📚 Properties of Ionising Radiation | 电离辐射的性质
Ionising radiation is a cornerstone topic in A-Level Physics, encompassing the particles and electromagnetic waves emitted by unstable nuclei. Understanding the properties of alpha, beta, and gamma radiation – their nature, ionising ability, penetrating power, and behaviour in electric and magnetic fields – is essential for explaining radioactive decay, detection methods, and practical applications. This article provides a detailed, exam-focused account of these properties, tailored to the CIE syllabus.
电离辐射是 A-Level 物理中的核心主题，涵盖了不稳定原子核所发出的粒子和电磁波。理解 α、β 和 γ 辐射的性质——它们的本质、电离能力、穿透本领以及在电场和磁场中的行为——对于解释放射性衰变、探测方法和实际应用至关重要。本文紧扣 CIE 考纲，详细阐述这些性质，助力备考。
1. Introduction to Ionising Radiation | 电离辐射简介
Ionising radiation is any form of radiation that carries enough energy to knock electrons out of atoms, producing positive ions and free electrons. In nuclear physics, the three main types originate from the decay of unstable nuclei: alpha (α) particles, beta (β) particles, and gamma (γ) rays. Each type differs fundamentally in its physical nature, charge, mass, and ability to interact with matter.
电离辐射是指任何具有足够能量从原子中打出电子、产生正离子和自由电子的辐射。在核物理中，三种主要类型源自不稳定原子核的衰变：α 粒子、β 粒子和 γ 射线。它们在物理本质、电荷、质量以及与物质的相互作用能力上存在根本差异。
The detection and measurement of ionising radiation rely on its ionising effect, which forms the basis of instruments such as the Geiger–Muller tube. Understanding how these radiations ionise, penetrate, and deflect is key to designing shielding, medical therapies, and industrial gauges.
电离辐射的探测和测量依赖于其电离效应，这是盖革-缪勒管等仪器的基础。理解这些辐射如何电离、穿透和偏转，是设计防护、医学治疗和工业仪表的关键。
2. Nature of Alpha Particles | α 粒子的本质
An alpha particle is identical to a helium nucleus, consisting of two protons and two neutrons. It is written as ⁴₂He or ⁴₂α. With a mass number of 4 and a charge of +2e, it is the most massive form of common nuclear radiation. Alpha particles are emitted during alpha decay, typically by heavy nuclei such as uranium-238 or radon-222.
α 粒子等同于一个氦核，由两个质子和两个中子组成，写作 ⁴₂He 或 ⁴₂α。它的质量数为 4，带 +2e 的电荷，是常见核辐射中质量最大的粒子。α 粒子在重核（如铀-238 或氡-222）发生 α 衰变时发射出来。
Because of their relatively large mass and double positive charge, alpha particles produce intense ionisation over a very short path. They quickly lose kinetic energy when passing through matter, creating many ion pairs per millimetre of travel. This strong ionising ability makes them easy to detect but limits their range to a few centimetres in air.
由于质量较大且带双正电荷，α 粒子在短路径上产生强烈的电离。它们在穿过物质时迅速失去动能，每毫米行程会产生大量离子对。这种强电离能力使它们易于探测，但也将空气中的射程限制在几厘米内。
3. Nature of Beta Particles | β 粒子的本质
Beta particles are high-speed electrons (β⁻) or positrons (β⁺) ejected from a nucleus during beta decay. A β⁻ particle is represented as ⁰₋₁e, while a β⁺ particle is ⁰₊₁e. Beta decay involves the transformation of a neutron into a proton (emitting an electron and an antineutrino) or a proton into a neutron (emitting a positron and a neutrino).
β 粒子是 β 衰变中从原子核发射出的高速电子（β⁻）或正电子（β⁺）。β⁻ 粒子表示为 ⁰₋₁e，β⁺ 粒子表示为 ⁰₊₁e。β 衰变涉及中子转变为质子（发射电子和反中微子）或质子转变为中子（发射正电子和中微子）。
Beta particles have much smaller mass than alpha particles and carry a single charge (+e or –e). They produce less intense ionisation per unit length, but their greater speed and smaller size allow them to penetrate further – typically a few millimetres of aluminium or several metres of air. The energy spectrum of beta particles is continuous, which provided early evidence for the existence of the neutrino.
