📚 Radioactive Decay for GCSE CCEA Physics | GCSE CCEA 物理：放射性衰变 考点精讲
Radioactive decay is a spontaneous process in which an unstable atomic nucleus loses energy by emitting radiation. For the CCEA GCSE Physics specification, you need to understand the three main types of radiation – alpha, beta and gamma – as well as half-life, background radiation, detection methods, nuclear equations and the uses and dangers of radioactivity. This guide breaks down every key point with clear English–Chinese pairs to help you revise effectively.
放射性衰变是一种不稳定的原子核通过发出辐射来损失能量的自发过程。根据 CCEA GCSE 物理大纲，你需要掌握三种主要的辐射类型——α、β 和 γ——以及半衰期、背景辐射、探测方法、核方程以及放射性的应用与危害。本文用清晰的中英文对照分解每一个关键点，帮助你高效复习。
1. What is Radioactivity? | 什么是放射性？
Radioactivity is the spontaneous emission of particles or high-energy electromagnetic waves from an unstable nucleus. This process is random and cannot be influenced by external conditions such as temperature or pressure. The nucleus becomes more stable after emitting radiation.
放射性是指不稳定的原子核自发地发射粒子或高能电磁波的过程。这一过程是随机的，不受温度或压力等外部条件影响。原子核在发出辐射后变得更加稳定。
An unstable nucleus contains either too many neutrons, too many protons, or has too much energy. Decay continues until a stable isotope is formed. The rate of decay is measured by the activity, which is the number of decays per second, with the unit becquerel (Bq).
不稳定的原子核要么含有过多的中子，要么含有过多的质子，要么能量过高。衰变会一直持续，直到形成稳定的同位素。衰变速率用活度来衡量，即每秒衰变的次数，单位是贝克勒尔（Bq）。
2. Alpha Decay (α) | α 衰变
Alpha decay occurs when an unstable nucleus emits an alpha particle. An alpha particle is identical to a helium nucleus, consisting of two protons and two neutrons (⁴₂He). Alpha radiation has a relative charge of +2e and a mass number of 4.
α 衰变发生时，不稳定的原子核放出一个 α 粒子。α 粒子等同于一个氦核，由两个质子和两个中子组成（⁴₂He）。α 辐射的相对电荷为 +2e，质量数为 4。
Because alpha particles are relatively large and heavy, they are highly ionising – they can knock electrons out of atoms over a short distance. However, they have low penetrating power and can be stopped by a sheet of paper or a few centimetres of air.
由于 α 粒子相对较大、较重，它们的电离能力很强——能在很短距离内将电子从原子中击出。但它们的穿透能力很弱，可以被一张纸或几厘米的空气阻挡。
²²₈₆Rn + ⁴₂He. Note that the atomic number decreases by 2 and the mass number decreases by 4.
α 衰变的一个例子是镭-226 衰变为氡-222：²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He。注意原子序数减少 2，质量数减少 4。
3. Beta Decay (β) | β 衰变
Beta decay occurs when a neutron in the nucleus turns into a proton while emitting a beta particle – a high-speed electron (⁰₋₁e). The mass number stays the same but the atomic number increases by 1. Beta particles are much lighter than alpha particles and have a charge of -1e.
β 衰变发生时，原子核中的一个中子转变为一个质子，同时放出一个 β 粒子——一个高速电子（⁰₋₁e）。质量数保持不变，但原子序数增加 1。β 粒子比 α 粒子轻得多，电荷为 -1e。
nium. In the nucleus, the process can be represented as: n → p + e⁻ + antineutrino (the antineutrino is not required for the decay equation in this specification).
β 粒子具有中等的电离能力和穿透能力。它们在空气中能穿行几米，可被几毫米厚的铝阻挡。在原子核中，这个过程可以表示为：n → p + e⁻ + 反中微子（在该大纲的衰变方程中不要求写出反中微子）。
₆C → ¹⁴₇N + ⁰₋₁e. The mass number remains 14, but the atomic number increases from 6 to 7.
