📚 Radioactivity and Nuclear Physics | 放射性与核物理
Radioactivity is a fundamental topic in Edexcel IGCSE Science that explains how unstable atomic nuclei emit radiation. Understanding the structure of the atom, the different types of radiation and their properties, half‑life, and the practical applications and risks of radioactive materials forms a core part of the physics syllabus. This article covers all essential concepts, from nuclear equations to safety precautions, to support your revision.
放射性是爱德思 IGCSE 科学中的一个基础主题，解释了不稳定的原子核如何发出辐射。理解原子结构、不同类型的辐射及其性质、半衰期以及放射性物质的实际应用与风险，是物理课程的核心内容。本文涵盖从核方程到安全防护的所有重要概念，为你的复习提供支持。
1. Structure of the Atom | 原子结构
All matter is made up of atoms, each consisting of a small, dense nucleus surrounded by orbiting electrons. The nucleus contains positively charged protons and neutral neutrons, collectively called nucleons. Electrons, which have a negative charge, travel in discrete energy levels around the nucleus. The number of protons (the atomic number, Z) defines the element, while the total number of protons and neutrons (the mass number, A) determines the isotope. Most of the atom is empty space, and the nucleus is about 10⁻¹⁵ m across, whereas the atom itself is about 10⁻¹⁰ m in diameter.
所有物质都由原子组成，每个原子由一个微小而致密的原子核以及绕核运动的电子构成。原子核包含带正电的质子和不带电的中子，统称为核子。带负电的电子在分立的能级上绕核运动。质子数（原子序数 Z）决定了元素种类，而质子与中子总数（质量数 A）决定了同位素。原子绝大部分是虚空，原子核的直径约为 10⁻¹⁵ 米，而整个原子的直径约为 10⁻¹⁰ 米。
2. Isotopes and Radioactivity | 同位素与放射性
Isotopes are atoms of the same element that have the same number of protons but differ in the number of neutrons. For example, carbon‑12 has six protons and six neutrons, while carbon‑14 has six protons and eight neutrons. Many isotopes are stable, but some have an unstable nucleus. An unstable nucleus will spontaneously decay, emitting radiation in order to become more stable – a process called radioactive decay. The instability arises from an imbalance between the number of protons and neutrons, or from having too much energy.
同位素是指质子数相同但中子数不同的同一种元素的原子。例如，碳‑12 有 6 个质子和 6 个中子，而碳‑14 有 6 个质子和 8 个中子。许多同位素是稳定的，但有些原子核不稳定。不稳定的原子核会自发衰变，通过发出辐射来变得更稳定——这一过程称为放射性衰变。不稳定性来源于质子数和中子数之间的不平衡，或者原子核具有过高的能量。
3. Types of Radiation: Alpha, Beta, Gamma | 辐射类型：α、β、γ射线
There are three main types of nuclear radiation: alpha (α), beta (β), and gamma (γ). An alpha particle is a helium nucleus, consisting of two protons and two neutrons, and has a relative charge of +2. Beta radiation is a fast‑moving electron (charge –1), emitted when a neutron in the nucleus turns into a proton. Gamma radiation is a high‑frequency electromagnetic wave, not a particle, and carries no charge or mass. These radiations differ in penetration power, ionising ability, and behaviour in electric and magnetic fields.
主要有三种核辐射：α射线、β射线和γ射线。α粒子是一个氦核，由两个质子和两个中子组成，相对电荷为 +2。β辐射是一种快速运动的电子（电荷 –1），当核内的一个中子转变为质子时发出。γ辐射是一种高频电磁波，不是粒子，不带电荷也没有质量。这些辐射在穿透能力、电离能力以及在电场和磁场中的行为各不相同。
4. Properties of Alpha, Beta, Gamma | α、β、γ射线特性
Alpha particles are the most ionising but the least penetrating: they can be stopped by a sheet of paper or a few centimetres of air. Beta particles are moderately ionising and can travel a few metres in air, being stopped by a thin sheet of aluminium (about 3 mm). Gamma rays are the least ionising but highly penetrating; they require several centimetres of lead or metres of concrete to be significantly reduced. All three types can be detected by a Geiger‑Müller tube. In an electric field, alpha particles are deflected towards the negative plate, beta particles towards the positive plate, and gamma rays are undeflected.
