Radioactivity & Particles | 放射性与粒子

📚 Radioactivity & Particles | 放射性与粒子

Radioactivity is the spontaneous disintegration of unstable atomic nuclei, accompanied by the emission of ionising radiation. This topic sits at the heart of modern physics and connects atomic structure, nuclear stability, and the fundamental building blocks of matter.

放射性是不稳定原子核自发衰变并伴随发射电离辐射的现象。这一主题处于现代物理学的核心位置，将原子结构、核稳定性与物质的基本组成紧密联系在一起。

1. Atomic Structure | 原子结构

An atom consists of a tiny, dense nucleus surrounded by orbiting electrons. The nucleus contains protons (positively charged) and neutrons (neutral), which together are called nucleons.

原子由一个微小而致密的原子核以及绕核运动的电子组成。原子核中包含带正电荷的质子和不带电的中子，二者统称为核子。

The proton carries a relative charge of +1 and has a mass of approximately 1.67 × 10⁻²⁷ kg. The neutron has almost the same mass but zero charge. The electron has a relative charge of −1 and a mass roughly 1/2000 that of a proton, so the electron contributes almost nothing to the total mass of the atom.

质子带 +1 相对电荷，质量约为 1.67 × 10⁻²⁷ 千克。中子的质量与质子几乎相同，但电荷为零。电子带 −1 相对电荷，质量约为质子的 1/2000，因此电子对原子的总质量几乎没有任何贡献。

Two key numbers define every nuclide: the atomic number Z, which is the number of protons, and the mass number A, which is the total number of protons plus neutrons. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, hence the same Z but different A.

两个关键数字定义每一种核素：原子序数 Z（质子数）和质量数 A（质子数加中子数）。同位素是指同一种元素中具有相同质子数但不同中子数的原子，因此它们的 Z 相同而 A 不同。

Example: ¹²₆C and ¹⁴₆C are isotopes of carbon — both have 6 protons, but carbon-14 has 8 neutrons.

例：¹²₆C 和 ¹⁴₆C 是碳的两种同位素——两者都有 6 个质子，但碳-14 含有 8 个中子。

2. Types of Radiation | 辐射类型

There are three main types of nuclear radiation: alpha particles, beta particles, and gamma rays. Each has distinct properties in terms of charge, mass, ionising ability, and penetrating power.

核辐射主要有三种类型：α 粒子、β 粒子和 γ 射线。每种辐射在电荷、质量、电离能力和穿透力方面都有各自不同的特性。

Property	Alpha (α)	Beta (β)	Gamma (γ)
Nature	Helium nucleus (2p + 2n)	High-speed electron	Electromagnetic wave
Charge	+2	−1	0
Ionising power	Strongest	Moderate	Weakest
Penetration	Stopped by paper / skin	Stopped by ~3 mm aluminium	Reduced by lead / concrete
Speed	~0.05c	Up to 0.9c	Speed of light

Alpha particles are the most highly ionising because of their large charge and mass, but they travel only a few centimetres in air. Because they are so strongly ionising, alpha particles are dangerous if they enter the body through inhalation or ingestion.

α 粒子因电荷和质量较大而具有最强的电离能力，但它们只能在空气中传播几厘米。由于电离能力极强，α 粒子一旦通过吸入或食入进入人体内部会非常危险。

Beta particles are electrons emitted at high speed when a neutron converts into a proton. They are moderately ionising and can pass through skin but are stopped by a few millimetres of aluminium.

β 粒子是中子转变为质子时发射出的高速电子。它们具有中等的电离能力，可以穿透皮肤，但会被几毫米厚的铝板阻挡。

Gamma rays are high-energy electromagnetic radiation emitted when an excited nucleus returns to a lower energy state. They are the most penetrating but least ionising; thick lead or several metres of concrete are needed to significantly reduce their intensity.

γ 射线是激发态原子核回到较低能态时发射的高能电磁辐射。它们穿透力最强但电离能力最弱；需要厚铅板或数米厚的混凝土才能显著减弱其强度。

3. Nuclear Equations | 核方程式

Nuclear equations must conserve both mass number A and atomic number Z on both sides of the arrow. In alpha decay, the parent nucleus loses 4 from its mass number and 2 from its atomic number.

核方程式必须在箭头两侧同时守恒质量数 A 和原子序数 Z。在 α 衰变中，母核的质量数减少 4，原子序数减少 2。

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He (α particle)

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He（α 粒子）

In beta decay, a neutron changes into a proton and an electron is ejected. The atomic number increases by 1 while the mass number stays the same.