一个例子是碳-14 衰变为氮-14：¹⁴₆C → ¹⁴₇N + ⁰₋₁e。质量数仍为 14，但原子序数从 6 增加到 7。
4. Gamma Radiation (γ) | γ 辐射
Gamma radiation is an electromagnetic wave of very high frequency and short wavelength. It is not a particle and has no mass or charge. Gamma rays are often emitted alongside alpha or beta particles when the daughter nucleus still has excess energy.
γ 辐射是一种频率极高、波长极短的电磁波。它不是粒子，没有质量也不带电荷。γ 射线通常伴随 α 或 β 粒子一起发射，此时子核仍具有多余的能量。
Gamma rays are weakly ionising but highly penetrating. They require thick lead or several metres of concrete to be absorbed. Unlike alpha and beta, gamma emission does not change the atomic number or mass number of the nucleus.
γ 射线的电离能力弱，但穿透能力极强。需要厚铅板或几米厚的混凝土才能将其吸收。与 α 和 β 不同，γ 辐射不会改变原子核的原子序数或质量数。
itting a gamma photon: e.g. ⁶⁰₂₇Co* → ⁶⁰₂₇Co + γ. The asterisk indicates an excited state.
在原子核中，γ 辐射可用符号 ⁰₀γ 表示。一个处于激发态的核通过发出 γ 光子失去能量：例如 ⁶⁰₂₇Co* → ⁶⁰₂₇Co + γ。星号表示激发态。
5. Comparing Alpha, Beta and Gamma | 比较 α、β 和 γ
It is essential to compare the three types of radiation in terms of their nature, ionising power, penetrating power, range in air, and ability to be deflected by electric and magnetic fields. The table below summarises these properties.
必须从本质、电离能力、穿透能力、在空气中的射程以及被电场和磁场偏转的能力等方面比较这三种辐射。下面的表格总结了这些性质。
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus (⁴₂He) | High-speed electron (⁰₋₁e) | Electromagnetic wave |
| Charge | +2e | -1e | 0 |
| Ionising power | Very high | Medium | Very low |
| Penetrating power | Stopped by paper or few cm of air | Stopped by a few mm of aluminium | Reduced by thick lead or concrete |
| Range in air | 2-5 cm | 1-3 metres | Several hundred metres (follows inverse-square law) |
| Deflection in electric/magnetic field | Slightly deflected towards negative plate (opposite to field lines) | Strongly deflected towards positive plate | No deflection |
Knowing these differences helps in selecting suitable radiation for applications and in interpreting the results of experiments, such as absorption tests or cloud chamber tracks.
了解这些差异有助于选择合适的辐射进行应用，以及解释实验的结果，如吸收试验或云室径迹。
6. Nuclear Decay Equations | 核衰变方程
Nuclear equations show the changes in the atomic number (subscript) and mass number (superscript) during a decay. The sum of mass numbers on both sides must be equal, and the sum of atomic numbers must also be equal. This reflects the conservation of mass-energy and charge.
核方程表示了衰变过程中原子序数（下标）和质量数（上标）的变化。两边的质量数之和必须相等，原子序数之和也必须相等。这体现了质能守恒和电荷守恒。
- Alpha decay: parent nucleus → daughter nucleus + ⁴₂He. Example: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He.
- Beta decay: parent nucleus → daughter nucleus + ⁰₋₁e. Example: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e.
- Gamma emission: excited nucleus → same nucleus + γ. No change in atomic or mass number.
- α 衰变：母核 → 子核 + ⁴₂He。例如：²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He。
- β 衰变：母核 → 子核 + ⁰₋₁e。例如：¹⁴₆C → ¹⁴₇N + ⁰₋₁e。
- γ 辐射：激发态核 → 相同核 + γ。原子序数和质量数不改变。
When balancing nuclear equations, always check that the total mass number (top) and total atomic number (bottom) are the same on both sides. This skill is frequently tested in CCEA exams.
在平衡核方程时，一定要检查两边的总质量数（上标）和总原子序数（下标）是否相等。这一技能在 CCEA 考试中经常考查。
7. Half-Life | 半衰期
The half-life of a radioactive isotope is the time taken for half of the unstable nuclei in a sample to decay, or equivalently, for the activity of the sample to fall to half its initial value. It is a constant for a given isotope and cannot be changed by physical or chemical means.
