📚 2 Nuclear physics | 2 核物理
Nuclear physics explores the heart of matter: the atomic nucleus. It uncovers the forces that bind protons and neutrons, the spontaneous transformations known as radioactive decay, and the enormous energies locked inside nuclei. This field underpins everything from carbon dating and medical imaging to nuclear power and the processes that fuel stars.
核物理学探索物质的核心——原子核。它揭示了束缚质子和中子的力、被称为放射性衰变的自发转变，以及封锁在核内的巨大能量。这一领域支撑着从碳年代测定和医学成像到核能和恒星能量来源的一切。
1. The Structure of the Nucleus | 原子核的结构
Every atom contains a tiny, dense nucleus at its centre, composed of nucleons – positively charged protons and electrically neutral neutrons. The number of protons, Z (the atomic number), defines the element, while the total number of nucleons, A (the mass number), determines the isotope. The nucleus is held together by the strong nuclear force, which overcomes the electrostatic repulsion between protons at extremely short ranges.
每个原子的中心都有一个致密的原子核，由核子组成——带正电的质子和电中性的中子。质子数Z（原子序数）定义了元素，而核子总数A（质量数）决定了同位素。原子核由强核力维系在一起，这种力在极短距离内克服了质子间的静电排斥。
his shows that nuclear matter has an approximately constant density, independent of the nucleus size.
原子核半径的数量级为10⁻¹⁵ m，可通过关系式 R = r₀A¹/³ 估算，其中 r₀ ≈ 1.2 fm。这表明核物质具有近似恒定的密度，与核的大小无关。
2. Isotopes and Nuclides | 同位素与核素
A nuclide is a specific combination of protons and neutrons, such as ¹²₆C or ²³⁵₉₂U. Isotopes are nuclides of the same element (same Z) but with different numbers of neutrons (different A). For instance, carbon has three naturally occurring isotopes: ¹²C, ¹³C and ¹⁴C. While ¹²C and ¹³C are stable, ¹⁴C is radioactive and decays via beta emission.
核素是质子和中子的特定组合，例如 ¹²₆C 或 ²³⁵₉₂U。同位素是同一元素（Z相同）但中子数不同（A不同）的核素。例如，碳有三种天然存在的同位素：¹²C、¹³C 和 ¹⁴C。¹²C 和 ¹³C 是稳定的，而 ¹⁴C 具有放射性，通过β发射衰变。
The notation for a nuclide is ᴬX, where X is the chemical symbol, A is the mass number, and Z is the atomic number. Since Z is fixed for a given element, it is sometimes omitted, but it remains essential for balancing nuclear equations.
核素的表示法为 ᴬX，其中 X 是化学符号，A 是质量数，Z 是原子序数。由于 Z 对于给定元素是固定的，有时可省略，但在平衡核方程时它仍然是必不可少的。
3. Radioactive Decay and Types of Radiation | 放射性衰变与辐射类型
Unstable nuclei spontaneously emit radiation to become more stable – a process called radioactive decay. There are three primary types of radiation: alpha (α), beta (β⁻ and β⁺), and gamma (γ). Alpha particles are helium nuclei, ⁴₂He, with two protons and two neutrons; they are highly ionising but have low penetrating power and can be stopped by paper or skin.
不稳定核会自发地发出辐射以变得更稳定——这一过程称为放射性衰变。有三种主要辐射类型：α、β（β⁻ 和 β⁺）以及 γ。α粒子是氦核 ⁴₂He，包含两个质子和两个中子；它们电离能力很强，但穿透力低，可被纸张或皮肤挡住。
Beta-minus (β⁻) decay occurs when a neutron transforms into a proton, emitting an electron (⁰₋₁e) and an antineutrino. Beta-plus (β⁺) decay involves a proton converting into a neutron, emitting a positron (⁰₊₁e) and a neutrino. Beta particles are moderately ionising and can be stopped by a few millimetres of aluminium.
β⁻ 衰变发生于中子转变为质子，放出一个电子（⁰₋₁e）和一个反中微子。β⁺ 衰变涉及质子转变为中子，放出一个正电子（⁰₊₁e）和一个中微子。β粒子电离能力中等，可被几毫米铝板挡住。
Gamma radiation is high-energy electromagnetic waves emitted from an excited nucleus after alpha or beta decay. Gamma rays have very low ionising power and extremely high penetration; they require several centimetres of lead or metres of concrete to be absorbed.
