📚 Radiation from Radioactive Substances and Their Properties | 放射性物质的辐射及其特性
Radioactive decay is a spontaneous and random process by which an unstable nucleus loses energy by emitting radiation. Understanding the nature and properties of this radiation is fundamental to nuclear physics and forms a core part of the CIE A-Level Physics syllabus.
放射性衰变是不稳定原子核通过发射辐射而失去能量的自发随机过程。理解这种辐射的本质和特性是核物理的基础，也是 CIE A-Level 物理大纲的核心内容。
1. The Nature of Radioactive Decay | 放射性衰变的本质
Radioactive decay occurs when an unstable nucleus undergoes a transformation to become more stable. This process is entirely spontaneous — it cannot be influenced by external factors such as temperature, pressure, or chemical state.
当不稳定原子核发生转变以变得更稳定时，就会产生放射性衰变。这一过程完全是自发的——温度、压力或化学状态等外部因素都无法影响它。
The decay is also a random process. It is impossible to predict exactly which nucleus in a sample will decay next, or precisely when a particular nucleus will decay. However, for a large number of nuclei, the decay follows a well-defined statistical pattern described by the decay constant λ and the half-life T₁/₂.
衰变也是一个随机过程。我们无法精确预测样品中哪个原子核接下来会衰变，也无法预知某个特定原子核何时衰变。然而，对于大量原子核而言，衰变遵循由衰变常数 λ 和半衰期 T₁/₂ 描述的明确统计规律。
A = A₀e⁻λᵗ
where A is the activity at time t, A₀ is the initial activity, and λ is the decay constant. The half-life is related to the decay constant by T₁/₂ = ln2/λ.
其中 A 是 t 时刻的活度，A₀ 是初始活度，λ 是衰变常数。半衰期与衰变常数的关系为 T₁/₂ = ln2/λ。
2. Types of Radiation: α, β and γ | 辐射类型：α、β 和 γ
There are three main types of nuclear radiation emitted during radioactive decay: alpha (α) particles, beta (β) particles, and gamma (γ) rays. Each has distinct physical characteristics arising from its nature.
放射性衰变过程中主要发射三种核辐射：α 粒子、β 粒子和 γ 射线。每种辐射因其本质不同而具有截然不同的物理特性。
| Property | α particle | β particle | γ ray |
| Nature | Helium nucleus (2p + 2n) | Fast-moving electron (or positron) | Electromagnetic wave / photon |
| Charge | +2e | −e (or +e for β⁺) | 0 |
| Rest mass | ≈ 4 u | ≈ 1/1836 u | 0 |
| Speed | ≈ 5% of c | Up to 99% of c | Exactly c |
3. Ionising Ability | 电离能力
All three types of radiation can ionise atoms by knocking electrons out of their orbits. The ionising ability depends on the charge, mass and speed of the particle.
三种辐射都能通过将电子撞出轨道来电离原子。电离能力取决于粒子的电荷、质量和速度。
Alpha particles are the most strongly ionising. Because they carry a charge of +2e and have a relatively large mass, they interact very strongly with atomic electrons, creating dense tracks of ionisation along their short path. They typically produce around 10⁴ ion pairs per millimetre of air.
α 粒子的电离能力最强。由于带有 +2e 的电荷且质量较大，它们与原子电子相互作用非常强烈，在短路径上产生密集的电离径迹。它们每毫米空气中约产生 10⁴ 对离子。
Beta particles are moderately ionising. Being lighter and carrying a single charge, they interact less strongly with matter, producing around 10² ion pairs per millimetre of air. Gamma rays are the least ionising because they have no charge and interact only via probabilistic processes such as the photoelectric effect, Compton scattering and pair production.
β 粒子的电离能力中等。由于质量更轻且只带单个电荷，它们与物质的相互作用较弱，每毫米空气约产生 10² 对离子。γ 射线的电离能力最弱，因为不带电荷，只能通过光电效应、康普顿散射和电子对产生等概率性过程相互作用。
4. Penetrating Power and Absorption | 穿透能力和吸收
The penetrating power of radiation is inversely related to its ionising ability. Radiation that ionises strongly loses its energy quickly and therefore penetrates only a short distance.
辐射的穿透能力与其电离能力成反比。电离能力强的辐射能量损失快，因此只能穿透很短的距离。
- Alpha particles: Stopped by a few centimetres of air or a sheet of paper. Their range in air is typically 3–8 cm depending on energy.
- Beta particles: Can penetrate several metres of air but are stopped by a few millimetres of aluminium. Their range depends on their initial kinetic energy.
- Gamma rays: Very penetrating; they require several centimetres of lead or metres of concrete for significant attenuation. Gamma radiation follows an exponential absorption law: I = I₀e⁻μˣ.
