📚 Radioactive Decay Revision for IGCSE WJEC Physics | IGCSE WJEC 物理：放射性衰变 考点精讲
Radioactive decay is a core topic in the IGCSE WJEC Physics syllabus, covering the nature of nuclear radiation, the properties of alpha, beta and gamma emissions, half-life calculations and real‑world applications. This article provides a bilingual, point‑by‑point breakdown of the essential knowledge, helping you master the concepts and prepare effectively for your examination.
放射性衰变是 IGCSE WJEC 物理大纲中的核心课题，内容涵盖核辐射的本质、α、β 和 γ 发射的性质、半衰期计算以及实际应用。本文以中英对照、要点逐一解析的方式梳理必考知识，帮助大家扎实掌握概念，为考试做好充分准备。
1. What is Radioactive Decay? | 什么是放射性衰变？
Radioactive decay is the spontaneous and random process by which an unstable atomic nucleus loses energy by emitting radiation. This process changes the nucleus into a more stable form, often a different element.
放射性衰变是原子核自发且随机地通过发射辐射来失去能量的过程。该过程使原子核转变为更稳定的形态，往往是另一种元素。
The decay is unaffected by external conditions such as temperature, pressure or chemical bonding. It is a purely nuclear event, and the rate depends only on the identity of the isotope.
衰变不受温度、压强或化学键等外部条件影响。它是纯粹的核事件，速率只取决于同位素本身。
In WJEC exams, you must be able to recognise that the process is both ‘spontaneous’ (it happens by itself) and ‘random’ (we cannot predict when a particular nucleus will decay).
在 WJEC 考试中，必须认识到该过程既是“自发的”（自行发生），又是“随机的”（无法预测某个原子核具体何时衰变）。
2. Alpha, Beta and Gamma Radiation | α、β 和 γ 辐射
There are three main types of nuclear radiation: alpha (α) particles, beta (β) particles and gamma (γ) rays. Each is emitted from the nucleus but has a different structure, charge and behaviour.
核辐射主要有三种类型：α 粒子、β 粒子和 γ 射线。它们均从原子核发出，但在结构、电荷及行为上各不相同。
An alpha particle is identical to a helium nucleus, composed of two protons and two neutrons. It carries a charge of +2 and is represented as ⁴₂He. A beta particle is a fast‑moving electron ejected from the nucleus when a neutron transforms into a proton. It has a charge of −1 and is written as ⁰₋₁e. Gamma radiation is high‑energy electromagnetic radiation with no mass and no charge.
α 粒子与氦核相同，由两个质子和两个中子组成，带 +2 电荷，记作 ⁴₂He。β 粒子是原子核内一个中子转变为质子时释放出的高速电子，带 −1 电荷，记作 ⁰₋₁e。γ 辐射是高频电磁波，无质量也不带电荷。
You should be able to identify each type by its symbol and recall that alpha and beta are deflected by electric and magnetic fields, while gamma continues straight.
应能根据符号辨认每种辐射，并记住 α 和 β 在电场和磁场中会发生偏转，而 γ 沿直线传播。
3. Properties and Penetration | 性质与穿透力
The penetrating power of each radiation is one of the most frequently examined properties. Alpha particles are the least penetrating: they can be stopped by a sheet of paper or a few centimetres of air.
每种辐射的穿透能力是最常考查的性质。α 粒子穿透力最弱：一张纸或几厘米厚的空气即可阻挡。
Beta particles can pass through paper but are absorbed by a few millimetres of aluminium. Gamma rays are the most penetrating; they require several centimetres of lead or metres of concrete to reduce their intensity significantly.
β 粒子能穿透纸张，但会被几毫米厚的铝片吸收。γ 射线穿透力最强，需要数厘米厚的铅板或数米厚的混凝土才能显著削弱其强度。
In the context of safety, this property determines the choice of shielding. For WJEC, you may be asked to match the radiation type with its absorber based on experimental diagrams.
在安全防护中，穿透力决定了屏蔽材料的选择。WJEC 考题可能会要求根据实验示意图将辐射类型与其阻挡材料匹配。
4. Ionising Power | 电离能力
Ionising ability refers to the capability of radiation to knock electrons out of atoms, creating ions. Alpha particles have the highest ionising power because of their large mass and double positive charge, which makes them strongly interact with matter.
电离能力指辐射从原子中击出电子、产生离子的能力。α 粒子由于质量大且带双正电荷，能与物质发生强烈作用，因而电离能力最强。
Beta particles are moderately ionising, while gamma rays have the lowest ionising power. This inverse relationship between ionising ability and penetrating power is a classic pattern: the more easily radiation creates ions, the faster it loses energy and gets absorbed.
β 粒子电离能力中等，γ 射线最弱。电离能力与穿透力之间成反比是一个经典模式：越容易产生离子的辐射，能量损失得越快，也就越容易被吸收。
Examination questions often ask you to explain why alpha is strongly ionising but poorly penetrating. The answer links large mass, charge, frequent collisions and rapid energy loss.
