📚 Radioactive Decay in IGCSE Physics: Key Exam Points Simplified | IGCSE 物理：放射性衰变 考点精讲
Radioactive decay is a random and spontaneous process in which an unstable atomic nucleus loses energy by emitting radiation. This topic is a cornerstone of the IGCSE Physics syllabus, requiring you to understand the nature of alpha, beta and gamma radiation, decay equations, half-life calculations, and practical applications as well as safety precautions. This revision guide breaks down every key point clearly and concisely, pairing each English explanation with its Chinese equivalent to maximise your learning efficiency.
放射性衰变是一种随机且自发的过程，不稳定的原子核通过释放辐射来失去能量。这个话题是 IGCSE 物理课程的核心内容之一，要求你理解 α、β 和 γ 辐射的本质、衰变方程、半衰期计算，以及实际应用和安全防护。这篇复习指南将每一个关键知识点清晰简洁地拆解，并配以中英对照讲解，帮助你高效掌握。
1. The Atom and Its Nucleus | 原子与原子核
All matter is made of atoms. Each atom contains a tiny central nucleus surrounded by electrons. The nucleus consists of positively charged protons and uncharged neutrons, collectively called nucleons. Almost all the mass of the atom is concentrated in the nucleus, yet its size is only about 1/10 000 of the atom’s radius. Electrons orbit the nucleus at relatively large distances.
所有物质都由原子组成。每个原子包含一个微小的中心原子核以及绕核运动的电子。原子核由带正电的质子和不带电的中子组成，统称为核子。原子的几乎全部质量都集中在原子核中，但原子核的大小仅约为原子半径的 1/10 000。电子在相对较远的距离上绕核运动。
The number of protons in the nucleus is the atomic number (Z), which defines the element. The total number of protons and neutrons is the mass number (A). A nuclide is often written as ²³⁸₉₂U, where 92 is the atomic number and 238 is the mass number. In exam questions, you must be able to identify these numbers and relate them to the nuclear model.
原子核内的质子数称为原子序数（Z），它决定了元素的种类。质子与中子的总和为质量数（A）。一个核素常写作 ²³⁸₉₂U，其中 92 为原子序数，238 为质量数。在考题中，你必须能够识别这些数字，并能将其与核模型联系起来。
2. Isotopes and Radioactivity | 同位素与放射性
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. For example, carbon-12 (¹²₆C) and carbon-14 (¹⁴₆C) are isotopes. While chemical properties are almost identical, some isotopes are unstable because the nucleus has an excess of energy or an imbalance of protons and neutrons. Such unstable isotopes are called radioisotopes and they undergo radioactive decay.
同位素是具有相同质子数但中子数不同的同种原子。例如，碳-12（¹²₆C）和碳-14（¹⁴₆C）就是同位素。虽然化学性质几乎相同，但有些同位素不稳定，因为原子核具有过剩的能量，或者质子与中子的比例失衡。这类不稳定的同位素称为放射性同位素，它们会发生放射性衰变。
Radioactive decay is entirely independent of temperature, pressure or chemical bonding. It is a random process at the level of a single nucleus, meaning we cannot predict exactly when a particular nucleus will decay, but we can describe the average behaviour of a large number of nuclei using half-life.
放射性衰变完全不受温度、压强或化学键的影响。对于单个原子核而言，这是一个随机过程，意味着我们无法确切预测某个特定原子核何时会衰变，但我们可以用半衰期来描述大量原子核的平均行为。
3. Types of Nuclear Radiation | 核辐射的种类
There are three main types of radiation emitted by radioactive sources: alpha (α) particles, beta (β) particles and gamma (γ) rays. Each has a distinct nature, charge and penetrating power. In IGCSE Physics, you are expected to know the composition, symbol, relative charge and approximate speed of each.
放射性源会释放三种主要的辐射：α粒子、β粒子和γ射线。每一种辐射都有不同的本质、电荷和穿透能力。在 IGCSE 物理中，你需要了解它们的组成、符号、相对电荷以及大概的速度。
- Alpha particle (α): A helium nucleus, consisting of 2 protons and 2 neutrons. Symbol: ⁴₂He or α. Charge: +2. Speed: up to about 10% of the speed of light.