β 粒子的质量远小于 α 粒子，且带单一电荷（+e 或 –e）。它们在单位长度上产生的电离较弱，但更快的速度和更小的尺寸使其能穿透更远——通常可穿过几毫米铝或几米空气。β 粒子的能谱是连续的，这为中微子的存在提供了早期证据。
4. Nature of Gamma Rays | γ 射线的本质
Gamma rays are electromagnetic waves of extremely short wavelength (less than 10⁻¹² m) and high photon energy. They have no mass and no charge, represented as ⁰₀γ. Gamma emission usually accompanies alpha or beta decay when the daughter nucleus is left in an excited state and releases excess energy as a photon.
γ 射线是波长极短（小于 10⁻¹² m）、光子能量极高的电磁波。它们没有质量、不带电荷，写作 ⁰₀γ。γ 辐射通常伴随 α 或 β 衰变，当子核处于激发态时，以光子形式释放多余的能量。
Because gamma rays are uncharged electromagnetic radiation, they interact weakly with matter, causing very low direct ionisation. However, their high penetrating power allows them to travel through thick materials, requiring several centimetres of lead or metres of concrete for effective shielding. Gamma rays are best attenuated by dense, high-atomic-number materials.
由于 γ 射线是不带电的电磁辐射，它们与物质的相互作用很弱，直接电离很少。然而，它们的高穿透力使其能穿过厚材料，需要几厘米铅或几米混凝土才能有效屏蔽。γ 射线最容易被高密度、高原子序数的材料衰减。
5. Ionising Ability and Penetrating Power | 电离能力与穿透力
The ionising ability and penetrating power of the three radiations are inversely related. Alpha particles produce the most ion pairs per unit length – up to 10 000 ion pairs per millimetre in air – because of their large mass and double charge. This strong interaction causes them to be absorbed by a sheet of paper or the outer layer of skin.
三种辐射的电离能力与穿透力呈反比关系。α 粒子在单位长度上产生的离子对最多——在空气中每毫米可达 10 000 个离子对——因其质量大且带双电荷。这种强相互作用使它们能被一张纸或皮肤外层吸收。
Beta particles have moderate ionising power, producing fewer ion pairs per millimetre, but they travel further and can be stopped by a few millimetres of aluminium. Gamma rays are the weakest ionisers, creating only a few ion pairs per metre, yet they are highly penetrating and require dense materials like lead for significant absorption.
β 粒子的电离能力中等，每毫米产生的离子对较少，但穿行距离更远，能被几毫米铝阻挡。γ 射线是电离最弱的，每米仅产生几个离子对，但穿透力极强，需要铅等致密材料才能显著吸收。
A summary comparison is shown in the table below:
下表对三种辐射进行了总结比较：
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | ⁴₂He nucleus | ⁰₋₁e or ⁰₊₁e | EM photon, ⁰₀γ |
| Mass (u) | 4 | 1/1840 | 0 |
| Charge | +2e | ±1e | 0 |
| Ionising power | High | Medium | Very low |
| Penetration | Stopped by paper, few cm in air | Stopped by ~3 mm Al | Reduced by cm of Pb |
| Typical speed | ~5% of c | Up to 99% of c | c |
These differences dictate how we handle sources: alpha emitters are dangerous only if ingested, beta emitters require modest shielding, while gamma sources demand substantial shielding and distance.
这些差异决定了我们处理放射源的方式：α 放射源仅在进入体内时才危险，β 放射源需适度屏蔽，而 γ 放射源则需要大量的屏蔽和足够的距离。
6. Deflection in Electric Fields | 在电场中的偏转
When ionising radiation passes through a uniform electric field, charged particles experience a force. Alpha particles, being positively charged, are deflected towards the negative plate, while beta-minus particles (negative) are deflected towards the positive plate. Gamma rays, having no charge, pass through undeviated.
当电离辐射通过匀强电场时，带电粒子会受到力的作用。带正电的 α 粒子向负板偏转，而带负电的 β⁻ 粒子向正板偏转。没有电荷的 γ 射线则无偏转地通过。
The degree of deflection depends on the charge, mass, and velocity. Alpha particles are deflected less than beta particles because their much larger mass gives them greater inertia, despite the larger charge. Beta particles, with very small mass, show significant curvature even in a moderate electric field.
偏转程度取决于电荷、质量和速度。α 粒子的偏转比 β 粒子小，因为尽管电荷更大，但其大得多的质量赋予它更大的惯性。β 粒子质量极小，即使在中等电场中也会呈现显著的弯曲。
F = qE
The direction of deflection for a positive charge follows the field direction; for an electron it is opposite to the field lines. This behaviour can be used to separate the three components of a radioactive source in a simple experiment.
