📚 Radioactivity and Nuclear Physics for Edexcel IGCSE Science | Edexcel IGCSE 科学：放射性与核物理
Radioactivity is a key topic in the Edexcel IGCSE Science (Double Award and Separate Physics) specification. It explores the structure of the atom, the nature of alpha, beta and gamma radiation, the concepts of half-life and background radiation, and the many uses of radioactive substances in medicine and industry. Mastering this unit requires a clear understanding of nuclear equations, detection methods, and safety precautions.
放射性是 Edexcel IGCSE 科学（双奖和独立物理）课程中的关键主题。它探讨原子结构、α、β 和 γ 辐射的性质、半衰期和背景辐射的概念，以及放射性物质在医学和工业中的多种用途。掌握本单元需要清楚地理解核反应方程式、探测方法和安全防护措施。
1. Atomic Structure and Isotopes | 原子结构和同位素
All matter is made of atoms, each consisting of a central nucleus containing protons and neutrons, surrounded by electrons in shells. Protons carry a positive charge, electrons carry a negative charge, and neutrons are neutral. The number of protons (atomic number) defines the element; the total of protons and neutrons gives the mass number.
所有物质都由原子构成，每个原子由包含质子和中子的中央原子核以及分层排布的电子组成。质子带正电荷，电子带负电荷，中子不带电。质子数（原子序数）决定了元素种类；质子数和中子数的总和就是质量数。
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. For example, carbon-12 (126C) and carbon-14 (146C) are isotopes. Some isotopes are unstable and eventually decay, giving out radiation – these are radioisotopes.
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 (126C) 和碳‑14 (146C) 就是同位素。有些同位素不稳定，最终会发生衰变并释放出辐射——这些就是放射性同位素。
2. What Is Radioactivity? | 什么是放射性？
Radioactivity is the spontaneous emission of particles or electromagnetic waves from the unstable nucleus of an atom. The nucleus undergoes radioactive decay to become more stable, often changing into a different element. This process is entirely random – we cannot predict when a particular nucleus will decay, but we can describe statistical patterns for large numbers of nuclei.
放射性是指不稳定原子核自发地发射粒子或电磁波的现象。原子核通过放射性衰变变得更加稳定，常常会变成另一种元素。这一过程完全是随机的——我们无法预测某个特定核何时衰变，但可以描述大量原子核的统计规律。
The three main types of radiation studied in Edexcel IGCSE are alpha (α), beta minus (β⁻) and gamma (γ) radiation. Each has a distinct nature, charge, penetration ability, and ionising power.
Edexcel IGCSE 中学习的三种主要辐射是 α 粒子、β⁻ 粒子和 γ 射线。它们各有不同的本质、电荷、穿透能力和电离本领。
3. Types of Radiation: Alpha, Beta and Gamma | 辐射类型：α、β 和 γ
An alpha particle (α) is a helium nucleus, consisting of 2 protons and 2 neutrons. It is written as 42He and carries a +2 charge. Alpha particles have a large mass (relative atomic mass 4) and are heavily ionising.
α 粒子是一个氦核，由 2 个质子和 2 个中子组成。它写作 42He，带 +2 电荷。α 粒子质量较大（相对原子质量为 4），电离能力很强。
A beta minus particle (β⁻) is a fast-moving electron emitted from the nucleus when a neutron turns into a proton. It is represented as 0-1e. Beta particles have a much smaller mass than alpha and carry a −1 charge. They are moderately ionising.
β⁻ 粒子是快电子，当中子转变为质子时从核内释放出来。它的符号是 0-1e。β 粒子的质量比 α 粒子小得多，带 −1 电荷，具有中等的电离能力。
Gamma radiation (γ) is not a particle but a high‑frequency electromagnetic wave released after alpha or beta decay. Gamma rays have no mass and no charge. They are weakly ionising but highly penetrating.