- β粒子 (α): 氦原子核，由2个质子和2个中子组成。符号：⁴₂He 或 α。电荷：+2。速度：最高可达光速的约10%。
- Beta particle (β⁻): A fast-moving electron emitted from the nucleus when a neutron turns into a proton. Symbol: ⁰₋₁e or β⁻. Charge: −1. Speed: up to about 90% of the speed of light.
- β粒子 (β⁻): 原子核内一个中子转变为质子时释放的快电子。符号：⁰₋₁e 或 β⁻。电荷：−1。速度：最高可达光速的约90%。
- Gamma ray (γ): and energy, with no mass and no charge. Symbol: γ. Speed: speed of light (3.0 × 10⁸ m/s).
- γ射线 (γ): 频率和能量极高的电磁波，没有质量，也不带电荷。符号：γ。速度：光速（3.0 × 10⁸ m/s）。
4. Properties of Alpha (α) Particles | α 粒子的性质
An alpha particle is relatively massive and highly charged. Because of its large mass and double positive charge, it causes strong ionisation when it passes through a material, knocking electrons out of atoms. Consequently, an α particle loses energy quickly and has a very short range in air – only a few centimetres. It can be stopped by a thin sheet of paper or the outer layer of human skin.
α粒子质量较大、带电量高。由于质量大且带双正电荷，它穿过物质时会引起强烈的电离，将原子中的电子撞出。因此，α粒子会迅速损失能量，在空气中的射程极短——仅几厘米。它可以被一张薄纸或人体皮肤表层阻挡。
In IGCSE exams, you must link the high ionising power with the short range and low penetrating ability. Additionally, because alpha sources emit only a narrow beam of particles, they are easily deflected by electric and magnetic fields. The deflection is slight due to the relatively large mass, and the direction can be predicted using Fleming’s left-hand rule for positive charge.
在 IGCSE 考试中，你需要将高电离本领与短射程和低穿透能力联系起来。此外，由于 α 源只发射出狭窄的粒子束，它们在电场和磁场中容易发生偏转。由于质量相对较大，偏转程度较小，且可利用左手定则根据正电荷判断偏转方向。
5. Properties of Beta (β) Particles | β 粒子的性质
Beta particles are fast-moving electrons. They have a mass about 1/2000 of an alpha particle and carry a single negative charge. Their ionising ability is moderate – they produce fewer ion pairs per unit length in air than α particles. However, because they are much lighter and faster, β particles penetrate further: they can travel up to about 1 metre in air and are stopped by a few millimetres of aluminium.
β粒子是高速运动的电子。其质量约为α粒子的1/2000，且带一个负电荷。它们的电离能力中等——在空气中每单位长度产生的离子对少于α粒子。然而，由于它们轻得多也快得多，β粒子的穿透距离更远：在空气中可以行进约1米，而几毫米厚的铝片就可以将其阻挡。
A crucial point for the exam is that beta emission occurs when a neutron decays into a proton, increasing the atomic number by 1 while the mass number stays the same. β particles are deflected much more strongly than α particles in electric and magnetic fields, and in the opposite direction because of the negative charge.
考试中的一个关键点是，β衰变发生于中子衰变为质子时，原子序数增加1，而质量数保持不变。在电场和磁场中，β粒子的偏转程度比α粒子大得多，而且由于带负电，偏转方向相反。
6. Properties of Gamma (γ) Rays | γ 射线的性质
Gamma radiation is not a particle but a form of electromagnetic wave, similar to X-rays but with higher energy. It has no mass and no charge, and therefore produces very weak direct ionisation. Gamma rays lose energy slowly as they pass through materials, making them highly penetrating. They can travel several metres in air and require several centimetres of lead or thick concrete to be significantly reduced.
γ辐射不是粒子，而是一种电磁波，类似于X射线但能量更高。它没有质量，也不带电荷，因此产生的直接电离非常微弱。γ射线在穿过物质时能量损失缓慢，这使它们具有极强的穿透性。它们在空气中可穿行数米，需要数厘米厚的铅或厚混凝土才能显著减弱。
Because γ rays carry no charge, they are not deflected by electric or magnetic fields. In the context of nuclear decay, gamma emission often accompanies alpha or beta decay when the daughter nucleus is left in an excited state. The excess energy is released as a γ photon, and there is no change in mass number or atomic number.