在 β 衰变中，一个中子转变为质子并释放一个电子。原子序数增加 1，而质量数保持不变。

¹⁴₆C → ¹⁴₇N + ⁰₋₁e (β particle)

¹⁴₆C → ¹⁴₇N + ⁰₋₁e（β 粒子）

Gamma emission accompanies alpha or beta decay when the daughter nucleus is left in an excited state. It involves no change in either A or Z — the nucleus simply loses excess energy.

当子核处于激发态时，γ 发射通常伴随 α 或 β 衰变而出现。γ 发射不会改变 A 或 Z——原子核只是释放多余的能量。

4. Half-Life | 半衰期

The half-life of a radioactive isotope is the time taken for half of the unstable nuclei to decay, or equivalently, the time taken for the activity (measured in becquerels, Bq) to fall to half its initial value.

放射性同位素的半衰期是指一半不稳定核发生衰变所需的时间，或者等价地，活度（以贝克勒尔 Bq 为单位）降至初始值一半所需的时间。

Radioactive decay is a random process — there is no way to predict which individual nucleus will decay next. However, for a large sample, the decay follows a precise exponential pattern.

放射性衰变是一个随机过程——我们无法预测下一个衰变的是哪个原子核。然而，对于大量原子核组成的样品，衰变遵循精确的指数规律。

After n half-lives, the fraction of undecayed nuclei remaining is given by:

经过 n 个半衰期后，未衰变原子核所占比例为：

Remaining = Initial × (½)ⁿ

剩余量 = 初始量 × (½)ⁿ

For example, if the half-life of iodine-131 is 8 days and you start with 80 g, after 24 days (3 half-lives) only 80 × (½)³ = 10 g remains. A graph of activity against time always produces a smooth exponential decay curve that asymptotically approaches zero.

例如，若碘-131 的半衰期为 8 天，初始质量为 80 克，经过 24 天（3 个半衰期）后，剩余量为 80 × (½)³ = 10 克。活度随时间变化的图像总是呈现一条平滑的指数衰减曲线，逐渐趋近于零。

5. Background Radiation | 背景辐射

Background radiation is the low-level ionising radiation that is always present in the environment. When measuring the activity of a radioactive source, the background count must be measured first and then subtracted from the total count.

背景辐射是环境中始终存在的低水平电离辐射。在测量放射源的活度时，必须先测量背景计数，然后从总计数中减去该值。

The main sources of background radiation include:

背景辐射的主要来源包括：

Radon gas released from rocks and soil, particularly granite

岩石和土壤释放的氡气，尤其是花岗岩中的氡
Cosmic rays from outer space, which interact with the atmosphere

来自外太空的宇宙射线，与大气层相互作用
Natural radioactivity in food and drink, such as potassium-40

食物和饮品中的天然放射性物质，如钾-40
Artificial sources including medical X-rays, nuclear power plants, and fallout from nuclear weapons testing

人工来源，包括医疗 X 射线、核电站以及核武器试验的沉降物

Background radiation levels vary with location and altitude; for instance, they are higher in mountainous areas and at high altitudes due to increased cosmic ray exposure.

背景辐射水平随地点和海拔而变化；例如，高山地区和高海拔地区的背景辐射更高，因为宇宙射线的照射更强。

6. Uses of Radioactivity | 放射性的应用

Despite the hazards, radioactivity has numerous valuable applications in medicine, industry, archaeology, and energy production.

尽管存在危害，放射性在医学、工业、考古和能源生产等领域有着许多宝贵的应用。

In medicine, gamma radiation is used in radiotherapy to destroy cancerous tumours. Since gamma rays kill living cells, a focused beam can shrink a tumour without major surgery. Radioactive tracers, such as iodine-131 for the thyroid or technetium-99m for imaging organs, allow doctors to monitor the function of organs using a gamma camera.

在医学中，γ 辐射被用于放射治疗以摧毁癌性肿瘤。由于 γ 射线能杀死活细胞，聚焦的射线束可以在不进行大手术的情况下缩小肿瘤。放射性示踪剂，如用于甲状腺的碘-131 或用于器官显像的锝-99m，使医生能够通过 γ 相机监测器官功能。

Carbon-14 dating is a famous application in archaeology. Living organisms absorb carbon-14 from the atmosphere; after death, the uptake stops and the ¹⁴C decays with a half-life of 5730 years. By measuring the remaining ¹⁴C fraction, the age of ancient wood, bone, or textiles can be determined.

碳-14 测年是考古学中一项著名的应用。生物体从大气中吸收碳-14；死亡后吸收停止，¹⁴C 以 5730 年的半衰期衰变。通过测量剩余的 ¹⁴C 比例，可以确定古代木材、骨骼或纺织品的年代。

In industry, radioactive sources are used in thickness gauges to monitor the production of paper or metal sheets — the amount of radiation passing through reveals the material’s thickness. Smoke detectors contain a small americium-241 alpha source; smoke particles absorb the alpha particles, triggering the alarm.