- α 粒子：几厘米空气或一张纸即可阻挡。它们在空气中的射程通常为 3–8 cm，取决于能量。
- β 粒子：可穿透数米空气，但几毫米铝即可阻挡。射程取决于初始动能。
- γ 射线：穿透力极强；需要数厘米铅或数米混凝土才能显著衰减。γ 辐射遵循指数吸收定律：I = I₀e⁻μˣ。
5. Deflection in Electric and Magnetic Fields | 在电场和磁场中的偏转
The behaviour of radiation in electric and magnetic fields provides strong evidence for the charge and mass of the particles involved.
辐射在电场和磁场中的行为为判断粒子的电荷和质量提供了有力证据。
In an electric field, α particles (positive) are deflected towards the negative plate, β particles (negative) towards the positive plate, and γ rays (neutral) pass undeflected. Because α particles are much more massive than β particles, their deflection is far smaller for the same field strength.
在电场中，带正电的 α 粒子向负极板偏转，带负电的 β 粒子向正极板偏转，而不带电的 γ 射线不偏转。由于 α 粒子比 β 粒子质量大得多，在相同场强下其偏转幅度远小得多。
In a magnetic field, the direction of curvature reveals the sign of the charge. Using Fleming’s left-hand rule, α particles curve one way, β particles curve the opposite way, and γ rays travel in a straight line. The radius of curvature r for a charged particle in a magnetic field is given by r = mv/(Bq), showing that for equal speeds, the heavier α particle has a much larger radius.
在磁场中，弯曲方向揭示电荷的符号。根据弗莱明左手定则，α 粒子向一侧弯曲，β 粒子向相反方向弯曲，而 γ 射线沿直线传播。带电粒子在磁场中的曲率半径 r = mv/(Bq)，表明在相同速度下，更重的 α 粒子具有大得多的半径。
6. Detection of Radiation | 辐射的探测
Several instruments are used to detect and measure nuclear radiation, each suited to different purposes.
有几种仪器用于探测和测量核辐射，每种仪器适用于不同目的。
The Geiger-Müller (GM) tube is the most common detector. Each incoming particle or photon causes a pulse of current, which is counted electronically. The GM tube is versatile and can detect all three types of radiation, provided the particles can enter the tube window. A thin mica window allows α particles to enter; thicker walls stop α but allow β and γ.
盖革-米勒（GM）管是最常用的探测器。每个入射粒子或光子都会引起电流脉冲，由电子设备计数。GM 管用途广泛，可以探测所有三种辐射，前提是粒子能进入管窗。薄云母窗允许 α 粒子进入；较厚的管壁阻挡 α 但允许 β 和 γ 通过。
The cloud chamber and spark chamber allow visual observation of radiation tracks. In a cloud chamber, ionisation along the path of a particle causes condensation of alcohol vapour, revealing the track. α tracks appear short, thick and straight; β tracks are longer, thinner and more erratic; γ rays produce faint, sparse tracks.
云室和火花室可以直观观察辐射径迹。在云室中，粒子路径上的电离使酒精蒸气凝结，从而显示径迹。α 径迹短而粗且笔直；β 径迹更长、更细且更曲折；γ 射线产生暗淡而稀疏的径迹。
7. The Random Nature of Decay and Background Radiation | 衰变的随机性和背景辐射
Because radioactive decay is random, measurements of count rate show fluctuations. If the count rate is measured repeatedly for equal time intervals, the results scatter around a mean value according to a Poisson distribution.
由于放射性衰变是随机的，计数率的测量会出现涨落。如果对相等时间间隔反复测量计数率，结果会按照泊松分布围绕平均值散布。
The standard deviation of counts N is √N. For example, if 1600 counts are recorded in one minute, the standard deviation is 40, so the result is 1600 ± 40. This statistical treatment is essential in experimental nuclear physics.
计数 N 的标准差为 √N。例如，如果一分钟记录到 1600 个计数，则标准差为 40，因此结果为 1600 ± 40。这种统计处理在实验核物理中至关重要。
Background radiation is the low-level radiation present everywhere from natural and artificial sources. Natural sources include cosmic rays, radon gas from rocks, and radioactive isotopes in soil and food. Artificial sources include medical procedures and nuclear fallout. When measuring a radioactive source, the background count must be measured separately and subtracted from the total count.
背景辐射是天然和人为来源产生的无处不在的低水平辐射。天然来源包括宇宙射线、岩石中的氡气以及土壤和食物中的放射性同位素。人为来源包括医疗程序和核沉降物。测量放射源时，必须单独测量背景计数并从总计数中减去。
8. Applications and Hazards | 应用与危害
Nuclear radiation has important applications in medicine, industry and research, but it also poses significant health risks.
核辐射在医学、工业和研究中有重要应用，但也带来显著的健康风险。
In medicine, γ radiation is used in radiotherapy to destroy cancerous tumours, and in medical imaging through techniques such as PET scans. Radioactive tracers with short half-lives are injected into the body to monitor organ function. In industry, radiation is used for thickness gauging, sterilisation of medical equipment, and food preservation.
