Nuclear Physics: IGCSE Physics Key Points | IGCSE 物理：核物理考点精讲

📚 Nuclear Physics: IGCSE Physics Key Points | IGCSE 物理：核物理考点精讲

Nuclear physics explores the structure of the atomic nucleus, the particles it contains and the energy released during nuclear reactions. In IGCSE Physics, you need to understand the composition of the nucleus, the notation used to describe nuclides, the nature of isotopes, the different types of radioactive decay, how to write decay equations, the concept of half-life, methods of detecting radiation, practical applications, associated hazards and safety measures, as well as the principles of nuclear fission and fusion. This article summarises all essential points in a bilingual format to help you master the topic.

核物理探讨原子核的结构、其所含的粒子以及核反应释放的能量。在 IGCSE 物理中，你需要理解原子核的组成、核素符号、同位素的性质、各类放射性衰变、衰变方程的书写、半衰期的概念、辐射探测方法、实际应用、相关的危害与安全措施，以及核裂变与核聚变的基本原理。本文用中英双语的形式总结所有重要考点，帮助你掌握这一主题。

1. Atomic Structure and Nucleons | 原子结构与核子

All atoms consist of a tiny central nucleus surrounded by electrons. The nucleus contains two types of nucleons: protons, which are positively charged, and neutrons, which have no charge. Electrons orbit the nucleus in energy levels and carry a negative charge equal in magnitude to the proton’s positive charge. The number of protons defines the element and is called the atomic number (Z). The total number of nucleons (protons + neutrons) is the mass number (A).

所有原子都由一个微小的中心原子核和绕核运动的电子组成。原子核包含两类核子：带正电的质子和不带电的中子。电子在能级上绕核运动，所带负电荷的大小与质子的正电荷相等。质子的数量决定了元素的种类，称为原子序数（Z）。核子总数（质子数 + 中子数）称为质量数（A）。

2. Nuclide Notation | 核素符号

A nuclide is a specific atomic species characterised by its proton number and neutron number. The standard notation places the mass number as a left superscript and the atomic number as a left subscript before the element symbol, e.g., ²³⁸₉₂U. For a uranium nucleus with 92 protons and 146 neutrons, the mass number is 238 and the atomic number is 92. The number of neutrons is calculated as A – Z.

核素是指由质子数和中子数确定的特定原子种类。标准符号将质量数放在元素符号左上角，原子序数放在左下角，例如 ²³⁸₉₂U。对于含有 92 个质子和 146 个中子的铀核，质量数为 238，原子序数为 92。中子数等于 A – Z。

3. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They share identical chemical properties because their electron configurations are the same, yet they differ in nuclear stability and mass. For example, carbon-12 (¹²₆C) has 6 protons and 6 neutrons, while carbon-14 (¹⁴₆C) has 6 protons and 8 neutrons. Some isotopes are stable, whereas others are radioactive and decay over time.

同位素是同一元素中质子数相同而中子数不同的原子。由于它们的电子排布相同，化学性质完全一致，但核稳定性和质量不同。例如，碳-12（¹²₆C）有 6 个质子和 6 个中子，而碳-14（¹⁴₆C）有 6 个质子和 8 个中子。有些同位素是稳定的，另一些则具有放射性，会随时间发生衰变。

4. Radioactive Decay | 放射性衰变

Radioactive decay is the spontaneous process by which an unstable nucleus loses energy by emitting radiation. There are three main types of radiation: alpha (α), beta (β) and gamma (γ). In alpha decay, the nucleus emits a helium nucleus (⁴₂He). In beta-minus decay, a neutron transforms into a proton, emitting an electron (⁰₋₁e) and an antineutrino. Gamma decay involves the emission of high-energy electromagnetic waves from an excited nucleus, often after alpha or beta decay.

放射性衰变是不稳定核自发地通过发射辐射而失去能量的过程。辐射主要有三种类型：α、β 和 γ 射线。在 α 衰变中，原子核放出一个氦核（⁴₂He）；在 β⁻ 衰变中，一个中子转变为质子，释放出一个电子（⁰₋₁e）和一个反中微子；γ 衰变则包含激发态原子核发射高能电磁波，通常伴随在 α 或 β 衰变之后。

5. Alpha, Beta, Gamma Properties | α、β、γ 辐射的特性

The three types of ionising radiation differ markedly in their ionising ability, penetrating power and behaviour in electric and magnetic fields. The table below summarises these properties.

