📚 Radioactive Decay for IGCSE OCR Physics | 放射性衰变 IGCSE OCR 考点精讲
Radioactive decay is a fundamental concept in IGCSE OCR Physics. It describes the spontaneous breakdown of unstable atomic nuclei, emitting radiation in the process. This revision guide covers everything you need to know: the types of radiation, decay equations, half-life, background radiation, and practical applications. Mastering these topics is essential not only for your exams but also for understanding how radioactivity is used safely in medicine, industry, and energy production.
放射性衰变是 IGCSE OCR 物理中的基础概念，它描述了不稳定的原子核自发分裂并释放辐射的过程。这篇考点精讲涵盖了你需要掌握的全部内容：辐射的类型、衰变方程、半衰期、背景辐射以及实际应用。掌握这些知识不仅对考试至关重要，也有助于理解如何安全地将放射性应用于医学、工业和能源领域。
1. What is Radioactivity? | 什么是放射性？
Radioactivity is the process by which an unstable atomic nucleus loses energy by emitting radiation. This process is spontaneous and random – it is impossible to predict exactly when a particular nucleus will decay. However, for a large number of nuclei, the average rate of decay can be measured and is characteristic of each isotope. The radiation emitted can be alpha (α), beta (β) or gamma (γ), and they have very different properties.
放射性是指不稳定的原子核通过释放辐射而失去能量的过程。这个过程是自发且随机的——无法精确预测某个特定的原子核何时衰变。但对于大量原子核而言，平均衰变速率是可以测量的，并且是每种同位素的特征。释放出的辐射可以是 α、β 或 γ，它们具有截然不同的性质。
2. Atomic Structure and Isotopes | 原子结构与同位素
All atoms consist of a central nucleus containing protons and neutrons, surrounded by electrons in energy levels. The atomic number (Z) is the number of protons, which defines the element. The mass number (A) is the total number of protons and neutrons. Isotopes are atoms of the same element (same Z) but with different numbers of neutrons (different A). Some isotopes are stable, while others are unstable and radioactive, such as carbon-14 (¹⁴₆C) used in dating or uranium-238 (²³⁸₉₂U) used in nuclear fuel.
所有原子都由一个包含质子和中子的中心原子核以及围绕原子核在能级上运动的电子组成。原子序数（Z）是质子的数目，决定了元素的种类。质量数（A）是质子与中子数的总和。同位素是指质子数相同（Z 相同）但中子数不同（A 不同）的同种元素原子。有些同位素是稳定的，而另一些则不稳定且具有放射性，例如用于测年的碳-14（¹⁴₆C）或用作核燃料的铀-238（²³⁸₉₂U）。
3. Types of Radioactive Decay | 放射性衰变的类型
There are three main types of radiation emitted during radioactive decay: alpha (α) particles, beta (β) particles, and gamma (γ) rays. They differ in their nature, their ability to ionise matter, and their penetrating power. Alpha radiation consists of helium nuclei, beta radiation consists of high-speed electrons (or positrons), and gamma radiation is a form of electromagnetic wave with very short wavelength. In an electric or magnetic field, alpha and beta particles are deflected in opposite directions, while gamma rays remain unaffected.
放射性衰变过程中主要释放三种辐射：α 粒子、β 粒子和 γ 射线。它们在本质、电离能力和穿透能力上都不同。α 辐射由氦原子核组成，β 辐射由高速电子（或正电子）组成，而 γ 辐射是一种波长极短的电磁波。在电场或磁场中，α 和 β 粒子会朝相反的方向偏转，γ 射线则不受影响。
4. Alpha Decay: Properties and Equations | α衰变：性质与方程式
An alpha particle is identical to a helium nucleus ⁴₂He, composed of two protons and two neutrons. When an alpha particle is emitted, the parent nucleus loses 2 protons and 2 neutrons, so its atomic number decreases by 2 and its mass number decreases by 4. Alpha particles have a high ionising ability because of their +2 charge and relatively large mass, but they can travel only a few centimetres in air and can be stopped by a sheet of paper or human skin. A typical alpha decay equation is:
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
α 粒子与氦原子核 ⁴₂He 完全相同，由两个质子和两个中子组成。当释放出一个 α 粒子时，母核失去 2 个质子和 2 个中子，因此其原子序数减少 2，质量数减少 4。α 粒子由于带有 +2 电荷且质量较大，具有很高的电离能力，但它们在空气中的射程仅为几厘米，可以被一张纸或人的皮肤挡住。一个典型的 α 衰变方程是：
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
5. Beta Decay: Properties and Equations | β衰变：性质与方程式
Beta-minus (β⁻) decay occurs when a neutron in the nucleus turns into a proton and emits a high-speed electron and an antineutrino (ν̅ₑ). The mass number remains the same, but the atomic number increases by 1. Beta particles have a much smaller mass than alpha particles and carry a single negative charge, so they have a lower ionising ability but can travel further (up to about one metre in air) and are stopped by a few millimetres of aluminium. The decay of carbon-14 is a classic example:
¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̅ₑ
A less common type is beta-plus (β⁺) decay, in which a proton turns into a neutron and a positron (⁰₊₁e) is emitted. This is used in positron emission tomography (PET) scans.