由于γ射线不带电荷，它们在电场和磁场中不会发生偏转。在核衰变过程中，当子核处于激发态时，γ发射常伴随α或β衰变发生。多余的能量以γ光子的形式释放，这时质量数和原子序数都不变。
7. Penetrating Power and Ionising Ability | 穿透能力与电离能力
There is an inverse relationship between penetrating power and ionising ability. Alpha particles ionise most strongly but are the least penetrating. Gamma rays are the most penetrating but cause the least direct ionisation. Beta particles lie between the two extremes. In the GCSE examination, you may be asked to compare these properties or to state appropriate absorbers for each type of radiation.
穿透能力与电离能力之间存在反比关系。α粒子电离最强，但穿透能力最弱。γ射线穿透能力最强，但直接电离能力最弱。β粒子介于两者之间。在 GCSE 考试中，你可能需要比较这些性质，或说明每种辐射的合适阻挡材料。
| Radiation 辐射 | Range in air 空气中射程 | Absorber 阻挡物 | Ionising power 电离本领 |
| Alpha (α) | 3–5 cm | Paper, skin | Very high 非常高 |
| Beta (β) | ~1 m | 3–5 mm aluminium | Medium 中等 |
| Gamma (γ) | Several metres 数米 | Several cm lead, thick concrete 数厘米铅/厚混凝土 | Very low 非常低 |
Remember that even though gamma rays have the lowest ionising ability, they are still extremely dangerous because they penetrate the body deeply and can damage internal organs and DNA. The ionisation caused by α and β particles inside the body is even more hazardous if a source is ingested or inhaled.
请记住，虽然γ射线电离能力最低，但它们依然非常危险，因为能深入穿透人体并损伤内脏器官与 DNA。如果放射源被摄入或吸入体内，α和β粒子在体内产生的电离危害则更为严重。
8. Nuclear Decay Equations | 核衰变方程
A nuclear decay equation represents the transformation of a parent nucleus into a daughter nucleus plus emitted radiation. The total mass number (A) and the total atomic number (Z) must be conserved before and after the decay.
核衰变方程表示母体核转变为子体核并放出辐射的过程。衰变前后的总质量数（A）与总原子序数（Z）必须守恒。
Alpha decay example 示例：
²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He
Radium-226 decays to radon-222 by emitting an alpha particle. Notice that the mass number decreases by 4 and the atomic number decreases by 2.
镭-226 通过释放一个α粒子衰变为氡-222。注意，质量数减少4，原子序数减少2。
Beta decay example 示例：
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Carbon-14 decays to nitrogen-14 via beta emission. The mass number remains 14, while the atomic number increases by 1 because a neutron has changed into a proton.
碳-14 通过β辐射衰变为氮-14。质量数保持14，而原子序数增加1，因为一个中子转变为了一个质子。
art of the equation when the nucleus loses excess energy, e.g. ⁶⁰₂₇Co → ⁶⁰₂₈Ni + ⁰₋₁e + γ.
γ发射不改变A或Z；当核失去多余能量时，常在方程中加入γ表示，例如 ⁶⁰₂₇Co → ⁶⁰₂₈Ni + ⁰₋₁e + γ。
9. Half-Life Concept and Calculations | 半衰期的概念与计算
Half-life (t½) is defined as the time taken for half the unstable nuclei in a sample to decay, or equivalently for the activity of the sample to fall to half its initial value. It is a measure of the rate of decay. The half-life for a given isotope is constant and unaffected by external conditions.
半衰期（t½）定义为样品中一半的不稳定原子核发生衰变所需的时间，或者等价地说，样品的放射性活度降至初始值一半的时间。它是衰变速率的量度。某一同位素的半衰期是一个常数，不受外界条件影响。
s. If a sample has initial activity A₀, after n half-lives the activity becomes A₀ × (½)ⁿ.
常见的考试任务包括：根据衰变曲线确定半衰期；计算经过几个半衰期后原始样品剩余的比例；求出进行特定数量衰变所需的时间。如果样品的初始活度为 A₀，经过 n 个半衰期后，活度变为 A₀ × (½)ⁿ。
ion. The mass of the sample changes very little because the majority of atoms are still present – only a fraction have decayed.
