📚 Radioactive Decay Exam Points for IGCSE Edexcel Physics | 放射性衰变考点精讲
Radioactive decay is a key topic in the IGCSE Edexcel Physics syllabus, exploring how unstable nuclei become stable by emitting radiation. Mastering the types of decay, half-life calculations, detector principles and real-world applications will set you up for high marks on exam questions.
放射性衰变是 IGCSE Edexcel 物理考纲核心内容，研究不稳定原子核如何通过释放辐射变得稳定。掌握衰变类型、半衰期计算、探测器原理和实际应用，能帮助你在考试中拿到高分。
1. Atomic Structure and Isotopes | 原子结构与同位素
Atoms consist of a nucleus containing protons and neutrons, with electrons orbiting in energy levels. The number of protons (atomic number, Z) defines the element, while the total of protons and neutrons gives the mass number (A).
原子由包含质子和中子的原子核与分层排布的电子组成。质子数（原子序数 Z）决定元素种类，质子数与中子数之和为质量数（A）。
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Many isotopes are stable, but some are unstable (radioisotopes) because the neutron-to-proton ratio falls outside the zone of stability.
同位素是指质子数相同而中子数不同的同一种元素的原子。许多同位素是稳定的，但当中子-质子比超出稳定区域时，原子核就会不稳定（放射性同位素）。
In unstable nuclei, the imbalance leads to spontaneous decay, releasing energy and particles in order to move toward a more stable configuration.
不稳定原子核的内部失衡会导致自发衰变，通过释放能量和粒子趋向更稳定的结构。
2. Types of Radioactive Decay: Alpha, Beta, Gamma | 放射性衰变类型：α、β、γ
Alpha (α) decay: an alpha particle, which is identical to a helium nucleus (⁴₂He), is ejected from a heavy nucleus. The mother nucleus loses 2 protons and 2 neutrons, so its mass number drops by 4 and its atomic number drops by 2.
α 衰变：从重核中射出一个 α 粒子，等同于氦原子核(⁴₂He)。母核减少 2 个质子和 2 个中子，质量数减 4，原子序数减 2。
Beta (β⁻) decay: a neutron inside the nucleus transforms into a proton, emitting a fast-moving electron (β⁻ particle, represented as ⁰₋₁e) and an antineutrino. The proton stays in the nucleus, so the atomic number increases by 1 while the mass number remains unchanged.
β⁻ 衰变：核内一个中子转变为质子，并发射出一个高速电子（β⁻ 粒子，记作 ⁰₋₁e）和一个反中微子。质子留在核内，因此原子序数增加 1，质量数不变。
Gamma (γ) decay: gamma radiation is a form of high-energy electromagnetic wave (⁰₀γ) released from an excited nucleus, often after alpha or beta decay. It carries away excess energy without changing the atomic or mass number.
γ 衰变：γ 射线是一种高能电磁波(⁰₀γ)，通常在 α 或 β 衰变后从激发态核中释放。它只带走多余能量，不改变原子序数或质量数。
3. Penetrating Power and Ionising Ability | 穿透力与电离能力
Alpha particles have the highest ionising ability because they carry a +2 charge and move relatively slowly, strongly pulling electrons off nearby atoms. Their penetrating power is very low – they are stopped by a few centimetres of air or a sheet of paper.
α 粒子的电离能力最强，因其带 +2 电荷且运动较慢，能强力剥离邻近原子的电子。它的穿透力很弱，几厘米空气或一张纸就能阻挡。
Beta particles are moderately ionising; they are lighter and faster than alpha particles. They can travel about a metre in air and are absorbed by a few millimetres of aluminium.
β 粒子的电离能力中等，比 α 粒子轻且快。它们在空气中可穿行约 1 米，几毫米厚的铝即可将其吸收。
Gamma rays have very low ionising ability but extremely high penetrating power. They are electromagnetic waves that can only be significantly reduced by many centimetres of lead or metres of concrete.
γ 射线的电离能力很弱，但穿透力极强。它们属于电磁波，只能被数厘米厚的铅或数米厚的混凝土明显衰减。
4. Nuclear Equations for Decay | 衰变方程
Nuclear equations show the rearrangement of nucleons. The total mass number and total atomic number must be conserved on both sides of the arrow.