放射性同位素的半衰期是指样本中一半的不稳定原子核衰变所需的时间，或者说样本的活度降至初始值一半所需的时间。对于给定的同位素，半衰期是一个常数，不能通过物理或化学方法改变。
tial × (½)ⁿ, where n is the number of half-lives. Graphs of activity against time show an exponential decay curve, and half-life can be read directly from such graphs.
半衰期可用来计算经过一定数量半衰期后样本剩余的质量或活度，公式为：剩余量 = 初始量 × (½)ⁿ，其中 n 是半衰期的个数。活度-时间图呈现指数衰减曲线，半衰期可以直接从该图中读取。
00 × (½)³ = 100 Bq. CCEA questions often involve calculating the fraction remaining or the time taken for a certain decay level.
例如，如果一个碘-131 样本的初始活度为 800 Bq，半衰期为 8 天，那么 24 天（3 个半衰期）后，活度将为 800 × (½)³ = 100 Bq。CCEA 的考题通常会涉及计算剩余的比例或达到某一衰变水平所需的时间。
8. Background Radiation | 背景辐射
Background radiation is the low-level radiation that is always present in the environment. It comes from natural sources such as cosmic rays from space, radon gas from the ground, rocks and soil containing radioactive isotopes, and from living things (e.g. carbon-14). There is also a small contribution from man-made sources like medical X-rays and nuclear power.
背景辐射是环境中始终存在的低水平辐射。它来自自然来源，如来自太空的宇宙射线、从地面释放的氡气、含有放射性同位素的岩石和土壤，以及生物体（如碳-14）。还有一小部分来自人造来源，如医疗 X 射线和核能。
Measuring background radiation is important because it must be subtracted from experimental readings when investigating radioactive sources. The unit of radiation dose is the sievert (Sv), but often millisievert (mSv) is used. The average annual background dose in the UK is about 2.5 mSv.
测量背景辐射很重要，因为在研究放射源时，必须将背景辐射从实验读数中减去。辐射剂量的单位是希沃特（Sv），但常用毫希沃特（mSv）。英国年平均背景辐射剂量大约为 2.5 mSv。
9. Detection of Radioactivity | 放射性的探测
Two main methods detect radioactivity for GCSE: the Geiger-Müller (GM) tube and photographic film. The GM tube detects the ionising effect of radiation. Each particle or gamma photon entering the tube causes a pulse of current, which is counted by a counter or ratemeter. The count rate is usually measured in counts per minute (cpm) or counts per second.
GCSE 阶段探测放射性的方法主要有两种：盖革-米勒（GM）管和照相胶片。GM 管检测辐射的电离效应。每个进入管的粒子或 γ 光子会产生一个电流脉冲，由计数器或速率计进行计数。计数率通常以每分钟计数（cpm）或每秒计数表示。
Photographic film darkens when exposed to radiation. It is used in film badges worn by radiation workers to monitor their cumulative dose. Different materials in the badge help distinguish between types of radiation.
照相胶片在受到辐射照射时会变黑。辐射工作人员佩戴的胶片徽章就使用这种方法来监测他们的累积剂量。徽章中不同的材料有助于区分辐射的类型。
A cloud chamber can also be used to show the tracks of alpha and beta particles. Alpha particles produce short, thick tracks; beta particles produce longer, thinner tracks; gamma rays usually produce no visible track but may release electrons that create faint tracks.
云室也可用于显示 α 和 β 粒子的径迹。α 粒子产生短而粗的径迹；β 粒子产生较长、较细的径迹；γ 射线通常不产生可见径迹，但可能会释放电子从而形成微弱的径迹。
10. Uses of Radioactivity | 放射性的应用
Radioactive isotopes have many practical uses based on their type of radiation and half-life. In medicine, gamma-emitting isotopes such as technetium-99m are used as tracers to produce images of organs. The gamma rays can be detected externally because they penetrate the body. The short half-life (6 hours) minimises the radiation dose to the patient.