这三种电离辐射在电离能力、穿透能力以及电场和磁场中的表现上有明显区别。下表对这些特性进行了总结。

Property 特性	Alpha (α) α 射线	Beta (β) β 射线	Gamma (γ) γ 射线
Nature 本质	Helium nucleus ⁴₂He 氦原子核 ⁴₂He	Fast electron ⁰₋₁e 高速电子 ⁰₋₁e	Electromagnetic wave 电磁波
Ionising ability 电离能力	Very high 很高	Medium 中等	Low 很低
Penetrating power 穿透能力	Stopped by paper or skin 被纸或皮肤挡住	Stopped by a few mm of aluminium 被几毫米铝板挡住	Reduced by several cm of lead or thick concrete 被数厘米铅板或厚混凝土减弱
Deflection in electric field 电场中偏转	Slightly deflected towards negative plate 略微向负极板偏转	Strongly deflected towards positive plate 强烈向正极板偏转	No deflection 不偏转

Because alpha particles are large and doubly charged, they ionise strongly but travel only a few centimetres in air. Beta particles are lighter and faster, so they penetrate further. Gamma rays are uncharged and highly penetrating, requiring dense materials for shielding.

因为 α 粒子质量大且带两个正电荷，所以电离作用强，但在空气中射程只有几厘米。β 粒子更轻、更快，因而穿透能力更强。γ 射线不带电且穿透力极强，需要高密度材料进行屏蔽。

6. Radioactive Decay Equations | 放射性衰变方程

In nuclear equations, both mass number and atomic number are conserved. For alpha decay: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. The mass number drops by 4 and the atomic number drops by 2. For beta-minus decay, a neutron changes into a proton, so the mass number stays the same while the atomic number increases by 1: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e. Gamma emission does not alter either number and is often written as a separate photon symbol ⁰₀γ accompanying another decay.

在核反应方程中，质量数和原子序数均守恒。α 衰变：²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He，质量数减少 4，原子序数减少 2。β⁻ 衰变时，一个中子转变为质子，因此质量数不变，原子序数增加 1：¹⁴₆C → ¹⁴₇N + ⁰₋₁e。γ 辐射不会改变质量数或原子序数，常常写成伴随其他衰变的光子符号 ⁰₀γ。

7. 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. It is a constant for a given isotope and cannot be altered by temperature, pressure or chemical changes. The activity of a sample halves over each half-life. If the initial count rate is 800 counts per second and the half-life is 2 hours, after 2 hours the count rate will be 400 s⁻¹, after 4 hours 200 s⁻¹, and so on. The relationship can be expressed as:

放射性同位素的半衰期是指样品中一半的原子核发生衰变所需的时间。对于给定的同位素，半衰期是一个常量，不受温度、压强或化学变化的影响。每经过一个半衰期，样品的计数率减半。如果起始计数率为 800 次/秒，半衰期为 2 小时，则 2 小时后计数率为 400 s⁻¹，4 小时后为 200 s⁻¹，以此类推。这一关系可以用下式表示：

N = N₀ × (½)^(t / T₁/₂)

where N is the remaining quantity, N₀ the original quantity, t the elapsed time and T₁/₂ the half-life. Graphs of activity against time show an exponential decay curve, from which half-life can be determined by reading the time for the count rate to halve.

其中 N 是剩余量，N₀ 是初始量，t 是经过的时间，T₁/₂ 是半衰期。计数率–时间图像呈指数衰减曲线，从图上可以找到计数率减半所对应的时间，从而求出半衰期。

8. Detecting Radiation | 探测辐射

Ionising radiation is invisible but can be detected using devices that respond to ionisation. The Geiger-Müller (GM) tube produces an audible click or electrical pulse each time radiation enters the tube and ionises the gas inside. Photographic film darkens when exposed to radiation and is used in film badges worn by radiation workers. Cloud chambers make the tracks of alpha and beta particles visible as tiny trails of condensation. Scintillation counters and semiconductor detectors are also used in more advanced settings.

电离辐射是看不见的，但可以利用对电离有响应的设备进行探测。盖革–米勒（GM）管在每次有辐射进入并电离管内气体时，会发出咔嗒声或电脉冲。照相胶片受辐射照射后会变黑，常用于辐射工作人员佩戴的胶片徽章。云室能够使 α 和 β 粒子的径迹以微小冷凝尾迹的形态显现出来。在更专业的场景中还使用闪烁计数器和半导体探测器。

9. Uses of Radiation | 辐射的应用

Radioactive isotopes have many practical applications. In medicine, gamma rays from cobalt-60 are used in radiotherapy to destroy cancerous tumours. Tracers such as technetium-99m emit gamma radiation that can be detected outside the body to image organs. Beta emitters are used in paper thickness gauges: if the detected count rate drops, the paper is too thick. In industry, gamma rays are used to inspect metal welds and to sterilise medical equipment by killing bacteria. Carbon-14 dating relies on the known half-life of 5730 years to estimate the age of archaeological artefacts containing organic material.