γ辐射是从α或β衰变后处于激发态的原子核发出的高能电磁波。γ射线电离能力很低但穿透性极强；需要数厘米铅或数米混凝土才能吸收。
4. Decay Equations and Conservation Laws | 衰变方程与守恒定律
In all nuclear decay processes, mass number (A) and proton number (Z) are conserved, as are energy, momentum and charge. Alpha decay reduces A by 4 and Z by 2. For example, uranium-238 decays to thorium-234:
在所有核衰变过程中，质量数（A）和质子数（Z）守恒，能量、动量和电荷也守恒。α衰变使A减少4，Z减少2。例如，铀-238衰变为钍-234：
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
In β⁻ decay, A stays the same while Z increases by 1. An example is carbon-14 decaying to nitrogen-14:
在 β⁻ 衰变中，A保持不变，Z增加1。例如碳-14衰变为氮-14：
¹⁴₆C → ¹⁴₇N + ⁰₋₁e + v̄
In γ decay, there is no change in A or Z; the nucleus simply loses energy by emitting a photon. Understanding these equations allows scientists to trace decay chains and predict daughter products.
在 γ 衰变中，A和Z都不变；原子核仅仅通过放出光子失去能量。理解这些方程能让科学家追踪衰变链并预测子产物。
5. Half-Life and Activity | 半衰期与活度
number of decays per second, measured in becquerels (Bq), where 1 Bq = 1 decay per second.
半衰期（T₁/₂）是给定放射性样品中一半核发生衰变所需的时间。它是一个统计性质：每个原子核在单位时间内有恒定的衰变概率，由衰变常量 λ 描述。样品的活度 A 是每秒衰变次数，以贝可勒尔（Bq）为单位，1 Bq = 1 次衰变每秒。
where N is the number of undecayed nuclei. The decay follows an exponential law: N = N₀e⁻λt /λ. For instance, ¹⁴C has a half-life of about 5730 years, making it useful for archaeological dating.
关系式为 A = λN，其中 N 是未衰变核数。衰变遵循指数规律：N = N₀e⁻λt。半衰期与 λ 的关系为 T₁/₂ = ln2/λ。例如，¹⁴C 的半衰期约为 5730 年，这使其可用于考古年代测定。
6. Detection of Radiation | 辐射的探测
Radioactive emissions are invisible but can be detected using devices such as the Geiger–Müller (GM) tube, which produces an electrical pulse each time a particle or photon ionises the gas inside. Cloud chambers and bubble chambers make tracks visible through condensation or bubbles along the path of ionising particles.
放射性辐射不可见，但可用盖革-米勒（GM）管等设备探测，该装置每次有粒子或光子电离管内气体时就会产生一个电脉冲。云室和气泡室则通过沿电离粒子路径产生的凝结或气泡来显示径迹。
Semiconductor detectors and scintillation counters offer higher precision and energy resolution. The detection method is chosen based on the type of radiation and the experimental requirements, such as count rate or identification of specific isotopes.
半导体探测器和闪烁计数器提供更高的精度和能量分辨率。探测方法的选择取决于辐射类型和实验需求，例如计数率或对特定同位素的鉴别。
7. Mass–Energy Equivalence (E = mc²) | 质能等价（E = mc²）
reveals that mass and energy are interchangeable. In nuclear physics, the total mass of a nucleus is slightly less than the sum of the masses of its individual nucleons. This ‘ , the mass defect, has been converted into binding energy that holds the nucleus together.
爱因斯坦著名的方程 E = mc² 揭示了质量和能量是可以互换的。在核物理中，原子核的总质量略小于其各核子质量之和。这一“缺失”的质量，即质量亏损，已被转化为将核束缚在一起的结合能。
E = mc²
iple explains why nuclear reactions release energies millions of times greater than chemical reactions.
即使微小的质量损失也对应着巨大的能量释放，因为 c² 的数量级为 9×10¹⁶ J/kg。这一原理解释了为何核反应释放的能量比化学反应高出数百万倍。
8. Binding Energy and Mass Defect | 结合能与质量亏损
nding energy per nucleon indicates a more stable nucleus. The graph of binding energy per nucleon against mass number shows a peak around iron-56, meaning that fusion of light nuclei and fission of heavy nuclei are both energetically favourable.
原子核的结合能是将其分离成质子和中子所需的能量。它可以通过质量亏损 Δm 计算：E_结合 = Δm c²。每个核子的结合能越高，原子核越稳定。每个核子结合能随质量数变化的曲线在铁-56附近达到峰值，这意味着轻核聚变和重核裂变在能量上都是有利的。
For example, the binding energy per nucleon for ⁴₂He is about 7.1 MeV, while for ²³⁸₉₂U it is around 7.6 MeV. Nuclear power exploits the tendency of nuclei to move towards the iron peak, releasing energy in the process.
