📚 GCSE WJEC Physics: Radioactive Decay Key Points | GCSE WJEC 物理：放射性衰变考点精讲
Welcome to this in-depth WJEC GCSE Physics revision guide on radioactive decay. This article walks you through every essential concept you need to master — from atomic structure and the three main types of radiation to half-life calculations, real-world applications, and safety precautions. Each section is carefully aligned with the WJEC specification, making it a perfect companion for your exam preparation.
欢迎阅读这篇 WJEC GCSE 物理放射性衰变考点精讲。本文将带你逐一掌握每一个核心概念——从原子结构与三种主要辐射类型，到半衰期计算、实际应用与安全防护。每一节都紧扣 WJEC 考试大纲，是备考复习的理想伙伴。
1. Atomic Structure and Isotopes | 原子结构与同位素
Atoms consist of a tiny central nucleus containing protons and neutrons, surrounded by electrons moving in energy levels. The number of protons (atomic number) determines the element, while the total number of protons and neutrons gives the mass number.
原子由一个微小的中心原子核（包含质子和中子）以及绕核运动的电子组成。质子数（原子序数）决定了元素的种类，而质子数与中子数之和就是质量数。
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. For example, carbon-12 (¹² ₆C) and carbon-14 (¹⁴ ₆C) are isotopes. Many isotopes are stable, but some are unstable and will decay.
同位素是同一种元素中质子数相同但中子数不同的原子。例如碳-12 (¹² ₆C) 和碳-14 (¹⁴ ₆C) 就是同位素。许多同位素是稳定的，但也有一些不稳定，会发生衰变。
Radioactivity arises from an unstable nucleus that contains an imbalance of protons and neutrons. To become more stable, the nucleus emits radiation, a process known as radioactive decay.
放射性来源于不稳定的原子核（质子与中子数量不平衡）。为了趋于稳定，原子核会发射辐射，这个过程就是放射性衰变。
2. Types of Radioactive Decay | 放射性衰变类型
There are three main types of radiation emitted during decay: alpha (α), beta (β), and gamma (γ) radiation. Each differs in its nature, ionising power, penetrating ability, and range in air.
衰变过程中主要发射三种辐射：α (alpha)、β (beta) 和 γ (gamma) 辐射。它们各自的性质、电离能力、穿透本领和在空气中的射程均不相同。
Alpha particles are helium nuclei, consisting of two protons and two neutrons, with a charge of +2 and a mass number of 4. They are highly ionising but have very low penetration – stopped by a few centimetres of air or a sheet of paper.
α 粒子是氦核，由两个质子和两个中子组成，带 +2 电荷，质量数为 4。它的电离能力很强，但穿透本领极弱——几厘米的空气或一张纸就能将其挡住。
Beta particles are fast-moving electrons emitted from the nucleus when a neutron turns into a proton. They carry a charge of –1 and have virtually no mass. Beta radiation is moderately ionising and can penetrate paper but is stopped by a few millimetres of aluminium.
β 粒子是从原子核中发射出来的高速电子，产生于一个中子转变为质子的过程。它带 –1 电荷，质量几乎为零。β 辐射的电离能力中等，能穿透纸张，但几毫米厚的铝片即可将其阻挡。
Gamma radiation is a high-energy electromagnetic wave with no mass and no charge. It is weakly ionising but extremely penetrating, requiring thick lead or several metres of concrete to reduce its intensity significantly.
γ 辐射是一种高能电磁波，没有质量也不带电荷。它的电离能力很弱，但穿透能力极强，需要厚铅板或数米厚的混凝土才能有效减弱其强度。
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus ⁴ ₂He | Fast electron ⁰ ₋₁e | Electromagnetic wave |
| Charge | +2 | –1 | 0 |
| Ionising power | Very high | Medium | Low |
| Penetration | Stopped by paper | Stopped by Al foil | Reduced by lead/concrete |
| Range in air | A few cm | About a metre | Effectively unlimited |
In an electric field, alpha particles are deflected towards the negative plate, beta particles towards the positive plate (much more deflection due to their smaller mass), and gamma rays pass straight through undeflected.
在电场中，α 粒子会偏向负极板，β 粒子偏向正极板（因其质量更小而偏转更大），γ 射线则直线穿过、不发生偏转。
3. Writing Nuclear Decay Equations | 核衰变方程书写
Nuclear equations show the changes in the nucleus during radioactive decay. Both the total mass number and the total atomic number must balance on each side of the equation.