放射性同位素根据其辐射类型和半衰期有许多实际应用。在医学上，像锝-99m 这样的 γ 放射源被用作示踪剂来生成器官图像。γ 射线可以穿透身体，从而在体外被探测到。其短半衰期（6 小时）可使患者受到的辐射剂量最小化。
In radiotherapy, cobalt-60 emits gamma rays to destroy cancer cells. Beta emitters, such as phosphorus-32, can be used to treat certain types of cancer directly inside the body. Alpha emitters like americium-241 are used in smoke alarms – the alpha particles ionise air, and smoke particles disrupt the current, triggering the alarm.
在放疗中，钴-60 发出 γ 射线来摧毁癌细胞。β 放射源，如磷-32，可用于直接在体内治疗某些癌症。α 放射源如镅-241 被用于烟雾报警器中——α 粒子使空气电离，烟雾粒子干扰电流，从而触发警报。
In industry, beta sources are used for thickness monitoring of paper or metal foil. If the amount of beta radiation passing through changes, the material is too thick or too thin. Gamma sources are used to inspect welds and metal castings for cracks, similar to X-ray imaging.
在工业上，β 放射源被用于监测纸张或金属箔的厚度。如果穿透的 β 辐射量发生变化，就意味着材料过厚或过薄。γ 放射源则用于检查焊缝和金属铸件中的裂纹，类似于 X 射线成像。
Carbon-14 dating is used to estimate the age of ancient organic materials. The half-life of carbon-14 is about 5730 years, making it suitable for dating objects up to about 60 000 years old.
碳-14 测年法用于估算古代有机物质的年龄。碳-14 的半衰期约为 5730 年，这使其适用于测定最远约 60000 年的物体。
11. Dangers and Safety Precautions | 危害与安全防护
Ionising radiation can damage living cells by breaking chemical bonds and creating ions. High doses can cause radiation sickness, while low doses over a long time increase the risk of cancer. Alpha particles are especially dangerous if ingested or inhaled because they are highly ionising and all their energy is absorbed by a small volume of tissue.
电离辐射可以通过破坏化学键和产生离子来损伤活细胞。高剂量可导致辐射病，而长期低剂量照射会增加患癌风险。α 粒子如果被摄入或吸入，尤其危险，因为它们电离能力很强，所有能量都会被一小块组织吸收。
Safety precautions include: using shielding appropriate to the radiation type (lead for gamma, aluminium for beta), minimising exposure time, increasing distance from the source (intensity follows inverse-square law), and handling sources with remote tools such as tongs. Never point a source at a person and store sources in labelled, shielded containers.
安全防护措施包括：使用与辐射类型相适应的屏蔽（γ 射线用铅，β 辐射用铝），尽量减少照射时间，增加与源的距离（强度遵循平方反比定律），以及用钳子等远程工具处理源。切勿将源指向人，并将源存放在贴有标签的屏蔽容器中。
Radioactive waste must be stored securely for many half-lives until its activity falls to a safe level. High-level waste from nuclear reactors may need to be stored for thousands of years.
放射性废物必须安全存放许多个半衰期，直到其活度降至安全水平。来自核反应堆的高放废物可能需要存放数千年。
12. Summary and Revision Tips | 总结与复习建议
Mastering radioactive decay for CCEA GCSE Physics requires you to be confident in describing the three types of radiation, writing balanced nuclear equations, performing half-life calculations, explaining applications and dangers, and interpreting detection experiments. Practice drawing and reading decay graphs, and always remember to subtract background count rate from your measurements.
要掌握 CCEA GCSE 物理的放射性衰变，你需要能够自如地描述三种辐射类型，书写平衡的核方程，进行半衰期计算，解释应用与危害，并理解探测实验。多练习绘制和阅读衰变图，并始终记得从测量值中减去背景计数率。
Use the table of properties to compare alpha, beta and gamma quickly. When revising half-life, work through several examples of calculating remaining activity or mass, and be comfortable finding the half-life from a graph by choosing the time taken for the count rate to halve. Ensure you can give both an advantage and a disadvantage for a particular isotope in a given use.
使用性质表快速比较 α、β 和 γ。复习半衰期时，多做几个计算剩余活度或质量的例题，并熟练地从图中找出计数率减半所需的时间来求半衰期。确保你能针对特定用途给出同位素的一个优点和一个缺点。
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