α粒子电离能力最强但穿透力最弱：一张纸或几厘米厚的空气就能将其挡住。β粒子电离能力中等，在空气中能行进几米，被薄铝片（约 3 毫米）挡住。γ射线电离能力最弱但穿透性极强；需要几厘米厚的铅或几米厚的混凝土才能明显减弱。这三种射线都可以用盖革‑米勒计数管检测。在电场中，α粒子向负极板偏转，β粒子向正极板偏转，γ射线不偏转。
5. Nuclear Decay Equations | 核衰变方程
decay, a neutron changes to a proton and an electron (β⁻) is emitted, so A stays the same and Z increases by 1. Example: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e. Gamma emission usually accompanies alpha or beta decay and does not change A or Z.
放射性衰变可以用配平的核方程表示，总质量数（A）和总原子序数（Z）守恒。在α衰变中，母核失去两个质子和两个中子，因此 A 减少 4，Z 减少 2。例如：²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He。在β⁻衰变中，一个中子变成一个质子和一个电子（β⁻），因此 A 保持不变，Z 增加 1。例如：¹⁴₆C → ¹⁴₇N + ⁰₋₁e。γ辐射通常伴随α或β衰变，不改变 A 和 Z。
6. Half‑Life and Decay Curves | 半衰期与衰变曲线
The half‑life of a radioactive isotope is the time taken for the number of undecayed nuclei (or the count rate) to fall to half of its initial value. Half‑life is constant for a given isotope and cannot be changed by physical or chemical means. By plotting a graph of count rate (or number of nuclei) against time, the half‑life can be read off directly as the time interval between any two points where the count rate halves. Half‑life values range from fractions of a second to billions of years. This concept is crucial for applications such as radioactive dating and medical treatments.
放射性同位素的半衰期是指未衰变原子核数目（或计数率）降至初始值一半所需的时间。对于给定的同位素，半衰期是恒定的，无法通过物理或化学手段改变。通过绘制计数率（或原子核数目）随时间变化的曲线，可以直接读出半衰期，即计数率减半的任意两点间的时间间隔。半衰期从不到一秒到数十亿年不等。这一概念对于放射性测年和医学治疗等应用至关重要。
7. Measuring Radiation | 辐射测量
Radiation is commonly detected using a Geiger‑Müller (GM) tube connected to a counter or rate meter. When radiation enters the tube, it ionises the gas inside, producing a short pulse of current that is counted. The count rate is measured in counts per second (cps) or counts per minute (cpm). For accurate measurement, background radiation must be subtracted. Schools often use sealed radioactive sources with a GM tube to investigate penetration, absorption, and the inverse‑square law for gamma rays. A cloud chamber can also show the tracks of alpha and beta particles.
辐射通常用连接至计数器或计数率仪的盖革‑米勒（GM）管检测。当辐射进入管内时，会使内部气体电离，产生一个短暂的电流脉冲并被计数。计数率以每秒计数（cps）或每分钟计数（cpm）为单位。为了准确测量，必须扣除本底辐射。学校常使用密封放射源和 GM 管研究穿透能力、吸收以及γ射线的平方反比定律。云室也可以显示α和β粒子的径迹。
8. Background Radiation | 背景辐射
Background radiation is the low‑level radiation that we are exposed to continuously. It comes from natural sources such as radon gas from rocks, cosmic rays from space, and radioactive materials in food and soil. Artificial sources include medical X‑rays, nuclear power, and fallout from past nuclear weapons tests. The typical background count rate depends on location and altitude, but is often around 0.5 counts per second. Understanding background radiation is important for interpreting experimental results and assessing radiation risks.
背景辐射是我们持续受到的低水平辐射。它来自天然来源，如岩石释放的氡气、来自太空的宇宙射线，以及食物和土壤中的放射性物质。人工来源包括医用 X 射线、核能以及过去核武器试验的沉降物。典型的背景计数率取决于地点和海拔，但通常约为每秒 0.5 个计数。了解背景辐射对于解释实验结果和评估辐射风险非常重要。
9. Uses of Radioactivity | 放射性的应用
Radioisotopes have many practical applications. Alpha sources like americium‑241 are used in smoke detectors because alpha particles are easily stopped by smoke. Beta emitters such as carbon‑14 are employed in archaeological dating (radiocarbon dating) because the half‑life of about 5730 years is ideal for measuring the age of carbon‑containing materials. Gamma rays from cobalt‑60 are used in radiotherapy to treat cancer, sterilise medical equipment, and detect cracks in pipelines and aircraft parts through industrial radiography. Tracers, often beta or gamma emitters with short half‑lives, are used in medicine to study organ function.
