Half-Life and Radioactive Decay | 半衰期与放射性衰变

📚 Half-Life and Radioactive Decay | 半衰期与放射性衰变

Radioactive decay is one of the most important yet least understood topics in IGCSE Science. Every unstable atomic nucleus carries a ‘clock of probability’: we never know exactly when any single nucleus will decay, but for a large sample the overall decay pattern is beautifully predictable. For Edexcel IGCSE Science, half-life is a guaranteed topic – it appears in Section 7 (Radioactivity) of the Physics specification and is examined regularly in Paper 1 and Paper 2. This article breaks down what half-life means, how to calculate it from data and graphs, and how it controls the practical uses of radioactive materials.

放射性衰变是 IGCSE 科学中十分重要却又最容易被误解的考点之一。每一个不稳定的原子核都携带着一个”概率时钟”：我们永远无法知道某一颗原子核究竟何时会衰变，但对于大量原子核来说，整体的衰变模式却具有惊人的可预测性。在 Edexcel IGCSE 科学中，半衰期是必考内容——它出现在物理部分的第 7 单元（放射性），并在 Paper 1 和 Paper 2 中反复考查。本文将深入讲解半衰期的含义，教你如何从数据和图像中计算半衰期，以及它如何决定放射性材料的实际用途。

1. What Is Radioactivity? | 什么是放射性

Some atomic nuclei are unstable. To become more stable, they spontaneously emit ionising radiation in the form of alpha particles, beta particles or gamma rays. The nucleus that decays is called the parent nucleus; the new nucleus that is formed is called the daughter nucleus. Isotopes that undergo this process are known as radioisotopes. Stability depends on the balance between protons and neutrons in the nucleus: if there are too many or too few neutrons relative to protons, the nucleus is likely to be unstable and will eventually decay.

某些原子核是不稳定的。为了变得更加稳定，它们会自发地以 α 粒子、β 粒子或 γ 射线的形式释放电离辐射。发生衰变的原子核称为母核，衰变后形成的新原子核称为子核。会发生这种过程的同位素叫作放射性同位素。稳定性取决于原子核内质子与中子的平衡：如果相对于质子来说中子过多或过少，原子核就很可能不稳定，并最终发生衰变。

The activity of a radioactive source is the rate at which nuclei decay, measured in becquerels (Bq). One becquerel means exactly one decay per second. The activity of a source decreases over time as the number of unstable nuclei becomes smaller.

放射源的活度是指原子核发生衰变的速率，单位是贝克勒尔（Bq）。1 贝克勒尔表示每秒恰好发生一次衰变。随着不稳定原子核数量逐渐减少，放射源的活度会随时间降低。

2. The Three Types of Radiation | 三种辐射类型

Each type of radiation has different properties of mass, charge and penetration. You need to know these differences well because questions about penetrating power and safety appear frequently in the Edexcel exams.

每种辐射在质量、电荷和穿透能力方面都有不同的特性。你需要熟练掌握这些差异，因为关于穿透能力和安全性的问题在 Edexcel 考试中经常出现。

Alpha decay reduces the mass number by 4 and the atomic number by 2. Beta decay does not change the mass number but increases the atomic number by 1 because a neutron changes into a proton. Gamma emission changes neither the mass number nor the atomic number; it simply removes excess energy from the nucleus.

α 衰变会使质量数减少 4、原子序数减少 2。β 衰变不改变质量数，但会使原子序数增加 1，因为一个中子转变为一个质子。γ 辐射既不改变质量数也不改变原子序数，它只是让原子核释放多余的能量。

3. Why Decay Is Random | 为何衰变是随机的

Radioactive decay is a completely random process. This means two important things. First, we cannot predict which particular nucleus in a sample will decay next, nor exactly when it will decay. An individual nucleus may decay in the next second, or it may survive for millions of years – there is no way to know in advance. Second, external conditions such as temperature, pressure, density and chemical state have absolutely no effect on the decay rate. Heating a radioactive substance does not make it decay faster.

放射性衰变是一个完全随机的过程。这意味着两件重要的事情。第一，我们无法预测样品中哪一颗原子核会下一个发生衰变，也无法预测它究竟何时衰变。一颗原子核可能在下一秒就衰变，也可能存活数百万年——我们事先无法知道。第二，温度、压强、密度和化学状态等外部条件对衰变速率完全没有影响。加热放射性物质并不会让它衰变得更快。

Although individual nuclei behave randomly, a large sample behaves predictably because of statistics. If you have millions of unstable nuclei, a very steady and predictable fraction will decay each second. This is why we can draw smooth decay curves and calculate half-life precisely, even though each individual decay event is random.

虽然单个原子核的行为是随机的，但由于统计规律的作用，大量原子核的整体行为是可预测的。如果你拥有数百万颗不稳定原子核，那么每秒发生衰变的比例会非常稳定、可预测。这就是尽管每一次衰变事件本身是随机的，我们仍能画出平滑的衰变曲线、精确计算半衰期的原因。

4. Defining Half-Life | 半衰期的定义

The half-life (symbol T½) of a radioactive isotope is the time taken for half of the unstable nuclei in a sample to decay. It can also be defined as the time taken for the activity, or corrected count rate, of a sample to fall to half of its original value. Half-life is a fixed property of a particular isotope and never changes, no matter how old the sample is or how much material you have.

放射性同位素的半衰期（符号 T½）是指样品中一半的不稳定原子核发生衰变所需的时间。它也可以定义为样品的活度（或修正后的计数率）降至初始值一半所需的时间。半衰期是某一特定同位素的固定属性，无论样品存在多久、材料有多少，它都永远不会改变。

Half-life can be measured in any unit of time: seconds for very short-lived isotopes, minutes or hours for medical tracers, days for some artificial isotopes, and thousands of years for isotopes used in dating. The key idea is always the same: after one half-life, half remains; after two half-lives, one quarter remains; and so on.

半衰期可以用任何时间单位来度量：极短寿命的同位素用秒，医用示踪剂用分钟或小时，一些人工同位素用天，用于断代测年的同位素则用数千年。核心思想始终相同：一个半衰期后剩下二分之一，两个半衰期后剩下四分之一，依此类推。

T½ = time for the number of undecayed nuclei (or activity) to halve

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