核方程用来表示放射性衰变过程中原子核的变化。方程两边的总质量数和总原子序数必须守恒（平衡）。
In alpha decay, the nucleus loses two protons and two neutrons. The mass number decreases by 4, and the atomic number decreases by 2. Example: ²³⁸ ₉₂U → ²³⁴ ₉₀Th + ⁴ ₂He.
在 α 衰变中，原子核失去两个质子和两个中子。质量数减少 4，原子序数减少 2。例如：²³⁸ ₉₂U → ²³⁴ ₉₀Th + ⁴ ₂He。
In beta decay, a neutron becomes a proton, so the atomic number increases by 1 while the mass number stays the same. A high-speed electron is emitted. Example: ¹⁴ ₆C → ¹⁴ ₇N + ⁰ ₋₁e.
在 β 衰变中，一个中子转变为质子，因此原子序数增加 1，而质量数不变。同时发射出一个高速电子。例如：¹⁴ ₆C → ¹⁴ ₇N + ⁰ ₋₁e。
Gamma emission often accompanies alpha or beta decay and does not change the mass number or atomic number. It simply releases excess energy from the nucleus.
γ 辐射通常伴随 α 或 β 衰变而产生，它不会改变质量数或原子序数，仅仅是释放原子核中多余的能量。
When writing equations for WJEC exams, always include the correct symbols for each particle and check that the top numbers and bottom numbers balance.
在 WJEC 考试中书写方程时，务必使用每种粒子的正确符号，并检查左上角的质量数和左下角的原子序数是否平衡。
4. Half-Life | 半衰期
The half-life of a radioactive isotope is the time taken for half the unstable nuclei in a sample to decay, or for the count rate (activity) to fall to half its initial value. It is a characteristic property of each isotope and cannot be changed by physical or chemical means.
放射性同位素的半衰期是指样品中半数不稳定的原子核发生衰变所需的时间，或者计数率（活度）下降到初始值一半所需的时间。半衰期是每种同位素的特征属性，不受物理或化学手段影响。
After one half-life, half the original radioactive nuclei remain; after two half-lives, one quarter remain; after three half-lives, one eighth, and so on. This leads to the useful formula:
经过一个半衰期后，剩余一半原放射性核；两个半衰期后剩四分之一；三个半衰期后剩八分之一，依此类推。由此可以得到一个实用的公式：
remaining activity = initial activity × (½)ⁿ
where n is the number of half-lives passed. You can calculate n by dividing the total time by the half-life period.
其中 n 是所经过的半衰期个数。可以用总时长除以半衰期来计算 n。
For example, a source has an initial count rate of 640 counts per second and a half-life of 2 hours. After 6 hours, n = 6/2 = 3, so the remaining count rate is 640 × (½)³ = 640 × ⅛ = 80 counts/s. This type of calculation is frequently tested.
例如，一个放射源初始计数率为 640 次/秒，半衰期为 2 小时。6 小时后，n = 6/2 = 3，剩余计数率为 640 × (½)³ = 640 × ⅛ = 80 次/秒。这种计算在考试中很常见。
You may also be asked to determine half-life from a graph of activity against time. Simply find how long it takes for the activity to drop by half, then check that it takes the same time to halve again.
你也可能需要从活度-时间图线中确定半衰期。只需找出活度下降一半所用的时间，然后验证再次减半所用的时间是否相同即可。
5. Random Nature of Decay | 衰变的随机性
Radioactive decay is a random process. This means you cannot predict exactly which nucleus will decay next, or when a particular nucleus will decay. All you can state is the overall probability that any nucleus will decay in a given time.
放射性衰变是一个随机过程。这意味着你无法准确预测下一个是哪个原子核衰变，或者某个特定的核何时会衰变。你只能给出任意一个核在给定时间内衰变的整体概率。
Because of this randomness, repeated measurements of count rate from the same source will show slight variations around an average value. A large number of decays must be recorded to obtain a reliable estimate of the activity.
正因为这种随机性，对同一放射源重复测量计数率会得到围绕平均值轻微波动的结果。必须记录大量的衰变次数才能得到可靠的活度估计值。
The half-life of a large sample appears constant because the random decays of many billions of nuclei average out. This statistical behaviour underpins all half-life predictions and safety calculations.
大量样品的半衰期之所以显得恒定，是因为数十亿个原子核的随机衰变在统计上被平均了。这种统计行为是所有半衰期预测和安全计算的基础。
6. Background Radiation | 本底辐射
Background radiation is the low-level radiation that is around us all the time. It comes from natural and artificial sources. When measuring activity in the lab, you must subtract the background count to find the corrected count rate from the source alone.