考题常要求解释为何 α 电离能力强而穿透力弱。答案应联系到质量大、带电、频繁碰撞以及能量快速损失。
5. Nuclear Decay Equations | 核衰变方程
Nuclear equations use atomic numbers (Z) and mass numbers (A) to show how a nucleus changes during decay. The sum of mass numbers and the sum of atomic numbers must be conserved on both sides of the equation.
核方程利用原子序数（Z）和质量数（A）来表示衰变过程中原子核如何变化。方程两边的质量数之和以及原子序数之和必须守恒。
Alpha decay: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He
α 衰变：²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He
Beta decay: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e
β 衰变：¹⁴₆C → ¹⁴₇N + ⁰₋₁e
In alpha decay, the mass number decreases by 4 and the atomic number decreases by 2. In beta decay, the mass number remains unchanged while the atomic number increases by 1. Gamma emission usually accompanies either alpha or beta decay and does not alter the mass or atomic numbers.
α 衰变中，质量数减少 4，原子序数减少 2。β 衰变中，质量数不变，原子序数增加 1。γ 辐射通常伴随 α 或 β 衰变，不改变质量数与原子序数。
You may have to complete or balance nuclear equations in the exam. Practise identifying the unknown product by using conservation rules.
考试中可能要补全或配平核方程。练习利用守恒规则推断未知产物。
6. Half-life | 半衰期
The half‑life of a radioactive isotope is the time taken for half the nuclei in a sample to decay, or equivalently for the activity (count rate) to fall to half its initial value. It is a constant for a given isotope.
放射性同位素的半衰期是指样本中一半的原子核发生衰变，或活度（计数率）下降到初始值一半所需的时间。对于给定的同位素，它是常数。
WCEC candidates must be confident in using the half‑life equation and performing simple calculations. Typical questions provide initial activity and ask for the activity after a certain number of half‑lives. For instance, after n half‑lives, the remaining fraction is (½)ⁿ.
WJEC 考生必须熟练运用半衰期公式并完成简单计算。典型题目给出初始活度，要求计算经过若干半衰期后的活度。例如，经过 n 个半衰期后，剩余比例为 (½)ⁿ。
- Initial activity 800 Bq, half‑life 5 hours. After 3 half‑lives (15 hours), activity = 800 × (½)³ = 100 Bq.
- 初始活度 800 Bq，半衰期 5 小时。3 个半衰期（15 小时）后，活度 = 800 × (½)³ = 100 Bq。
Be careful with units: half‑life can be given in seconds, minutes, hours or years. Always check the time interval matches the half‑life unit.
注意单位：半衰期可能以秒、分钟、小时或年给出。务必检查时间间隔是否与半衰期单位一致。
7. Random Nature and Decay Curves | 随机性与衰变曲线
Radioactive decay is a random process, which means it is impossible to predict exactly which nucleus will decay next. However, for a large number of nuclei, the overall decay pattern is predictable and follows an exponential decrease.
放射性衰变是随机过程，这意味着无法准确预测下一个衰变的是哪个原子核。但对大量原子核而言，整体衰变规律是可预测的，并遵循指数衰减规律。
A graph of activity or number of undecayed nuclei against time produces a characteristic curve that decreases by half over each half‑life. In WJEC papers, you may need to read half‑life values from such a graph or sketch the curve.
活度或未衰变原子核数目随时间变化的曲线具有特征形状：每经过一个半衰期，数量减半。WJEC 试卷中可能要求从图中读取半衰期值或绘制曲线。
Students often confuse the random nature of individual decays with the predictable overall curve. Remember: the decay curve is an average outcome for many nuclei, while the timing of a single decay remains unpredictable.
学生常将单个衰变的随机性与整体可预测的曲线混淆。请记住：衰变曲线是大量原子核的平均结果，单个衰变时刻依然不可预测。
8. Background Radiation | 背景辐射
Background radiation is the low‑level radiation that surrounds us all the time. Its sources include cosmic rays from space, radioactive rocks in the ground (such as granite containing uranium), radon gas, and even certain foods (like bananas containing potassium‑40).
背景辐射是指时刻存在于我们周围的低水平辐射。其来源包括宇宙射线、地下岩石（如含铀的花岗岩）、氡气，甚至某些食物（如含钾‑40 的香蕉）。
When conducting an experiment to measure the count rate from a radioactive source, you must always subtract the background count to get the corrected count rate. The standard procedure is to measure the background count with the source absent, then subtract this value from all subsequent readings.
在测量放射源计数率的实验中，必须扣除背景计数以得到修正后的计数率。标准做法是：在没有放射源的情况下测量背景计数，再从后续所有读数中减去该值。
WJEC may ask about the significance of background radiation in practical work and how to minimise its effect, for example by using lead shielding or taking longer counting times.
WJEC 可能考查背景辐射在实验中的意义，以及如何减小其影响，例如采用铅屏蔽或延长计数时间。
9. Uses of Radioisotopes | 放射性同位素的应用
Radioactive isotopes have many practical uses across medicine, industry and archaeology, primarily because they can be detected easily and their half‑lives can be matched to the required task.
