📚 IGCSE CIE Physics: Nuclear Physics Key Points | IGCSE CIE 物理：核物理考点精讲
Mastering nuclear physics for the IGCSE CIE Physics exam requires a clear understanding of atomic structure, radioactive decay processes, half-life calculations, and the practical applications of radiation. This revision guide breaks down every key concept you need, with paired English and Chinese explanations, tables, and decay equations to ensure thorough preparation.
要在 IGCSE CIE 物理考试中掌握核物理，需要清楚理解原子结构、放射性衰变过程、半衰期计算以及辐射的实际应用。这份复习指南拆分了你需要掌握的每一个关键概念，配以中英对照讲解、表格和衰变方程，确保全面备考。
1. Atomic Structure and Nuclide Notation | 原子结构与核素符号
Every atom consists of a tiny, dense nucleus containing protons and neutrons, orbited by electrons in shells. The nucleus holds most of the mass, while electrons determine chemical properties.
每个原子都有一个微小、致密的原子核，核内含有质子和中子，核外有电子分层绕行。原子核集中了大部分质量，而电子则决定化学性质。
The nuclide notation summarises the composition of a nucleus: AZX, where X is the element symbol, A is the mass number (nucleon number) = protons + neutrons, and Z is the atomic number (proton number). For example, 23892U represents uranium with 92 protons and 146 neutrons (238 – 92).
核素符号概括了原子核的组成：AZX，X为元素符号，A为质量数（核子数）= 质子数 + 中子数，Z为原子序数（质子数）。例如，23892U 表示铀，有92个质子和146个中子（238 – 92）。
Protons carry a positive charge (+1), neutrons are neutral, and electrons carry a negative charge (-1). In a neutral atom, the number of electrons equals Z.
质子带正电 (+1)，中子不带电，电子带负电 (-1)。在中性原子中，电子数等于Z。
2. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons (same Z) but different numbers of neutrons, hence different mass numbers. For instance, 126C, 136C and 146C are three isotopes of carbon.
同位素是指质子数（Z相同）相同而中子数不同的同种元素的原子，因此质量数不同。例如，126C、136C 和 146C 是碳的三种同位素。
Isotopes display identical chemical behaviour because chemical reactions depend on electrons, which are determined by the proton number. However, their nuclear stability may differ; some isotopes are radioactive and decay spontaneously.
同位素表现出完全相同的化学性质，因为化学反应取决于电子，而电子数由质子数决定。然而，它们的核稳定性可能不同；有些同位素具有放射性，会自发衰变。
3. Radioactivity and Nuclear Decay | 放射性与核衰变
Radioactivity is the spontaneous and random emission of radiation from an unstable nucleus. The nucleus becomes more stable by releasing particles or energy. This process is independent of physical conditions like temperature or pressure.
放射性是指不稳定原子核自发、随机地发出辐射的过程。原子核通过释放粒子或能量变得更稳定。这一过程不受温度或压力等物理条件影响。
Three main types of radiation are emitted: alpha (α) particles, beta (β) particles, and gamma (γ) rays. Each originates from the nucleus and has distinct properties. Additionally, a neutron can decay to produce a proton, an electron (β⁻), and an antineutrino.
辐射主要分为三种：α 粒子、β 粒子和 γ 射线。它们都源自原子核，且各自具有独特的性质。此外，中子可以衰变成一个质子、一个电子（β⁻）和一个反中微子。
4. Alpha (α) Decay | α 衰变
An alpha particle is identical to a helium nucleus, composed of 2 protons and 2 neutrons. Its symbol is 42He or α. When an unstable nucleus emits an alpha particle, its mass number decreases by 4 and its atomic number decreases by 2.
α 粒子等同于一个氦原子核，由 2 个质子和 2 个中子组成。其符号为 42He 或 α。当不稳定原子核放出一个 α 粒子时，其质量数减少 4，原子序数减少 2。
Alpha particles have a high ionising power because of their +2 charge and relatively large mass. They travel only a few centimetres in air and can be stopped by a sheet of paper or the outer layer of skin. They are strongly deflected by electric and magnetic fields.
α 粒子因带+2电荷且质量较大，具有高电离能力。它们在空气中只能传播几厘米，可以被一张纸或皮肤外层阻挡。在电场和磁场中，α 粒子会发生显著偏转。
5. Beta (β) Decay | β 衰变
A beta particle is a fast-moving electron emitted from the nucleus when a neutron transforms into a proton. It is denoted as 0-1e or β⁻. The mass number remains unchanged, but the atomic number increases by 1.
