²²⁶Ra) undergoes α decay. What are the nucleon number and proton number of the new nucleus formed?
Medium- ANucleon number 222, proton number 86
- BNucleon number 222, proton number 88
- CNucleon number 224, proton number 87
- DNucleon number 230, proton number 90
8.Which type of radiation is most suitable for measuring the thickness of thin aluminium foil in a manufacturing process?
Medium- Aβ particles
- Bα particles
- CGamma rays
- DNeutrons
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