放射性同位素在医学、工业和考古学中有广泛的实际用途，主要因其易于检测，且半衰期可与任务需求匹配。
Medical tracers: Technetium‑⁹⁹m emits gamma rays and has a half‑life of 6 hours, making it ideal for imaging organs without causing excessive damage. Iodine‑¹³¹ is used to investigate thyroid function.
医学示踪剂： 锝‑⁹⁹m 发射 γ 射线，半衰期 6 小时，非常适合器官显像又不会造成过度损伤。碘‑¹³¹ 用于检查甲状腺功能。
Industrial gauging: Beta emitters like strontium‑⁹⁰ are used to monitor the thickness of paper or metal foil. The amount of radiation passing through indicates thickness changes.
工业测厚： 锶‑⁹⁰ 等 β 发射体用于监测纸张或金属箔的厚度。穿过的辐射量变化指示厚度差异。
Carbon dating: Carbon‑¹⁴ (half‑life 5730 years) is used to estimate the age of once‑living materials. The ratio of ¹⁴C to ¹²C decreases over time, allowing dating up to about 50 000 years.
碳定年法： 碳‑¹⁴（半衰期 5730 年）用于估算曾存活体的年龄。¹⁴C 与 ¹²C 的比值随时间下降，可测定距今约 5 万年的样品。
For WJEC, you should be ready to suggest a suitable isotope for a given scenario, justifying your choice based on radiation type and half‑life.
在 WJEC 考试中，应准备为给定场景推荐合适的同位素，并根据辐射类型和半衰期说明理由。
10. Safety and Handling | 安全与处理
Because ionising radiation can damage living cells and DNA, strict safety precautions must be followed when handling radioactive sources. The three key principles are time, distance and shielding.
由于电离辐射会损伤活细胞和 DNA，处理放射源时必须严格遵循安全措施。三大原则是：时间、距离和屏蔽。
Minimise exposure time: experiments should be planned so that handling is as brief as possible. Increase distance: using tongs or remote handling tools keeps the source away from the body, as intensity follows the inverse‑square law. Use appropriate shielding: lead for gamma, thick aluminium for beta, and even a simple container for alpha emitters.
尽量缩短暴露时间：实验应提前规划，操作越快越好。增大距离：使用镊子或遥控工具使放射源远离身体，因为辐射强度遵循平方反比定律。采用合适的屏蔽：γ 用铅板，β 用厚铝板，α 放射源甚至用简单的容器即可屏蔽。
Never point a source at anyone and always store sources in labelled lead‑lined containers when not in use. WJEC practical questions often test your understanding of safe practice.
切勿将放射源指向任何人，不使用时必须将其存放在标有识的铅衬容器内。WJEC 实验题常测试对安全操作的理解。
11. WJEC Exam Focus | WJEC 考试聚焦
WJEC examination papers regularly feature questions that combine factual recall with data analysis. Typical tasks include: identifying radiation type from penetration tests, completing decay equations, calculating half‑life from graphs or tabulated data, and explaining uses and safety.
WJEC 试卷经常出现将知识记忆与数据分析相结合的题目。典型任务包括：根据穿透实验辨别辐射类型、补全衰变方程、从图表中计算半衰期，以及解释用途和安全措施。
Command words such as ‘state’, ‘describe’, ‘explain’ and ‘calculate’ demand different levels of response. ‘Explain’ questions usually require linking a property (e.g. high ionisation of alpha) to a consequence or application. Always refer back to the data if provided.
指令词如“陈述”“描述”“解释”“计算”要求不同层次的作答。“解释”题通常需要将某一性质（如 α 的高电离能力）与结果或应用联系起来。若题目提供了数据，一定要结合数据分析。
Calculations must show working steps clearly. Even if the final answer is wrong, marks are awarded for correct equations and substitution. Pay close attention to units and significant figures.
计算题必须清晰展示计算步骤。即使最终答案错误，正确的方程和代入过程也能得分。注意单位与有效数字。
12. Key Points Summary | 关键要点总结
Radioactive decay is spontaneous and random, emitting α, β or γ radiation. Alpha is highly ionising but weakly penetrating; gamma is the opposite. Nuclear equations must balance mass and atomic numbers. Half‑life is the time for activity to halve, used in carbon dating, medical imaging and industry. Background radiation must be subtracted in experiments, and safety relies on reducing time, increasing distance and using shielding.
放射性衰变是自发且随机的，可放出 α、β 或 γ 辐射。α 电离能力强而穿透力弱，γ 则相反。核方程必须满足质量数与原子序数守恒。半衰期是活度减半所需的时间，应用于碳定年、医学成像和工业。实验需扣除背景辐射，安全依靠缩短时间、增加距离和使用屏蔽。
Mastering these fundamentals will give you confidence in tackling any WJEC question on radioactive decay. Regular practice with past papers and careful review of the definitions will secure your success.
掌握这些基础知识将使你在应对所有 WJEC 放射性衰变题目时充满信心。通过反复练习历年真题、仔细复习定义，定能成功应对考试。
Published by TutorHao | Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导