在工业中，放射源被用于测厚仪，以监测纸张或金属板材的生产——穿过材料的辐射量可以反映材料的厚度。烟雾探测器内含有一个微小的镅-241 α 放射源；烟雾颗粒吸收 α 粒子，从而触发警报。

Nuclear power stations use the fission of uranium-235 to generate electricity, providing a concentrated energy source with no direct carbon dioxide emissions during operation.

核电站利用铀-235 的裂变来发电，提供高密度能源，且在运行过程中不直接排放二氧化碳。

7. Hazards & Safety | 危害与安全

It is essential to distinguish between contamination and irradiation. Contamination occurs when radioactive material is introduced into or onto an object, person, or the environment. Irradiation occurs when an object is exposed to radiation from a source without physical contact.

必须区分污染与照射。污染是指放射性物质进入或附着在物体、人体或环境中。照射是指物体在没有物理接触的情况下受到来自放射源的辐射。

Alpha particles are especially dangerous when inhaled or ingested because their strong ionising ability can damage DNA and increase the risk of cancer. Beta particles can cause skin burns, while gamma radiation penetrates deep into the body and can damage internal organs.

α 粒子被吸入或食入时尤其危险，因为它们强烈的电离能力会破坏 DNA 并增加癌症风险。β 粒子可造成皮肤灼伤，而 γ 辐射能深入人体内部并损害内脏器官。

Standard safety precautions include:

标准安全预防措施包括：

Using tongs or remote handling tools to keep a safe distance from the source

使用镊子或远程操作工具，与放射源保持安全距离
Minimising exposure time — the total dose is proportional to time spent near the source

尽量减少暴露时间——总剂量与在放射源附近停留的时间成正比
Storing sources in lead-lined containers to absorb radiation

将放射源存放在内衬铅的容器中以吸收辐射
Wearing protective clothing and dosimeter badges to monitor accumulated dose

穿戴防护服并佩戴剂量计徽章以监测累积剂量
Never eating or drinking in the laboratory to prevent ingestion of radioactive material

严禁在实验室饮食，以防止摄入放射性物质

The three fundamental principles of radiation protection are summed up as distance, time, and shielding.

辐射防护的三项基本原则概括为：距离、时间和屏蔽。

8. Particles & The Standard Model | 粒子与标准模型

Modern particle physics describes matter in terms of fundamental particles. These are classified into quarks and leptons, which are the building blocks of all visible matter.

现代粒子物理学用基本粒子来描述物质。基本粒子分为夸克和轻子两大类，它们是所有可见物质的基本组成单元。

Quarks come in six flavours: up (u), down (d), charm (c), strange (s), top (t), and bottom (b). Protons are composed of two up quarks and one down quark (uud), while neutrons consist of one up quark and two down quarks (udd).

夸克有六种“味”：上夸克 (u)、下夸克 (d)、粲夸克 (c)、奇夸克 (s)、顶夸克 (t) 和底夸克 (b)。质子由两个上夸克和一个下夸克（uud）组成，而中子由一个上夸克和两个下夸克（udd）组成。

Leptons include the electron, muon, tau, and their associated neutrinos. The electron is the most familiar lepton and is stable; muons and taus are heavier and decay rapidly. Neutrinos are extremely light, neutral particles that interact very weakly with matter.

轻子包括电子、μ 子、τ 子及其相关的中微子。电子是最常见的轻子且稳定；μ 子和 τ 子质量更大且衰变迅速。中微子是极轻的中性粒子，与物质的相互作用非常微弱。

Hadrons are particles made of quarks. Baryons (such as protons and neutrons) contain three quarks, while mesons contain one quark and one antiquark. Antiparticles have the same mass as their corresponding particles but opposite charge — for example, the positron is the antiparticle of the electron, with charge +1.

强子是由夸克组成的粒子。重子（如质子和中子）包含三个夸克，而介子包含一个夸克和一个反夸克。反粒子与其对应粒子具有相同的质量但相反的电荷——例如，正电子是电子的反粒子，电荷为 +1。

Fundamental forces are mediated by exchange particles: photons carry the electromagnetic force, gluons carry the strong nuclear force, and W and Z bosons carry the weak nuclear force. The strong force holds quarks together inside protons and neutrons and also binds nucleons within the nucleus.