本底辐射是我们周围始终存在的低水平辐射，来自天然和人工来源。在实验室测量活度时，你必须扣除本底计数，得到仅由放射源产生的修正计数率。
Natural sources include radon gas from the ground, cosmic rays from space, rocks and building materials, and even our own bodies (from carbon-14 and potassium-40). Artificial sources include medical X-rays and fallout from nuclear testing.
天然来源包括地下释放的氡气、来自太空的宇宙射线、岩石和建筑材料，甚至我们人体自身（来自碳-14 和钾-40）。人工来源包括医用 X 射线和核试验的沉降物。
In the UK, about half of the average background radiation dose comes from radon gas. The dose is measured in sieverts (Sv), but for GCSE you should be aware of the concept of radiation dose and that background varies with location.
在英国，平均本底辐射剂量约有一半来自氡气。剂量单位是希沃特 (Sv)，在 GCSE 阶段你需要了解辐射剂量的概念，并知道本底辐射水平随地点变化。
7. Detecting Radiation | 辐射探测
Radioactive emissions can be detected using a Geiger-Müller (GM) tube connected to a counter or ratemeter. When radiation enters the tube, it ionises the gas inside, creating a pulse of current that is counted.
放射性辐射可以使用连接至计数器或计数率计的盖革-米勒管（GM 管）进行探测。辐射进入管中会使内部气体电离，产生一次电流脉冲并被计数。
Photographic film is also used – it darkens when exposed to radiation. This principle was used historically by Henri Becquerel and is still applied in personal dosimeters worn by radiation workers.
照相胶片也会被用于探测——胶片受到辐射照射会变黑。这一原理历史上曾被贝克勒尔使用，如今仍用于辐射工作人员佩戴的个人剂量计中。
In cloud chambers, the paths of alpha and beta particles appear as visible trails of condensation. Alpha particles leave thick, straight tracks, while beta particles produce thin, irregular tracks.
在云室中，α 和 β 粒子的径迹会呈现出可见的凝结尾迹。α 粒子留下粗而直的径迹，β 粒子则产生细而不规则的径迹。
Understanding detection methods helps relate the properties of each type of radiation to real-world measurement and safety monitoring.
理解探测方法有助于将各种辐射的性质与实际测量和安全监测联系起来。
8. Uses of Radioactive Sources | 放射源的应用
Radioactive isotopes have a wide range of practical uses, chosen carefully according to the type of radiation emitted and the isotope’s half-life.
放射性同位素有着广泛的实际用途，人们根据所发射辐射的类型和同位素的半衰期来谨慎选择。
Americium-241, an alpha emitter, is used in smoke detectors. Alpha particles ionise air molecules, allowing a small current to flow. When smoke enters, it absorbs the alpha particles, reducing the current and triggering the alarm.
镅-241 是一种 α 放射源，用于烟雾探测器。α 粒子使空气分子电离，产生微小电流。当烟雾进入时，它会吸收 α 粒子，使电流减小并触发警报。
Beta emitters such as strontium-90 are used in thickness gauges to control the production of paper, plastic, and metal sheets. The amount of beta radiation passing through the material indicates its thickness.
锶-90 等 β 放射源被用于厚度计中，以控制纸张、塑料和金属薄板的生产。穿透材料的 β 辐射量反映了材料的厚度。
Gamma-emitting isotopes like cobalt-60 are used to sterilise surgical instruments and to treat cancer through radiotherapy, where carefully directed gamma rays kill cancer cells. Iodine-131, a beta and gamma emitter, is used as a tracer to check thyroid function.
钴-60 等 γ 放射源被用来给手术器械灭菌，以及通过放射治疗来治疗癌症——精密定向的 γ 射线可以杀死癌细胞。碘-131 同时发射 β 和 γ，可用作示踪剂检查甲状腺功能。
Carbon-14 has a half-life of about 5730 years and is used in radiocarbon dating to determine the age of ancient organic materials. The short half-lives of certain medical tracers ensure they decay quickly, reducing long-term exposure to the patient.
碳-14 的半衰期约为 5730 年，用于放射性碳定年，以确定古代有机物质的年代。某些医用示踪剂的短半衰期确保它们迅速衰变，减少患者的长期辐射暴露。
9. Hazards of Radiation | 辐射的危害
Ionising radiation can damage living cells by knocking electrons out of atoms, breaking chemical bonds, and disrupting DNA. This can lead to cell death, mutations, or uncontrolled cell division (c
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