核反应方程式体现了核子的重排。箭号两侧的总质量数和总原子序数必须守恒。
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
The equation above represents the alpha decay of uranium-238 into thorium-234. Notice the sum of mass numbers (238 = 234 + 4) and atomic numbers (92 = 90 + 2) are equal.
上式表示铀-238 经 α 衰变生成钍-234。注意质量数之和 (238 = 234 + 4) 与原子序数之和 (92 = 90 + 2) 均相等。
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Carbon-14 undergoes beta decay to become nitrogen-14. A neutron in the carbon nucleus changes into a proton, so the atomic number increases by 1, while the mass number stays 14.
碳-14 发生 β 衰变转变为氮-14。碳核中一个中子变为质子，原子序数增加 1，质量数保持 14。
When gamma emission accompanies a decay, it is often written by adding ⁰₀γ to the products. No change occurs to the mass or atomic numbers.
若衰变伴随 γ 射线，通常在产物一侧添加 ⁰₀γ，此时质量数和原子序数均保持不变。
5. Half-life: Definition and Calculations | 半衰期：定义与计算
Half-life (T₁/₂) is the time taken for half of the radioactive nuclei in a sample to decay, or equivalently for the activity (or count rate) to fall by half. It is constant for a given isotope and unaffected by physical conditions.
半衰期 (T₁/₂) 是指样品中一半放射性核发生衰变所需的时间，或者说活度（或计数率）减半的时间。对特定同位素它是个常量，不受物理条件影响。
N = N₀ × (1/2)^(t/T₁/₂)
To solve problems, you can use the equation N = N₀ × (1/2)^(t/T₁/₂), where N₀ is the initial number of undecayed nuclei (or activity), N is the number remaining after time t. The same relationship holds for count rate corrected for background.
解题时可使用公式 N = N₀ × (1/2)^(t/T₁/₂)，其中 N₀ 为初始未衰变核数（或活度），N 为时间 t 后剩余的核数。经背景值修正的计数率也满足该关系。
For example, if a sample starts with an activity of 800 Bq and has a half-life of 2 hours, after 6 hours (three half-lives) the activity will be 800→400→200→100 Bq. Plotting a decay curve shows the characteristic exponential shape.
例如，某样品活度初始为 800 Bq，半衰期为 2 小时，则 6 小时（3 个半衰期）后活度依次为 800→400→200→100 Bq。画出衰变曲线可得到典型的指数形状。
6. Background Radiation | 背景辐射
Background radiation is the low-level radiation that is always present from natural and artificial sources. Natural sources include cosmic rays, radioactive rocks such as granite, radon gas from the ground, and naturally occurring radioisotopes in food.
背景辐射是指始终存在的低水平辐射，来自天然和人工源。天然源包括宇宙射线、花岗岩等放射性岩石、地下释放的氡气，以及食物中天然存在的放射性同位素。
Artificial sources include medical X-rays, fallout from nuclear weapons testing, and small releases from nuclear power stations. When measuring the count rate from a radioactive source, you must subtract the background count rate to obtain the corrected count rate.
人工源包括医疗 X 射线、核武器试验沉降物以及核电站的微量排放。测量放射源的计数率时，必须扣除背景计数率得到修正值。
7. Detecting Radiation | 辐射探测
A Geiger-Müller (GM) tube connected to a scaler or rate meter is the most common detector in school labs. Radiation enters the tube and ionises the gas inside, producing an electrical pulse that is counted.
学校实验室最常见的探测器是盖革-米勒计数管，连接定标器或计数率计。射线进入管内电离气体，产生电脉冲并计数。
A cloud chamber reveals tracks: alpha particles leave short, thick, straight trails; beta particles leave thinner, wispy, often curved trails; gamma rays are invisible but may produce secondary electrons.
云室能显示径迹：α 粒子留下短、粗、直的轨迹；β 粒子轨迹较细、模糊且常弯曲；γ 射线不可见，但可能产生次级电子轨迹。
Photographic film darkens when exposed to radiation; it is used in film badges worn by workers to monitor cumulative dose. The degree of darkening indicates the amount of exposure.
