📚 Atomic, Nuclear and Particle Physics | 原子、核与粒子物理
From the discovery of the electron to the Higgs boson, the subatomic world has continuously challenged and refined our understanding of nature. This article reviews the key concepts of IB Physics Topic 7 – Atomic, Nuclear and Particle Physics – including atomic structure, radioactivity, nuclear energy and the Standard Model. Each section pairs English and Chinese explanations to support bilingual learners.
从电子的发现到希格斯玻色子的证实，亚原子世界不断挑战并完善着我们对自然的认识。本文梳理了 IB 物理专题7——原子、核与粒子物理的核心概念，涵盖原子结构、放射性、核能以及粒子物理标准模型。每部分均采用中英双语对照，以帮助双语学习者加深理解。
1. Structure of the Atom | 原子结构
Rutherford's gold foil experiment revealed that the atom consists of a tiny, dense, positively charged nucleus surrounded by electrons. Most of the atom is empty space, which explained why most alpha particles passed straight through the foil.
卢瑟福的金箔实验揭示了原子由微小、致密、带正电的原子核及环绕的电子组成。原子内绝大部分是空的，这解释了为什么大多数 α 粒子径直穿过金箔。
of specific energies. The energy of a photon is given by ΔE = hf = hc/λ, where h is Planck's constant, f is frequency and λ is wavelength.
随后，玻尔提出电子处于分立的能级上，只能通过吸收或发射特定能量的光子在能级间跃迁。光子能量由 ΔE = hf = hc/λ 给出，其中 h 为普朗克常数，f 为频率，λ 为波长。
ΔE = hf = hc/λ
Emission spectra consist of bright lines corresponding to electron transitions to lower energy levels, while absorption spectra show dark lines where photons are absorbed to raise electrons to higher levels. These line spectra are unique for each element.
发射光谱由亮线组成，对应于电子向低能级跃迁；吸收光谱则显示暗线，对应于光子被吸收将电子激发到高能级。这些线状光谱是每种元素的独特指纹。
2. Radioactive Decay and Half-life | 放射性衰变与半衰期
ma (γ) radiation. The decay is random and spontaneous, governed by the decay law: N = N₀ e–λt, where λ is the decay constant.
不稳定原子核发生放射性衰变，释放 α 粒子、β⁻ 或 β⁺ 粒子或 γ 辐射。衰变是随机且自发的，遵循衰变定律：N = N₀ e–λt，其中 λ 为衰变常量。
N = N₀ e–λt
half the nuclei in a sample to decay. Activity A = λN also decays exponentially: A = A₀ e–λt. Background radiation must be subtracted when measuring count rates.
半衰期 T₁/₂ = ln2/λ 是样品中半数原子核衰变所需的时间。活度 A = λN 也呈指数衰减：A = A₀ e–λt。测量计数率时需扣除本底辐射。
Three types of decay are distinguished: alpha decay reduces the mass number by 4 and atomic number by 2; beta-minus decay increases atomic number by 1 with emission of an electron and an antineutrino; gamma decay releases excess energy without changing the nuclear composition.
三种衰变各有特点：α 衰变使质量数减4、原子序数减2；β⁻ 衰变使原子序数加1，同时发射电子和反中微子；γ 衰变释放多余能量，不改变核组成。
3. Nuclear Reactions and Binding Energy | 核反应与结合能
². The binding energy per nucleon peaks around iron-56, indicating the most stable nuclei.
原子核的质量小于其组成核子质量之和，这一质量亏损通过 E = mc² 转化为结合能。比结合能（每核子结合能）在铁-56 附近达到峰值，意味着这些核最稳定。
E = Δm c²
can be represented by equations such as ²H + ³H → ⁴He + n + energy. In any reaction, mass– ge and nucleon number are conserved. The Q-value of a reaction is the net energy released.
核反应可用方程表示，例如 ²H + ³H → ⁴He + n + 能量。在任何反应中，质能、电荷和核子数均守恒。反应的 Q 值即净释放能量。
Fusion of light nuclei and fission of heavy nuclei both release energy because the products are closer to the peak of the binding energy curve, resulting in a more stable configuration.
轻核聚变与重核裂变都会释放能量，因为产物更接近比结合能曲线的峰值，形成更稳定的结构。
4. Fission and Fusion | 裂变与聚变
Nuclear fission is the splitting of a heavy nucleus (e.g., uranium-235) into lighter fragments, releasing energy and neutrons that can sustain a chain reaction. Controlled fission in reactors provides low-carbon electricity.
核裂变是重核（如铀-235）分裂为较轻碎片的过程，释放能量和中子并可维持链式反应。反应堆中的受控裂变用于提供低碳电力。
Nuclear fusion combines light nuclei (e.g., deuterium and tritium) to form a heavier nucleus, releasing enormous energy as in the Sun. The main challenge for fusion power on Earth is confining the plasma at extremely high temperatures.