基本力由交换粒子传递：光子传递电磁力，胶子传递强核力，W 和 Z 玻色子传递弱核力。强力将夸克束缚在质子和中子内部，同时将核子束缚在原子核中。

9. Fission & Fusion | 裂变与聚变

Nuclear fission is the splitting of a heavy nucleus, such as uranium-235, into two smaller nuclei when it absorbs a neutron. The reaction releases a large amount of energy and typically produces two or three additional neutrons, which can trigger a chain reaction.

核裂变是指重核（如铀-235）在吸收一个中子后分裂成两个较小的原子核。反应释放大量能量，通常还会产生两到三个额外中子，这些中子可以触发链式反应。

²³⁵₉₂U + ¹₀n → ¹⁴⁴₅₆Ba + ⁹⁰₃₆Kr + 3¹₀n + energy

²³⁵₉₂U + ¹₀n → ¹⁴⁴₅₆Ba + ⁹⁰₃₆Kr + 3¹₀n + 能量

In a nuclear reactor, control rods absorb surplus neutrons to maintain a steady, controlled rate of fission. In an atomic bomb, the chain reaction is left uncontrolled, releasing an enormous amount of energy in a fraction of a second.

在核反应堆中，控制棒吸收多余的中子以维持稳定、可控的裂变速率。在原子弹中，链式反应不受控制，在瞬间释放出巨大的能量。

Nuclear fusion is the process in which two light nuclei combine to form a heavier nucleus. For example, deuterium (²₁H) and tritium (³₁H) fuse to form helium-4 and a neutron:

核聚变是两个轻核结合形成一个较重原子核的过程。例如，氘（²₁H）和氚（³₁H）聚变形成氦-4 和一个中子：

²₁H + ³₁H → ⁴₂He + ¹₀n + energy

²₁H + ³₁H → ⁴₂He + ¹₀n + 能量

Fusion requires extremely high temperatures (millions of degrees) to overcome the electrostatic repulsion between nuclei, and it releases far more energy per kilogram of fuel than fission. Fusion is the process that powers the Sun and stars; achieving controlled fusion on Earth remains a major engineering challenge.

聚变需要极高的温度（数百万度）来克服原子核之间的静电排斥力，并且单位质量燃料释放的能量远大于裂变。聚变是太阳和恒星能量来源；在地球上实现可控聚变仍然是重大的工程挑战。

The energy released in both fission and fusion comes from the conversion of mass into energy, described by Einstein’s famous equation E = mc². The total mass of the products is slightly less than the total mass of the reactants, and this mass defect is converted into kinetic energy.

裂变和聚变释放的能量都来自于质量向能量的转化，由爱因斯坦著名的方程 E = mc² 描述。产物的总质量略小于反应物的总质量，这个质量亏损被转化为动能。

10. Exam Tips | 考试技巧

In the IGCSE Edexcel examination, students often lose marks on nuclear notation and half-life calculations. Always write nuclear equations with both the mass number (superscript) and atomic number (subscript) explicitly shown, and check that both are balanced on both sides of the arrow.

在 Edexcel IGCSE 考试中，学生常在核素符号和半衰期计算上失分。写核方程式时，始终明确标出质量数（上标）和原子序数（下标），并检查箭头两侧是否都守恒。

When comparing alpha, beta, and gamma, quote specific evidence: paper stops alpha, aluminium stops beta, lead reduces gamma.

比较 α、β、γ 时，引用具体证据：纸可阻挡 α，铝可阻挡 β，铅可减弱 γ。
In half-life problems, identify the initial activity, count how many half-lives have passed, and apply (½)ⁿ rather than simply guessing.

在半衰期问题中，确定初始活度，计算经过了多少个半衰期，并应用 (½)ⁿ 而不是随意猜测。
For background radiation questions, remember to subtract the background count from each reading before calculating the corrected activity.

对于背景辐射问题，切记在计算修正活度之前从每个读数中减去背景计数。
When asked about safety, distinguish clearly between contamination and irradiation — this is a common two-mark question.

被问及安全问题时，明确区分污染和照射——这是一个常见的两分题。
Understand how the strong nuclear force overcomes electrostatic repulsion between protons — this explains why nuclei are stable and why fusion needs extreme conditions.

理解强力如何克服质子之间的静电排斥力——这解释了原子核为何稳定，以及聚变为何需要极端条件。

Mastering these core ideas — atomic notation, the properties of the three radiations, half-life, background correction, and the particle model — will give you the confidence to tackle any radioactivity question on the paper.

扎实掌握这些核心概念——核素符号、三种辐射的性质、半衰期、背景修正以及粒子模型——将使你自信地应对试卷上任何放射性相关的问题。

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Radioactivity & Particles | 放射性与粒子