照相胶片受辐照会变黑，用于工作人员佩戴的胶片剂量计来监测累积剂量。变黑程度反映受照量。
8. Uses of Radioactivity | 放射性的应用
Medical tracers: a radioisotope with a short half-life, such as technetium-99m (which emits gamma rays), is injected into the body. The gamma rays can be detected externally to track blood flow or organ function without surgery.
医用示踪剂：半衰期短的放射性同位素（如发射 γ 射线的锝-99m）注入体内，在体外探测 γ 射线即可追踪血流或器官功能，无需手术。
Radiotherapy: gamma rays from cobalt-60 are focused on cancerous tumours to destroy malignant cells while minimising harm to surrounding healthy tissue.
放射治疗：将钴-60 的 γ 射线聚焦于癌变肿瘤，破坏恶性细胞，同时尽量减少对周边健康组织的损伤。
Industrial thickness control: a beta source is placed on one side of paper or metal foil and a detector on the other; changes in count rate indicate variations in thickness. Smoke alarms use a weak alpha source (americium-241) to ionise air. Smoke particles disrupt the ionisation current, triggering the alarm.
工业测厚：在纸张或金属箔一侧放置 β 源，另一侧放置探测器；计数率的变化反映厚度偏差。烟雾报警器利用弱 α 源（镅-241）电离空气，烟雾颗粒破坏电离电流从而触发警报。
Carbon-14 dating: living organisms maintain a constant ratio of carbon-14 to carbon-12. After death, the carbon-14 decays with a half-life of about 5730 years; measuring the remaining ¹⁴C gives an estimate of the sample’s age.
碳-14 测年：活有机体的碳-14 与碳-12 比值恒定，死后碳-14 以约 5730 年半衰期衰变，测定剩余 ¹⁴C 可估算样品年代。
9. Hazards and Safety Precautions | 危害与安全措施
Ionising radiation can damage DNA and kill cells, causing radiation sickness, genetic mutations, or cancer. High doses are particularly dangerous. Alpha sources are especially hazardous if ingested or inhaled because their strong ionisation occurs inside the body.
电离辐射会损伤 DNA 并杀死细胞，引起辐射病、基因突变或癌症。高剂量尤其危险。α 源若被吸入或食入尤其危险，因为其强电离效应会在体内发生。
The three fundamental protective measures are: Time – limit exposure duration; Distance – increase distance from the source (intensity follows an inverse-square law for gamma); Shielding – use appropriate absorbers (e.g. lead for gamma, aluminium for beta).
三项基本防护措施是：时间——限制受照时长；距离——远离放射源（γ 射线强度遵循平方反比律）；屏蔽——使用适当吸收材料（如铅屏蔽 γ，铝屏蔽 β）。
When handling sources in a laboratory, use long-handled tongs, never point a source at anyone, label containers clearly, and store sources in lead-lined boxes when not in use.
实验室操作放射源时，应使用长柄钳，切勿将源指向他人，清晰标记容器，不用时存放在铅衬盒内。
10. Random Nature and Decay Series | 随机性与衰变系列
Radioactive decay is a random process. It is impossible to predict exactly which nucleus will decay at a particular moment, but with a large number of nuclei the statistical pattern emerges as a constant half-life.
放射性衰变是随机过程。无法准确预测哪个核在某一时刻衰变，但对大量核而言，统计规律呈现为恒定的半衰期。
Some heavy nuclei undergo a decay series – a sequence of alpha and beta decays that eventually lead to a stable isotope of lead. Uranium-238, for example, decays through many steps to lead-206. Each step has its own characteristic half-life.
某些重核经历衰变系列——一连串 α 和 β 衰变最终达到稳定的铅同位素。例如铀-238 经过多个步骤衰变为铅-206，每一步有其特有的半衰期。
In exam questions, you may be asked to explain the random nature or interpret graphs showing decay. Remember: although individual decays are random, the overall trend is predictable and the half-life can be determined from the graph.
考试中可能要求解释随机性或解读衰变曲线图。需记住：单个衰变虽然随机，整体趋势却可预测，且可从图中求出半衰期。
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