γ 辐射不是粒子，而是在 α 或 β 衰变后释放的高频电磁波。γ 射线无质量、无电荷，电离能力很弱，但穿透力极强。
4. Penetrating Power and Ionisation | 穿透力和电离能力
Ionisation is the process of removing electrons from atoms, creating charged ions. The more ionising a radiation is, the more energy it loses per collision and the shorter its range in materials. Penetrating power is inversely related to ionising ability.
电离是指从原子中移走电子，产生带电离子的过程。辐射的电离能力越强，每次碰撞损失的能量就越多，在物质中的射程也就越短。穿透力与电离能力呈反比关系。
| Property | Alpha (α) | Beta (β⁻) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus | Electron | EM wave |
| Charge | +2 | −1 | 0 |
| Penetration | Stopped by paper or a few cm of air | Stopped by a few mm of aluminium | Reduced by thick lead or concrete; never fully stopped |
| Ionising power | Very high | Moderate | Low |
Alpha particles can be stopped by a sheet of paper or even the outer layer of skin, but they are very dangerous if ingested or inhaled because they cause severe local damage. Beta particles require aluminium shielding. Gamma rays need thick lead or concrete and even then some may still get through.
α 粒子可以被一张纸甚至皮肤的表皮挡住，但一旦被吸入或摄入则极其危险，因为它们会造成严重的局部损伤。β 粒子需要用铝板防护，而 γ 射线需要很厚的铅板或混凝土，即便如此仍会有一部分穿透。
5. Writing Nuclear Equations | 书写核反应方程式
In alpha and beta decay, both mass number and atomic number must be conserved. For alpha decay, the nucleus loses 2 protons and 2 neutrons, so the mass number decreases by 4 and the atomic number by 2.
在 α 衰变和 β 衰变中，质量数和原子序数都必须守恒。α 衰变时，原子核失去 2 个质子和 2 个中子，因此质量数减少 4，原子序数减少 2。
23892U → 23490Th + 42He
For beta minus decay, a neutron changes into a proton and an electron (beta particle) is ejected. The atomic number increases by 1, but the mass number stays the same.
对于 β⁻ 衰变，一个中子变成质子，同时释放出一个电子（β 粒子）。原子序数增加 1，而质量数保持不变。
146C → 147N + 0-1e
Gamma decay does not change the mass or atomic numbers; it simply carries away excess energy, often written alongside the daughter nucleus.
γ 衰变不改变质量数或原子序数；它只是带走多余的能量，通常与子核一起表示。
6. Half-Life and Decay Calculations | 半衰期和衰变计算
The half-life of a radioisotope is the time taken for half of the unstable nuclei in a sample to decay, or equivalently, for the count rate to fall to half its initial value. It is a constant for a given isotope and is unaffected by temperature, pressure or chemical state.
放射性同位素的半衰期是指样本中一半不稳定原子核发生衰变所需的时间，或者说计数率下降到初始值一半所用的时间。半衰期是给定同位素的常数，不受温度、压力或化学状态的影响。
You can determine half-life from decay graphs. From a curve of activity against time, read the time for the activity to drop from, say, 800 Bq to 400 Bq, then from 400 Bq to 200 Bq – the time interval should be the same. If a substance has a half-life of 6 hours and starts with 80 g, after 18 hours (three half-lives) only 10 g remain.
你可以通过衰变图确定半衰期。在活度对时间的关系曲线上，读出活度从例如 800 Bq 下降到 400 Bq 的时间，再从 400 Bq 下降到 200 Bq 的时间——两个时间间隔应相同。若某物质的半衰期为 6 小时，初始质量为 80 g，则经过 18 小时（三个半衰期）后仅剩 10 g。
Background count must always be subtracted from measured counts to obtain the corrected count rate, ensuring accurate half-life determinations.
