📚 A-Level Edexcel Physics: Radioactive Decay – Key Concepts | A-Level Edexcel 物理：放射性衰变 考点精讲
Radioactive decay is a fundamental process in nuclear physics where an unstable atomic nucleus loses energy by emitting radiation. This topic is crucial for Edexcel A-Level Physics, covering nuclear structure, types of radiation, decay laws, half-life, activity, and practical applications. Mastering these concepts enables students to solve decay problems, interpret exponential graphs, and understand real-world uses such as carbon dating and medical imaging. This revision guide provides a comprehensive review of key points, ensuring you are fully prepared for the exam.
放射性衰变是核物理中的一个基本过程，即不稳定的原子核通过发射辐射而损失能量。该主题是 Edexcel A-Level 物理的重要内容，涵盖核结构、辐射类型、衰变定律、半衰期、活度及实际应用。掌握这些概念有助于学生解决衰变问题、解读指数曲线图，并理解碳定年及医学成像等实际用途。本复习指南提供了关键考点的全面梳理，确保你为考试做好充分准备。
1. Nuclear Structure and Notation | 原子核结构与符号
Isotopes have the same Z but different A. Nuclide notation places A as a superscript and Z as a subscript before the element symbol, e.g., ¹⁴₆C for carbon-14 or ²³⁸₉₂U for uranium-238. Unstable nuclides lie outside the stability band and undergo radioactive decay to become more stable.
原子核包含质子和中子（统称核子）。原子序数 Z 等于质子数，决定了元素种类。质量数 A = Z + N（中子数）。同位素具有相同的 Z 但不同的 A。核素符号将质量数 A 标为左上标，原子序数 Z 标为左下标，后接元素符号，例如碳-14 写作 ¹⁴₆C，铀-238 写作 ²³⁸₉₂U。不稳定核素处在稳定带之外，会通过放射性衰变变得更加稳定。
2. Types of Radiation: Alpha, Beta, Gamma | 辐射类型：α、β、γ
Three main types of radiation are emitted: alpha (α), beta (β), and gamma (γ). Alpha particles are helium-4 nuclei (⁴₂He), carrying a +2e charge. They have the highest ionising power but the lowest penetration (stopped by paper or a few cm of air). Beta particles are high-speed electrons (β⁻) or positrons (β⁺). Beta-minus decay emits an electron (⁰₋₁e); beta-plus decay emits a positron (⁰₊₁e). Beta radiation has moderate ionising ability and can be stopped by a few mm of aluminium. Gamma radiation is electromagnetic with no mass or charge. It has the highest penetrating power (requires thick lead or concrete) and the lowest ionising ability.
放射出的主要辐射有三种：α、β 和 γ。α 粒子是氦-4 核（⁴₂He），带 +2e 电荷。它的电离能力最强，但穿透力最弱（纸或几厘米空气即可阻挡）。β 粒子是高速电子（β⁻）或正电子（β⁺）。β⁻ 衰变发射电子（⁰₋₁e），β⁺ 衰变发射正电子（⁰₊₁e）。β 辐射具有中等的电离能力，可被几毫米铝片阻挡。γ 辐射是不带质量与电荷的电磁波，穿透力最强（需要厚铅或混凝土），电离能力最弱。
| Property | Alpha (α) | Beta (β⁻/β⁺) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus | Electron/positron | EM radiation |
| Charge | +2e | −1e / +1e | 0 |
| Penetration | Low (paper) | Medium (3 mm Al) | High (thick Pb) |
| Ionising ability | Very high | Medium | Very low |
上表清晰地总结了三种辐射的关键区别。在考试中常要求比较穿透力与电离能力，并说明各自防护材料。
3. Decay Equations and Conservation Laws | 衰变方程与守恒定律
In all decay equations, the total mass number (A) and total atomic number (Z) are conserved. For alpha decay: ᴬ Z X → ᴬ⁻⁴ Z₋₂ Y + ⁴₂He. Example: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He. Beta-minus decay converts a neutron into a proton, emitting an electron and an antineutrino: ¹⁴₆C → ¹⁴₇N + ⁰
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