例如，⁴₂He 的每个核子的结合能约为 7.1 MeV，而 ²³⁸₉₂U 约为 7.6 MeV。核能利用的是原子核趋向于铁峰的趋势，从而在过程中释放能量。
9. Nuclear Fission | 核裂变
Nuclear fission is the splitting of a heavy nucleus into two or more smaller nuclei, accompanied by the release of neutrons and a large amount of energy. Induced fission occurs when a nucleus, such as ²³⁵₉₂U, captures a slow neutron and becomes unstable, splitting into fragments like ¹⁴¹₅₆Ba and ⁹²₃₆Kr, plus two or three additional neutrons.
核裂变是一个重核分裂成两个或更多较小核的过程，同时释放出中子和大量能量。感生裂变发生在像 ²³⁵₉₂U 这样的核俘获一个慢中子并变得不稳定，分裂为例如 ¹⁴¹₅₆Ba 和 ⁹²₃₆Kr 的碎片，外加两到三个中子。
These secondary neutrons can go on to trigger further fission events, leading to a chain reaction. In a nuclear reactor, the chain reaction is controlled by control rods (e.g. boron or cadmium) that absorb neutrons, and a moderator (e.g. water or graphite) that slows them down to increase the probability of fission.
这些次级中子可以继续引发进一步裂变，从而形成链式反应。在核反应堆中，链式反应由吸收中子的控制棒（如硼或镉）和慢化剂（如水或石墨，用于降低中子速度以增加裂变概率）来控制。
10. Nuclear Fusion | 核聚变
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, with a release of energy because the product has a higher binding energy per nucleon. The most important fusion reaction in stars combines deuterium (²₁H) and tritium (³₁H) to form helium and a neutron:
核聚变是两个轻核结合形成一个更重的核，并释放能量，因为产物的每个核子结合能更高。恒星中最重要的聚变反应是将氘（²₁H）和氚（³₁H）结合成氦和一个中子：
²₁H + ³₁H → ⁴₂He + ¹₀n
Fusion requires extremely high temperatures (millions of degrees Kelvin) and pressures to overcome the Coulomb barrier between the positively charged nuclei. On Earth, achieving controlled fusion remains a major challenge, with experimental facilities like tokamaks using magnetic confinement to contain the hot plasma.
聚变需要极高的温度（数百万开尔文）和压力来克服带正电核之间的库仑势垒。在地球上，实现受控聚变仍是一个重大挑战，像托卡马克这样的实验设施使用磁约束来约束高温等离子体。
11. Applications of Nuclear Physics | 核物理应用
Nuclear physics has profound applications across many fields. In medicine, radioisotopes such as technetium-99m (⁹⁹₄₃Tc) are used for diagnostic imaging, while iodine-131 (¹³¹₅₃I) treats thyroid cancer. Radiotherapy uses focused gamma rays to destroy malignant cells. In archaeology and geology, radiocarbon dating (¹⁴C) and potassium-argon dating provide accurate chronologies.
核物理在众多领域有深远应用。在医学上，锝-99m（⁹⁹₄₃Tc）等放射性同位素用于诊断成像，而碘-131（¹³¹₅₃I）用于治疗甲状腺癌。放射疗法利用聚焦的γ射线摧毁恶性细胞。在考古学和地质学中，放射性碳（¹⁴C）和钾-氩测年提供了精确的年代序列。
Nuclear reactors are used for electricity generation, desalination, and the production of isotopes for research and industry. Smoke detectors, sterilisation of medical equipment, and food irradiation all rely on radioactive sources.
核反应堆用于发电、海水淡化和生产用于研究和工业的同位素。烟雾探测器、医疗设备消毒和食品辐照都依赖于放射源。
12. Safety and Handling of Radioactive Sources | 放射源的安全与处理
While ionising radiation has beneficial uses, it can also damage living tissue and DNA, so strict safety protocols are essential. The three fundamental principles of radiation protection are time (minimise exposure time), distance (maximise distance from the source), and shielding (use appropriate absorbing materials).
电离辐射虽有有益的用途，但也可能损伤活组织和DNA，因此严格的安全规程至关重要。辐射防护的三个基本原则是时间（尽量减少暴露时间）、距离（尽量远离源）和屏蔽（使用适当的吸收材料）。
Radioactive waste is classified by activity and half-life. High-level waste, such as spent fuel rods, must be stored underwater for years and eventually encased in geological repositories. Professionals monitor exposure using dosimeters and adhere to legally enforced dose limits to ensure safety for workers, patients, and the public.
放射性废物按活度和半衰期分类。高放废物如乏燃料棒必须在水下贮存多年，最终封存在地质处置库中。专业人员使用剂量计监测暴露量，并遵守法定剂量限值，以确保工作人员、患者和公众的安全。
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