放射性同位素有许多实际应用。α放射源如镅‑241 用于烟雾探测器，因为α粒子容易被烟雾阻挡。β发射体如碳‑14 用于考古定年（放射性碳定年法），因为约 5730 年的半衰期非常合适测定含碳材料的年代。来自钴‑60 的γ射线用于放射治疗治癌、灭菌医疗设备，以及通过工业射线照相检测管道和飞机部件中的裂缝。示踪剂通常为短半衰期的β或γ发射体，在医学上用于研究器官功能。
10. Safety and Dangers | 安全与危害
Ionising radiation can damage living cells, leading to radiation sickness, genetic mutations, or cancer. The risk depends on the type of radiation, the absorbed dose (measured in gray, Gy) and the equivalent dose (measured in sievert, Sv) which accounts for the biological effect. To minimise exposure, the three principles of radiation protection are followed: time (limit the time spent near sources), distance (stay as far away as possible, since intensity decreases with distance), and shielding (use appropriate absorbers such as lead for gamma, thick plastic for beta). Persons handling radioactive materials must wear protective clothing, use tongs, and store sources in lead‑lined containers clearly labelled with the radiation symbol.
电离辐射会损伤活细胞，导致放射病、基因突变或癌症。风险取决于辐射类型、吸收剂量（以戈瑞 Gy 为单位）和考虑生物效应的等效剂量（以希沃特 Sv 为单位）。为了尽量降低照射，遵循辐射防护三原则：时间（限制在放射源附近的停留时间）、距离（尽可能远离，因为强度随距离减弱）和屏蔽（使用适当的吸收体，如铅屏蔽γ射线、厚塑料屏蔽β射线）。处理放射性物质的人员必须穿戴防护服，使用镊子，并将放射源存放在标有辐射符号的铅衬容器内。
11. Nuclear Fission and Fusion | 核裂变与聚变
Nuclear fission is the splitting of a large, unstable nucleus (such as uranium‑235 or plutonium‑239) after absorbing a neutron. The nucleus breaks into two smaller nuclei, releasing two or three more neutrons and a large amount of energy. A controlled chain reaction in a nuclear reactor generates electricity, while an uncontrolled chain reaction produces the explosion of an atomic bomb. Nuclear fusion is the joining of two light nuclei, such as hydrogen isotopes, to form a heavier nucleus (helium) with the release of energy. Fusion powers stars but is very difficult to achieve on Earth because it requires extremely high temperatures and pressures to overcome the electrostatic repulsion between nuclei.
核裂变是大而不稳定的原子核（如铀‑235 或钚‑239）吸收一个中子后分裂的过程。原子核分裂成两个较小的核，同时释放两到三个中子及大量能量。核反应堆中的受控链式反应用于发电，而不受控的链式反应导致原子弹爆炸。核聚变是两个轻核（如氢的同位素）结合形成一个较重的核（氦）并释放能量的过程。聚变是恒星的能源，但在地球上很难实现，因为它需要极高的温度和压力来克服原子核间的静电排斥。
12. Comparing Fission and Fusion | 裂变与聚变的比较
and fusion release energy, but the conditions differ significantly. Fission produces long‑ ions and environmental implications. A typical fission equation: ²³⁵₉₂U + ¹₀n → ⁹²₃₆Kr + ¹⁴¹₅₆Ba + 3¹₀n + energy. A fusion example: ²₁H + ³₁H → ⁴₂He + ¹₀n + energy.
裂变与聚变都释放能量，但条件显著不同。裂变会产生长寿命放射性废物，并需要重核裂变材料。聚变产生的放射性废物少得多，使用氘和氚等丰富燃料，但目前在实际反应堆中，引发聚变所需的能量大于释放的能量。在 IGCSE 课程中，你需要了解基本方程和环境影响。典型的裂变方程：²³⁵₉₂U + ¹₀n → ⁹²₃₆Kr + ¹⁴¹₅₆Ba + 3¹₀n + 能量。聚变示例：²₁H + ³₁H → ⁴₂He + ¹₀n + 能量。
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