放射性同位素有许多实际应用。在医学上，钴-60 产生的 γ 射线用于放射治疗，摧毁癌变肿瘤；锝-99m 等示踪剂发射可被体外探测的 γ 射线，用于器官成像。β 放射源用于纸张测厚仪：若探测到的计数率下降，说明纸张过厚。工业中利用 γ 射线检测金属焊缝，并利用其杀死细菌以对医疗设备进行灭菌。碳-14 测年法依靠其 5730 年的已知半衰期，来估算含有有机物的考古文物的年代。

10. Hazards and Safety Precautions | 辐射的危害与安全防护

Ionising radiation can damage living cells by breaking chemical bonds and altering DNA. High doses can cause radiation sickness, burns, organ failure, and an increased risk of cancer. Alpha particles are particularly dangerous if inhaled or ingested, because they cause intense local ionisation. To minimise exposure, three principles are followed: reduce exposure time, increase distance from the source (intensity decreases according to the inverse square law), and use appropriate shielding — lead for gamma, aluminium for beta, and simply staying clear for alpha. Safety equipment includes lead aprons, tongs for handling sources, and never pointing a source at people.

电离辐射会破坏化学键并改变 DNA，从而损伤活细胞。高剂量可引发辐射病、灼伤、器官衰竭以及增加患癌风险。α 粒子若被吸入或摄入则尤为危险，因为它们会造成强烈的局部电离。为了尽量减少暴露，遵循三项原则：缩短照射时间、增大人体与源的距离（强度按平方反比律下降）以及使用适当的屏蔽——γ 用铅、β 用铝、对 α 只需远离即可。安全设备包括铅围裙、操作放射源的长柄夹具，并且绝对不要将源对准任何人。

11. Nuclear Fission | 核裂变

Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235 or plutonium-239) into two smaller nuclei after absorbing a neutron. This process releases a large amount of energy and two or three more neutrons, which can trigger further fission reactions in a chain reaction. The energy released comes from a tiny loss of mass, according to Einstein’s equation:

核裂变是指大的不稳定原子核（如铀-235 或钚-239）在吸收一个中子后分裂成两个较小的核。此过程释放出巨大能量，同时放出两到三个中子，这些中子可以诱发更多的裂变反应，形成链式反应。释放的能量来自微小的质量亏损，遵循爱因斯坦方程：

E = mc²

In a nuclear reactor, control rods (often made of boron or cadmium) absorb excess neutrons to keep the chain reaction steady. The heat generated is used to produce steam that drives turbines to generate electricity. An example fission equation is:

在核反应堆中，控制棒（通常用硼或镉制成）吸收多余的中子以维持链式反应的稳定。产生的高热用于产生蒸汽，推动汽轮机发电。一个裂变方程的例子是：

²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n

12. Nuclear Fusion | 核聚变

Nuclear fusion is the joining of two small nuclei to form a larger nucleus, releasing even more energy per unit mass than fission. This is the process that powers the Sun and other stars, where hydrogen nuclei fuse to form helium. For example:

核聚变是两个小核结合成一个较大核的过程，单位质量释放的能量比裂变更多。这是太阳和其他恒星的能量来源，其中氢核聚变形成氦。例如：

²₁H + ³₁H → ⁴₂He + ¹₀n

Fusion requires extremely high temperatures (millions of degrees Celsius) and pressures to overcome the electrostatic repulsion between positively charged nuclei. On Earth, achieving and containing these conditions is a major engineering challenge, although experimental reactors have achieved short-lived fusion. Fusion produces less long-lived radioactive waste than fission and uses abundant fuel such as isotopes of hydrogen from water.

聚变需要极高的温度（数百万摄氏度）和压强才能克服带正电的原子核之间的静电排斥。在地球上实现并约束这些条件是一项巨大的工程挑战，尽管实验性反应堆已经实现了短时间的聚变。与裂变相比，聚变产生的长寿命放射性废物更少，并且可以使用水中丰富的氢同位素作为燃料。

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Nuclear Physics: IGCSE Physics Key Points | IGCSE 物理：核物理 考点精讲