核聚变将轻核（如氘和氚）结合成较重核，释放巨大能量，正如太阳内部的反应。实现地球上聚变发电的主要挑战是在极高温度下约束等离子体。
In both processes, the energy release originates from the increase in binding energy per nucleon, which reduces the total mass of the products compared with the reactants.
在这两种过程中，能量释放源于比结合能的增加，使产物总质量小于反应物总质量。
5. Introducing the Particle Zoo | 粒子家族概览
In the mid-20th century, dozens of subatomic particles were discovered, leading to a classification scheme. Hadrons (baryons and mesons) are composed of quarks and feel the strong force. Leptons (e.g., electron, neutrino) do not experience the strong interaction.
20 世纪中叶，数十种亚原子粒子被发现，促使人们对其进行分类。强子（重子和介子）由夸克组成，参与强相互作用。轻子（如电子、中微子）不参与强相互作用。
n uud, neutron udd), while mesons consist of a quark and an antiquark (e.g., pion π⁺ = u d̄).
标准模型将这些粒子及其相互作用进行了系统组织。重子是由三个夸克组成的强子（如质子 uud，中子 udd），介子则由一个夸克和一个反夸克构成（如 π⁺ 介子 = u d̄）。
Leptons include the electron, muon, tau and their corresponding neutrinos, each with a lepton number +1. Antileptons have lepton number –1.
轻子包括电子、μ子、τ子和相应的中微子，每种轻子数为 +1，反轻子轻子数为 –1。
6. Quarks and Leptons | 夸克与轻子
There are six flavours of quarks: up (u), down (d), charm (c), strange (s), top (t) and bottom (b). Up-type quarks carry charge +⅔ e, down-type quarks carry charge –⅓ e. Antiquarks have opposite charges.
夸克有六种味：上夸克 (u)、下夸克 (d)、粲夸克 (c)、奇异夸克 (s)、顶夸克 (t) 和底夸克 (b)。上型夸克带 +⅔ e 电荷，下型夸克带 –⅓ e 电荷。反夸克的电荷符号相反。
Protons (uud) have charge +1 e, neutrons (udd) have charge 0. The baryon number for all quarks is +⅓, giving baryons a baryon number of +1. Leptons have no baryon number.
质子 (uud) 电荷为 +1 e，中子 (udd) 电荷为 0。所有夸克的重子数为 +⅓，因此重子的重子数为 +1。轻子无重子数。
The table below summarises some common particles and their quark content:
| Particle | Type | Quark composition | Charge |
|---|---|---|---|
| proton | baryon | uud | +1 e |
| neutron | baryon | udd | 0 |
| π⁺ | meson | u d̄ | +1 e |
| K⁺ | meson | u s̄ | +1 e |
Quarks are never found in isolation due to confinement; they are always bound inside hadrons.
夸克因色禁闭永远不会被单独观察到，它们总是束缚在强子内部。
7. Fundamental Forces and Exchange Particles | 基本相互作用与交换粒子
All interactions in nature are described by four fundamental forces: electromagnetic (photon γ), weak (W⁺, W⁻, Z⁰ bosons), strong (gluons g), and gravitational (graviton, hypothetical). Exchange particles mediate the forces between matter particles.
自然界的所有相互作用可由四种基本力描述：电磁力（光子 γ）、弱力（W⁺、W⁻、Z⁰ 玻色子）、强力（胶子 g）和引力（引力子，假设的）。交换粒子在物质粒子间传递这些力。
For example, the electromagnetic repulsion between two electrons is visualised by the exchange of virtual photons. The strong force, carried by gluons, binds quarks together and also holds protons and neutrons in the nucleus via residual strong force.
例如两个电子间的电磁排斥可通过交换虚光子来理解。由胶子传递的强力将夸克束缚在一起，并通过残余强力将质子和中子束缚在原子核内。
The weak force is responsible for beta decay and is mediated by W and Z bosons. Its short range is due to the large mass of these bosons (~80–90 GeV/c²).
弱力负责 β 衰变，由 W 和 Z 玻色子传递。因其质量很大（~80–90 GeV/c²），弱力的力程极短。
8. Antimatter and Pair Production | 反物质与对产生
Every particle has an antimatter counterpart, such as the positron (e⁺) for the electron. When a particle and its antiparticle meet, they annihilate, converting their mass into energy in the form of photons.
每种粒子都有反物质对应物，如电子的反粒子是正电子 (e⁺)。粒子与反粒子相遇时会湮灭，质量转化为光子形式的能量。
Conversely, energy can create particle–antiparticle pairs if the photon energy exceeds 2mc², a process called pair production. This must occur near a nucleus to conserve momentum.
反之，若光子能量超过 2mc²，能量可产生粒子–反粒子对，这一过程称为对产生。对产生需在原子核附近发生，以同时满足动量守恒。
γ → e⁻ + e⁺
Positrons are used in medical imaging (PET scans), where the annihilation radiation
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