β 粒子是从原子核内发射出的快速电子，由中子转变为质子时产生。符号为 0-1e 或 β⁻。衰变后质量数不变，原子序数增加 1。
Beta particles have moderate ionising power, less than alpha but more than gamma. They can travel about a metre in air and are stopped by a few millimetres of aluminium. Their deflection in electric and magnetic fields is larger than alpha particles due to their much smaller mass, and the direction is opposite because of the negative charge.
β 粒子电离能力中等，弱于 α 但强于 γ。它们在空气中可传播约一米，几毫米厚的铝片即可阻挡。由于质量极小，它们在电场和磁场中的偏转比 α 粒子大，且因带负电，偏转方向与 α 相反。
6. Gamma (γ) Radiation | γ 辐射
Gamma rays are electromagnetic waves of very high frequency and energy, emitted from a nucleus after alpha or beta decay to release excess energy. The symbol is 00γ. Gamma emission does not change the mass number or atomic number.
γ 射线是频率和能量极高的电磁波，通常在 α 或 β 衰变后由原子核释放过剩能量时发出。符号为 00γ。放出 γ 射线不会改变质量数或原子序数。
Gamma radiation has the lowest ionising power but the highest penetrating ability. It can travel several metres in air and requires thick lead or concrete to be substantially reduced. It is not deflected by electric or magnetic fields because it carries no charge.
γ 辐射电离能力最弱，但穿透能力最强。它在空气中能传播数米，需要厚铅板或混凝土才能有效屏蔽。因不带电，它在电场和磁场中不发生偏转。
7. Comparing Alpha, Beta, Gamma | 比较三种射线
It is essential to memorise the relative properties of the three radiations for the examination. The table below summarises key characteristics.
熟记三种射线的相对性质对考试至关重要。下表总结了关键特征。
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus 42He | Fast electron 0-1e | Electromagnetic wave |
| Charge | +2 | -1 | 0 |
| Ionising power | High | Medium | Low |
| Penetrating power | Stopped by paper/skin (a few cm in air) | Stopped by ~3 mm Al (about 1 m in air) | Reduced by thick Pb/concrete (several m in air) |
| Deflection in fields | Deflected slightly (positive) | Deflected greatly (negative) | No deflection |
中英对照版：
| 性质 | α | β | γ |
|---|---|---|---|
| 本质 | 氦核 42He | 高速电子 0-1e | 电磁波 |
| 电荷 | +2 | -1 | 0 |
| 电离能力 | 强 | 中等 | 弱 |
| 穿透能力 | 纸/皮肤可挡（空气中几cm） | ～3 mm 铝可挡（空气中约1 m） | 厚铅/混凝土减弱（空气中数米） |
| 场中偏转 | 小幅偏转（正电） | 大幅偏转（负电） | 不偏转 |
8. Deflection in Electric and Magnetic Fields | 电场和磁场中的偏转
When alpha and beta radiations pass through an electric field, they are attracted to the plate of opposite charge: alpha towards the negative plate, beta towards the positive plate. Gamma continues straight through. In a magnetic field, the direction of deflection is governed by Fleming’s left-hand rule, with alpha and beta bending in opposite directions.
当 α 和 β 射线通过电场时，会受到异号电极板的吸引：α 偏转向负极板，β 偏转向正极板。γ 射线径直通过。在磁场中，偏转方向由弗莱明左手定则决定，α 和 β 朝相反方向偏转。
The magnitude of deflection depends on the particle’s charge, mass, and speed. Beta particles, with very small mass and charge of -1, experience the greatest deflection per unit mass. Alpha particles, much heavier, show a smaller curve. Gamma, being uncharged, is completely unaffected.
偏转幅度取决于粒子的电荷、质量和速度。β 粒子质量极小且带-1电荷，单位质量的偏转最大。α 粒子质量大得多，偏转较小。γ 因不带电，完全不受影响。
9. Radioactive Decay Equations | 衰变方程
A nuclear equation balances both mass number and atomic number on each side. For alpha decay: AZX → A-4Z-2Y + 42He. Example: 23892U → 23490Th + 42He.