例如：碘-131 的半衰期为8天。如果一个样品最初含有2亿个放射性原子，经过24天（三个半衰期）后，剩余的数量为 200 × (½)³ = 2500万。样品的质量变化很小，因为大多数原子依然存在——只有一部分发生了衰变。
10. Background Radiation and Sources | 背景辐射与来源
Background radiation is the low-level radiation that is constantly present in our environment. It originates from both natural and artificial sources. Key natural sources include cosmic rays from space, radon gas from rocks and soil, and naturally occurring radioisotopes in food and the human body. Artificial sources include medical X-rays, nuclear power plants (under normal operation) and fallout from nuclear weapons testing.
背景辐射是环境中始终存在的低水平辐射。它既有自然来源，也有人工来源。主要的自然来源包括来自太空的宇宙射线、来自岩石和土壤的氡气，以及食物和人体内天然存在的放射性同位素。人工来源包括医用X射线、正常运行下的核电站以及核武器试验的沉降物。
In the UK, around 85% of the average annual radiation dose comes from natural sources, with radon being the largest single contributor. When discussing background radiation, it is important to correct experimental readings by subtracting the background count from the measured count.
在英国，平均年辐射剂量的大约85%来自自然来源，其中氡气是最大的单一贡献者。在讨论背景辐射时，重要的是要通过从测量计数中减去本底计数来对实验读数进行修正。
11. Detecting Radiation | 探测辐射
Radiation is detected using instruments that respond to ionisation. The Geiger–Müller (GM) tube is the most common detector in IGCSE experiments. When radiation enters the tube, it ionises the gas inside, producing a pulse of current that is counted and often displayed as a count rate (counts per minute or counts per second). A cloud chamber reveals visible tracks of α and β particles, while a photographic film badge can monitor cumulative exposure.
辐射是通过对电离有响应的仪器来探测的。盖革-米勒（GM）管是 IGCSE 实验中最常用的探测器。当辐射进入管内，它使管内的气体电离，产生一个电流脉冲，该脉冲被计数并通常显示为计数率（每分钟计数或每秒计数）。云室可以显示出α和β粒子的可见径迹，而照相胶片剂量计可以监测累积照射量。
Typical school experiments involve measuring the count rate from a source at different distances or through different absorbers, always taking care to subtract the background count. The random nature of decay can be demonstrated by observing fluctuations in the count rate over repeated short intervals.
典型的学校实验包括在不同距离或通过不同吸收材料测量放射源的计数率，务必扣减本底计数。通过观测在重复短时间间隔内计数率的涨落，可以演示衰变的随机本质。
12. Uses and Hazards of Radiation | 辐射的应用与危害
Radioactive materials are used widely in medicine, industry and research. Medical uses include radiotherapy for killing cancer cells (using focused gamma rays), diagnostic tracers (for example, iodine-131 for thyroid imaging), and sterilisation of medical equipment with gamma radiation. In industry, beta sources can measure the thickness of paper or aluminium foil, while gamma radiography inspects welds and metal castings.
放射性材料在医学、工业和研究领域有着广泛的应用。医学用途包括用聚焦的γ射线杀死癌细胞的放射治疗，诊断示踪剂（例如用碘-131做甲状腺成像），以及用γ辐射对医疗设备进行灭菌。在工业中，β源可用于测量纸张或铝箔的厚度，而γ射线照相术则能检查焊缝和金属铸件。
Hazards arise from the ability of radiation to ionise atoms. Acute exposure can cause radiation burns and sickness, while long-term low-level exposure increases the risk of cancer due to DNA damage. Safe handling practices always involve minimising time of exposure, maximising distance from the source (using tongs) and using appropriate shielding (lead aprons, lead glass screens). You must also know that radioactive sources should never be touched with bare hands and must be stored in sealed, labelled lead-lined containers.
辐射的危害源于它能使原子电离。急性照射可引起辐射灼伤和放射病，而长期低水平照射因DNA损伤会增加患癌风险。安全操作的原则始终包括：尽量减少暴露时间、尽可能加大与源的距离（使用镊子），并使用适当的屏蔽（铅围裙、铅玻璃屏）。你还须知道，绝不能徒手接触放射源，并应将其存放在密封、贴有标签的铅衬容器中。
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