β⁻ 衰变发生在原子核内的一个中子转变为一个质子时，同时释放出一个高速电子和一个反中微子（ν̅ₑ）。质量数保持不变，但原子序数增加 1。β 粒子的质量远小于 α 粒子，带一个负电荷，因此电离能力较低，但穿透距离更远（空气中可达约一米），可以被几毫米厚的铝板挡住。碳-14 的衰变是一个典型例子：
¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̅ₑ
还有一种不太常见的类型是 β⁺ 衰变，此时一个质子转变为中子，并释放出正电子（⁰₊₁e），这种方法被用于正电子发射断层扫描（PET）。
6. Gamma Radiation: A Wave, Not a Particle | γ辐射：波而非粒子
Gamma radiation is an electromagnetic wave with extremely high frequency and energy. It has no mass and no charge, so it does not change the atomic or mass number of the nucleus. Gamma rays are often emitted after an alpha or beta decay, when the daughter nucleus is left in an excited state. Because gamma rays are uncharged, they have very low ionising power but are extremely penetrating; thick lead or several metres of concrete are needed to reduce their intensity significantly.
γ 辐射是一种频率和能量极高的电磁波。它没有质量和电荷，因此不会改变原子核的原子序数或质量数。γ 射线通常在 α 或 β 衰变后产生，此时子核处于激发态。由于 γ 射线不带电，其电离能力非常低，但穿透力极强；需要厚铅板或数米厚的混凝土才能显著减弱其强度。
7. Penetrating Power and Ionising Ability | 穿透力与电离能力
The three types of radiation can be compared by their ability to penetrate materials and to ionise atoms. The table below summarises these properties, which are frequently examined in IGCSE OCR Physics.
可以通过辐射穿透材料以及电离原子的能力来比较这三种辐射。下表总结了这些性质，这些内容在 IGCSE OCR 物理考试中经常出现。
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus (⁴₂He) | Electron/positron (⁰₋₁e or ⁰₊₁e) | Electromagnetic wave |
| Charge | +2 | -1 (or +1) | 0 |
| Ionising power | Very high | Medium | Very low |
| Penetrating power | Stopped by paper or skin (few cm in air) | Stopped by ~3 mm aluminium | Reduced by thick lead or concrete |
| Deflection in electric/magnetic field | Towards negative plate (slight deflection) | Towards positive plate (large deflection) | No deflection |
Remember: alpha particles are the most dangerous when inhaled or ingested because of their high ionising power inside the body, even though they cannot penetrate skin. Gamma rays pose an external hazard due to their far reach.
记住：α 粒子虽然在体外无法穿透皮肤，但一旦被吸入或摄入，因其在体内的强电离能力而最危险。γ 射线由于传播距离远，会形成外部辐射危害。
8. Half-Life: Definition and Calculations | 半衰期：定义与计算
The half-life of a radioactive isotope is the time taken for half of the unstable nuclei in a sample to decay. It is a fixed property for a given isotope and is not affected by temperature, pressure or chemical bonding. The activity of a sample (measured in becquerels, Bq) also halves in each half-life. Half-life is used to determine how long a radioactive source will remain hazardous, and it is central to practical applications such as carbon dating.
放射性同位素的半衰期是指样品中一半的不稳定原子核发生衰变所需的时间。它是每种同位素的固定属性，不受温度、压力或化学键的影响。样品的活度（以贝克勒尔 Bq 为单位）在每个半衰期后也会减半。半衰期可用于判断一个放射源的危险持续时间，并在碳定年等实际应用中处于核心地位。
A typical graph of activity against time shows an exponential decay curve. You may be asked to read the half-life from a graph or calculate the remaining mass after a number of half-lives. For example, if you start with 80 g of a substance with a half-life of 10 days, after 30 days (three half-lives) you will have 80 ÷ 2 = 40 g (after 10 d), 40 ÷ 2 = 20 g (after 20 d), 20 ÷ 2 = 10 g (after 30 d). Always pay attention to units and show your working.