始终要从测量计数中扣除本底计数，得到校正后的计数率，以确保半衰期的测定准确。
7. Background Radiation | 背景辐射
Background radiation is the low-level radiation that constantly surrounds us. Its sources include cosmic rays from outer space, radioactive rocks in the ground (e.g. granite containing uranium), radon gas released from the soil, and even certain foods (e.g. bananas contain potassium‑40). Medical procedures such as X‑rays and nuclear medicine also contribute to our annual dose.
本底辐射是始终存在于我们周围的低水平辐射。其来源包括来自外层空间的宇宙射线、地表放射性岩石（如含铀的花岗岩）、土壤释放的氡气，甚至某些食物（例如香蕉含有钾‑40）。X 射线和核医学等医疗程序也会增加我们的年辐射剂量。
In the UK, the average annual dose from natural background radiation is about 2.4 mSv. Radiation dose is measured in sieverts (Sv), but millisieverts (mSv) are more practical for typical exposure levels.
在英国，天然本底辐射的年平均剂量约为 2.4 mSv。辐射剂量用希沃特（Sv）计量，但对于典型照射水平，更常用毫希沃特（mSv）。
8. Uses of Radioactivity | 放射性的应用
Radioisotopes have many practical applications in medicine, industry and research. In medicine, gamma emitters such as technetium‑99m are used as tracers to image organs; because gamma rays have good penetration and can be detected outside the body, while the isotope has a short half-life of about 6 hours, minimising the patient's exposure.
放射性同位素在医学、工业和研究中有许多实际应用。在医学上，锝‑99m 等 γ 放射源被用作示踪剂来对器官成像；由于 γ 射线穿透性好，可以在体外被探测到，而该同位素的半衰期只有大约 6 小时，因此能最大限度地减少患者的辐射暴露。
Radiotherapy uses concentrated beams of gamma rays (or sometimes beta sources applied directly) to destroy cancer cells. In industry, beta sources are employed to measure the thickness of paper or metal foil: a detector monitors how much radiation passes through; if the reading drops, the material is too thick.
放射治疗利用集中的 γ 射线束（有时直接使用 β 源）来杀灭癌细胞。工业上，β 源被用于测量纸张或金属箔的厚度：探测器监测有多少辐射穿过材料；若读数下降，则说明材料过厚。
Alpha sources are used in smoke detectors: the alpha particles ionise air, creating a small current; smoke particles absorb the ions, reducing the current and triggering the alarm. Carbon‑14 dating is another well‑known application for determining the age of archaeological samples containing organic material.
α 源用于烟雾探测器：α 粒子使空气电离，产生微小电流；烟雾颗粒吸收离子，使电流减小，从而触发警报。碳‑14 年代测定是另一个著名应用，用来确定含有机物质的考古样品的年龄。
9. Safety Precautions | 安全防护措施
Handling radioactive sources requires strict safety measures to minimise exposure. The three key principles are:
处理放射源需要严格的安全措施，以尽量减少暴露。三个关键原则是：
- Time: keep the exposure time as short as possible. 时间：尽可能缩短照射时间。
- Distance: increase distance from the source; radiation intensity decreases with the square of the distance. 距离：增大与放射源的距离；辐射强度与距离的平方成反比。
- Shielding: use appropriate absorbers (paper for α, aluminium for β, lead for γ). 屏蔽：使用适当的吸收材料（纸挡 α，铝挡 β，铅挡 γ）。
Additionally, sources should never be held with bare hands; use long-handled tongs. Radioactive waste must be stored in lead-lined containers and clearly labelled.
此外，绝不可徒手触摸放射源，而要使用长柄镊子。放射性废物必须存放在铅衬里的容器中，并有清晰标识。
In schools, sealed sources are used and students must always wear film badges or dosimeters to monitor received dose.
在学校里，使用的是密封源，学生必须始终佩戴胶片徽章或剂量计来监测所受剂量。
10. Detecting Radiation | 探测辐射
The most common instrument for detecting radiation is the Geiger–Müller (GM) tube connected to a counter or rate meter. When radiation enters the tube, it ionises the gas inside, creating a pulse of current that is registered as a count.