在医学领域，γ 辐射用于放射疗法以摧毁癌性肿瘤，并通过 PET 扫描等技术用于医学成像。短半衰期的放射性示踪剂注入体内以监测器官功能。在工业中，辐射用于厚度测量、医疗设备灭菌和食品保鲜。
The hazards of radiation arise from its ionising effect on living cells. Ionisation can damage DNA molecules, potentially causing mutations or cancer. The severity of damage depends on the absorbed dose, measured in grays (Gy), and the biological effectiveness of the radiation type, weighted to give the dose equivalent in sieverts (Sv). For the same absorbed energy, α particles cause far more biological damage than β or γ radiation due to their high linear energy transfer.
辐射的危害源于其对活细胞的电离作用。电离会损伤 DNA 分子，可能导致突变或癌症。损伤的严重程度取决于以戈瑞（Gy）为单位的吸收剂量，以及按辐射类型的生物有效性加权得到的以希沃特（Sv）为单位的剂量当量。在相同吸收能量下，α 粒子因其高传能线密度造成的生物损伤远大于 β 或 γ 辐射。
The three basic principles of radiation protection are time, distance and shielding. Minimising exposure time, maximising distance from the source, and using appropriate shielding (paper for α, aluminium for β, lead or concrete for γ) reduce the dose received.
辐射防护的三个基本原则是时间、距离和屏蔽。最小化暴露时间、最大化与源的距离，以及使用适当的屏蔽（α 用纸、β 用铝、γ 用铅或混凝土）可以减少所受剂量。
9. Worked Example: Identifying Radiation | 例题：鉴别辐射类型
A radioactive source emits radiation that is not deflected by a magnetic field, produces no tracks in a cloud chamber, but penetrates 5 cm of lead with reduced intensity. What type of radiation is it?
一个放射源发出的辐射在磁场中不偏转，在云室中不产生径迹，但能穿透 5 cm 铅且强度减弱。这是什么类型的辐射？
Not being deflected by a magnetic field rules out α and β particles, both of which are charged. Producing no tracks in a cloud chamber confirms it is not directly ionising. Penetrating 5 cm of lead with only partial absorption is characteristic of γ radiation. The answer is gamma rays.
在磁场中不偏转排除了 α 和 β 粒子，因为它们都带电。在云室中不产生径迹确认它不是直接电离的。穿透 5 cm 铅且仅部分吸收是 γ 辐射的特征。答案是 γ 射线。
To determine the type of an unknown source, the following systematic procedure is used. First, test with paper: if blocked, it is α. If it passes paper but is blocked by a few millimetres of aluminium, it contains β. If it penetrates aluminium significantly, it contains γ. Charged particles can be identified by their direction of curvature in a known magnetic field.
要确定未知源的类型，可使用以下系统程序。首先用纸测试：如果被阻挡，则为 α。如果能穿过纸但被几毫米铝阻挡，则含有 β。如果能显著穿透铝，则含有 γ。带电粒子可通过其在已知磁场中的弯曲方向来鉴别。
10. Common Misconceptions | 常见误解
Several misunderstandings frequently appear in student answers and must be corrected.
学生答案中经常出现几个误解，必须予以纠正。
- Misconception: “Alpha particles are dangerous because they penetrate everything.” Correction: α particles are easily stopped but are extremely damaging if the emitting source is ingested or inhaled.
- Misconception: “Beta particles are electrons from the electron shell.” Correction: β particles are created in the nucleus when a neutron converts into a proton (n → p + e⁻ + ν̄ₑ).
- Misconception: “Gamma rays are particles.” Correction: γ rays are electromagnetic photons with zero rest mass, emitted when an excited nucleus returns to ground state after α or β decay.
- 误解：“α 粒子很危险，因为它们能穿透一切。” 纠正：α 粒子容易阻挡，但如果摄入或吸入发射源则极具破坏性。
- 误解：“β 粒子来自电子壳层。” 纠正：β 粒子是在原子核内中子转化为质子时产生的（n → p + e⁻ + ν̄ₑ）。
- 误解：“γ 射线是粒子。” 纠正：γ 射线是静止质量为零的电磁光子，在 α 或 β 衰变后激发态核返回基态时发射。
Understanding the distinctions between α, β and γ radiation — their nature, ionising ability, penetrating power and behaviour in fields — is essential for solving exam questions. Remember the inverse relationship between ionising ability and penetrating power, and always consider the random nature of decay when interpreting experimental data.
理解 α、β 和 γ 辐射之间的区别——它们的本质、电离能力、穿透能力以及在场中的行为——对于解答考试题目至关重要。记住电离能力与穿透能力之间的反比关系，并在解释实验数据时始终考虑衰变的随机性。
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