一张典型的活度-时间图呈现指数衰减曲线。考试中可能会要求你从图中读出半衰期，或计算经过若干个半衰期后剩余的质量。例如，初始质量为 80 克的某种物质，半衰期为 10 天，那么经过 30 天（三个半衰期）后，剩余质量为：80 ÷ 2 = 40 g（10 天后），40 ÷ 2 = 20 g（20 天后），20 ÷ 2 = 10 g（30 天后）。一定要留意单位并写出计算步骤。
9. Background Radiation | 背景辐射
We are constantly exposed to background radiation from natural and artificial sources. Natural sources include cosmic rays from space, radioactive rocks (e.g., granite containing uranium), radon gas seeping from the ground, and tiny amounts of radioactive isotopes in our food and water. Artificial sources include medical X-rays, nuclear weapons testing, and nuclear power stations (very small contribution). The level of background radiation varies from place to place and must be subtracted when measuring the count rate of a radioactive source in the lab.
我们一直暴露在来自天然和人工源的背景辐射中。天然源包括来自太空的宇宙射线、含有放射性岩石（如含铀的花岗岩）、从地表渗出的氡气，以及食物和水中微量的放射性同位素。人工源包括医学 X 光、核武器试验和核电站（贡献极小）。背景辐射的水平因地而异，在实验室测量放射源的计数率时必须将其扣除。
10. Uses of Radioactive Isotopes | 放射性同位素的应用
Radioactive isotopes have many important uses, all relying on choosing the right type of radiation and a suitable half-life.
- Medical tracers: Technetium-99m emits gamma rays and has a half-life of about 6 hours, ideal for imaging internal organs without causing excessive dose. Gamma rays are detected outside the body.
医用示踪剂：锝-99m 发射 γ 射线，半衰期约 6 小时，非常适合体内器官成像，不会造成过量辐射。γ 射线可以在体外被探测到。 - Radiotherapy: High-energy gamma rays from cobalt-60 are focused to kill cancer cells.
放射治疗：来自钴-60 的高能 γ 射线被聚焦以杀死癌细胞。 - Industrial thickness monitoring: A beta source is used to measure the thickness of paper or metal foil; if the count rate drops, the material is too thick. Alpha would not penetrate, and gamma would pass right through without much change.
工业厚度监测：用 β 源测量纸张或金属箔的厚度；若计数率下降，说明材料太厚。α 无法穿透，而 γ 会直接穿过且变化不大。 - Carbon-14 dating: Living things contain a constant ratio of ¹⁴C to ¹²C. After death, the ¹⁴C decays with a half-life of 5730 years; measuring the remaining amount reveals the age of archaeological objects.
碳-14 定年：活体生物中含有恒定比例的 ¹⁴C 与 ¹²C。死亡后，¹⁴C 以 5730 年的半衰期衰变；测量其剩余量可以揭示考古物的年代。 - Smoke detectors: A tiny alpha source (e.g., americium-241) ionises air between two plates; smoke particles disrupt the current, triggering the alarm.
烟雾探测器：微小的 α 源（如镅-241）使两板之间的空气电离；烟雾颗粒干扰电流，从而触发警报。
11. Safety and Handling Precautions | 安全与防护措施
Because ionising radiation can damage living cells and DNA, strict safety precautions must be followed when handling radioactive materials. The three key principles are time, distance and shielding. Reduce the time of exposure, increase the distance from the source (intensity follows the inverse square law), and use appropriate shielding: thick lead for gamma, aluminium for beta, and sealed containers for alpha emitters. Always use tongs or remote handling equipment and store sources in a locked lead-lined box. Never point a source towards anyone and wear a radiation badge to monitor exposure.
由于电离辐射会损害活细胞和 DNA，处理放射性物质时必须遵循严格的安全预防措施。三个关键原则是时间、距离和屏蔽。缩短暴露时间，增大与源的距离（强度遵循平方反比定律），并使用适当的屏蔽：γ 用厚铅板，β 用铝板，α 发射体用密封容器。务必使用长柄钳或远程操作设备，并将放射源存放在带锁的铅衬里箱中。绝不要将源指向任何人，并佩戴辐射剂量计以监测暴露。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
In IGCSE OCR Physics exams, students often lose marks by confusing the properties of the three radiations. Remember: only gamma is an EM wave, only alpha has a +2 charge, and beta particles are much lighter and faster. When writing decay equations, always balance both mass number and atomic number. For half-life questions, check whether the question asks for mass or number of nuclei, and always show your method step by step. Do not forget to subtract background count from your measurements. Finally, always explain danger in terms of ionisation, not just ‘it is harmful’ – state that ionising radiation can cause cell mutation and cancer.
在 IGCSE OCR 物理考试中，学生常因混淆三种辐射的性质而失分。记住：只有 γ 是电磁波，只有 α 带 +2 电荷，β 粒子则更轻更快。书写衰变方程时，一定要配平质量数和原子序数。对于半衰期问题，看清题目问的是质量还是原子核数量，并始终逐步写出计算过程。不要忘记从测量值中扣除背景计数。最后，解释危害时要联系电离作用，而不要只是说“有害”——要指出电离辐射可能导致细胞突变和癌症。
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