最常用的辐射探测仪器是盖革‑米勒（GM）计数管，连接到计数器或率表上。当辐射进入计数管时，它会使管内气体电离，产生一个电流脉冲，记录为一次计数。
Photographic film can also detect radiation: it darkens when exposed, and film badges are worn by workers in nuclear facilities to monitor cumulative doses. Cloud chambers provide a visual display of tracks – alpha particles produce short, thick tracks, beta particles give longer, thinner ones, and gamma rays show faint, scattered tracks.
照相胶片也能探测辐射：曝光后会变黑，核设施的工作人员佩戴胶片徽章来监测累积剂量。云室可以直观地展示粒子径迹——α 粒子产生短而粗的轨迹，β 粒子产生较长、较细的轨迹，而 γ 射线则显示微弱、分散的径迹。
11. Nuclear Fission and Fusion | 核裂变与核聚变
Nuclear fission is the splitting of a large, unstable nucleus (e.g. uranium‑235) when it absorbs a neutron. The nucleus breaks into two smaller nuclei, releasing two or three more neutrons and a large amount of energy in the form of kinetic energy of the fragments and gamma radiation. These neutrons can trigger further fission events, leading to a chain reaction.
核裂变是指一个大的不稳定原子核（如铀‑235）在吸收一个中子后发生分裂。原子核分裂成两个较小的原子核，同时释放出两到三个中子以及巨大的能量，能量以碎片动能和 γ 辐射的形式出现。这些中子可以引发更多的裂变事件，形成链式反应。
Nuclear fusion is the joining of two light nuclei, such as hydrogen isotopes, to form a heavier nucleus (helium). This process releases far more energy than fission and is the source of the Sun's energy. Fusion requires extremely high temperature and pressure to overcome the electrostatic repulsion between the positive nuclei.
核聚变是两个轻核（例如氢的同位素）结合成一个较重核（氦）的过程。这一过程释放的能量远大于裂变，是太阳能量的来源。聚变需要极高的温度和压力，以克服带正电荷的原子核之间的静电排斥力。
ergy because the mass of the products is slightly less than the mass of the reactants; the 'lost' mass is converted into energy according to the equation E = mc2.
裂变和聚变都能释放能量，因为产物的总质量略小于反应物的总质量；‘丢失’的质量根据公式 E = mc2 转化为能量。
12. Key Equations and Graphs for Revision | 复习关键方程和图表
There is no universal 'decay equation' to memorise beyond the nuclear equations shown in Section 5, but you must be confident in representing alpha, beta and gamma emissions. Remember that the emission of a gamma ray does not change A or Z.
除了第 5 节中展示的核反应式，没有其他必须记忆的通用“衰变方程”，但你必须能够熟练表示 α、β 和 γ 发射。记住，发射 γ 射线不改变 A 或 Z。
-lives = total time / half-life. The remaining mass or count rate is initial value × (1/2)n, where n is the number of half-lives. Be ready to read half-life from a graph and to sketch decay curves.
半衰期问题常常涉及一个简单的公式：半衰期次数 = 总时长 / 半衰期。剩余质量或计数率 = 初始值 × (1/2)n，其中 n 为半衰期次数。要准备好根据图表读取半衰期，并绘制衰变曲线。
The inverse square law for gamma radiation intensity is sometimes required: intensity ∝ 1/d2, where d is distance from the source. This can be checked experimentally with a GM tube and a gamma source.
有时会用到 γ 射线强度的平方反比定律：强度 ∝ 1/d2，其中 d 是离源的距离。这可以通过 GM 计数管和 γ 源进行实验验证。
Finally, always subtract the background count from your raw readings before plotting graphs or calculating half-life.
最后，在绘制图表或计算半衰期之前，务必从原始读数中扣除本底计数。
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