核反应方程必须满足两边质量数和原子序数守恒。α 衰变通式：AZX → A-4Z-2Y + 42He。实例：23892U → 23490Th + 42He。
Beta decay involves the conversion of a neutron into a proton, so the atomic number rises by 1 while the mass number stays the same: AZX → AZ+1Y + 0-1e (+ antineutrino). Example: 146C → 147N + 0-1e. The antineutrino is often omitted in IGCSE equations.
β 衰变中，一个中子转化为质子，原子序数增加 1，质量数不变：AZX → AZ+1Y + 0-1e (+反中微子)。实例：146C → 147N + 0-1e。在 IGCSE 方程中，反中微子常被省略。
Gamma emission is often shown alongside alpha or beta decay to indicate the release of energy. The parent and daughter nuclides remain the same, with a gamma ray symbol 00γ added to the products.
γ 辐射常伴随 α 或 β 衰变出现，表示能量释放。母核与子核保持不变，产物中增加一个 γ 符号 00γ。
10. 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 count rate to fall to half its initial value. It is a constant characteristic of a particular nuclide.
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所需的时间，或等效地，计数率降至初始值一半所需的时间。这是特定核素的一个恒定特征。
Half-life calculations are common: starting with an initial mass or activity, after n half-lives, the remaining amount = initial amount × (½)ⁿ. For example, if a substance has a half-life of 2 days and starts with 80 g, after 6 days (3 half-lives), 80 × (½)³ = 10 g remain.
半衰期计算很常见：设初始质量或活度，经过 n 个半衰期后，剩余量 = 初始量 × (½)ⁿ。例如，某物质半衰期为 2 天，初始质量为 80 g，6 天后（3 个半衰期）剩余 80 × (½)³ = 10 g。
IGCSE problems may provide a decay curve and ask you to find half-life by reading the time for the count rate to halve. Always check twice: e.g., from 400 counts/s to 200 counts/s takes t seconds, and from 200 to 100 takes another t seconds, confirming the half-life is constant.
IGCSE 题目可能给出衰变曲线，要求通过读取计数率减半所用的时间来求半衰期。务必核实两次：例如，从 400 计数/秒降至 200 用了 t 秒，从 200 降至 100 又用了 t 秒，从而确认半衰期恒定。
11. Safety Precautions and Handling Radioactive Sources | 安全防护与放射源操作
When dealing with radioactive materials in the laboratory, strict safety rules must be followed to minimise exposure: keep sources in lead-lined containers, use tongs or remote handling tools, never point a source at anyone, and keep handling time as short as possible.
在实验室处理放射性物质时，必须遵守严格的安全规则以减少照射：把放射源存放在衬铅容器中，使用钳子或遥控工具，切勿将源指向任何人，并尽可能缩短操作时间。
The three key protective measures are: distance (increase distance from the source; intensity follows an inverse-square law), shielding (use appropriate absorbers – paper for alpha, aluminium for beta, thick lead for gamma), and time (limit exposure duration). Monitor radiation dose with a film badge or Geiger-Müller counter.
三项关键防护措施是：距离（加大与源的距离；强度遵循平方反比定律）、屏蔽（使用适当吸收体——α 用纸、β 用铝、γ 用厚铅板）和时间（限制照射时长）。可用胶片剂量计或盖革-米勒计数器监测辐射剂量。
12. Applications of Radioactivity | 放射性应用
Radioisotopes have widespread applications in medicine, industry, and research. In medicine, technetium-99m, a gamma emitter with a short half-life of about 6 hours, is used as a tracer to image organs. Iodine-131 treats thyroid disorders because it is absorbed by the thyroid gland.
放射性同位素在医疗、工业和科研中有广泛应用。在医学上，锝-99m 是一种释放 γ 射线、半衰期约 6 小时的示踪剂，用于器官成像。碘-131 可治疗甲状腺疾病，因为它会被甲状腺吸收。
In industry, gamma rays are used to detect flaws in welds and metal castings (radiography), while beta gauges measure the thickness of paper, plastic, and metal foils. Alpha sources in smoke detectors ionise air, allowing a small current to flow; smoke particles disrupt this current, triggering the alarm.
在工业上，γ 射线用于检测焊缝和金属铸件的缺陷（放射照相术），β 测厚仪则测量纸张、塑料和金属箔的厚度。烟雾探测器中的 α 源电离空气，形成微小电流；烟雾颗粒干扰电流，触发警报。
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