正电荷的偏转方向与电场方向相同；电子的偏转方向则与电场线相反。这一行为可用于在简单实验中分离放射源的三种组分。
7. Deflection in Magnetic Fields | 在磁场中的偏转
force given by F = Bqv sin θ, which acts perpendicular to both the field and the particle' d beta particles therefore move in circular arcs, with the direction determined by Fleming's left-hand rule.
在匀强磁场中，带电粒子受到磁力 F = Bqv sin θ 的作用，该力垂直于磁场和粒子的速度。因此 α 和 β 粒子沿圆弧运动，其方向由弗莱明左手定则决定。
For a positive alpha particle, the force direction is opposite to that for a negative beta particle, so they deflect in opposite directions. Gamma rays, being uncharged, are unaffected and continue in a straight line.
对于带正电的 α 粒子，受力方向与带负电的 β 粒子相反，因此它们向相反方向偏转。不带电的 γ 射线不受影响，继续沿直线运动。
r = mv / (Bq)
The radius of curvature r depends on momentum, charge, and magnetic flux density B. Because alpha particles have much greater mass, their radius of curvature is typically much larger than that of beta particles for comparable speeds and field strengths. Fast beta particles may leave very tight spirals in bubble chambers.
曲率半径 r 取决于动量、电荷和磁通量密度 B。由于 α 粒子的质量大得多，在类似的速度和磁场条件下，其曲率半径通常远大于 β 粒子。快速的 β 粒子可能在气泡室中留下非常紧密的螺旋轨迹。
8. Detection of Ionising Radiation | 电离辐射的探测
The most common detector in school laboratories is the Geiger–Muller (GM) tube connected to a counter. The tube contains a low-pressure gas and a central wire at high voltage. When ionising radiation enters, it ionises the gas, producing a pulse of current that is registered as a count. The GM tube detects all three types, though with varying efficiency.
学校实验室最常见的探测器是连接计数的盖革-缪勒（GM）管。管内有低压气体和施加高压的中心金属丝。当电离辐射进入时，它会电离气体，产生一个电流脉冲并被记录为一个计数。GM 管能探测所有三种辐射，但效率各不相同。
Cloud chambers and bubble chambers provide a visual trail of the paths of ionising particles. In a cloud chamber, supersaturated alcohol vapour condenses on ions left by the radiation, making tracks visible. Alpha particles produce thick, short tracks, beta particles show thin, wiggly tracks, and gamma rays tend to produce diffuse, faint streaks due to secondary electrons.
云室和气泡室可提供电离粒子路径的可视化轨迹。在云室中，过饱和的酒精蒸气在辐射留下的离子上凝结，使轨迹可见。α 粒子产生粗而短的径迹，β 粒子显示细而弯曲的径迹，而 γ 射线通常通过次级电子产生弥散的暗淡痕迹。
Semiconductor detectors and scintillation counters are also widely used in research for their energy resolution. However, the principle remains the same: interaction with matter generates an electrical signal proportional to the energy deposited.
半导体探测器和闪烁计数器因其能量分辨率在研究中也广泛使用。但其原理不变：与物质相互作用会产生与沉积能量成正比的电信号。
9. Radioactive Decay and Half-Life | 放射性衰变与半衰期
Radioactive decay is a random process; it is impossible to predict exactly when a given nucleus will decay. However, for a large number of nuclei, the activity A (decays per second) follows the exponential decay law:
放射性衰变是一个随机过程；无法准确预测某个原子核何时衰变。但对于大量原子核，活度 A（每秒衰变数）遵循指数衰变规律：
A = A₀ e–λt and N = N₀ e–λt
Here N is the number of undecayed nuclei, λ is the decay constant (probability of decay per unit time), and A₀, N₀ are the initial values. The half-life T½ is the time taken for half the nuclei to decay or for the activity to halve.
这里 N 是未衰变核的数量，λ 是衰变常量（单位时间衰变概率），A₀、N₀ 是初始值。半衰期 T½ 是半数核衰变或活度减半所需的时间。
T½ = ln 2 / λ
The half-life is constant for a given isotope and is unaffected by temperature, pressure, or chemical state. This property is exploited in radiometric dating, such as carbon‑14 dating, where the decay of ¹⁴C with a half-life of 5730 years is used to determine the age of organic materials.
半衰期对给定同位素是恒定的，不受温度、压力或化学状态影响。这一特性被用于放射性测年，例如碳-14 测年利用半衰期为 5730 年的 ¹⁴C 衰变来确定有机物的年龄。
10. Decay Equations | 衰变方程
In any nuclear decay, both mass number A and proton number Z are conserved. Alpha decay reduces A by 4 and Z by 2. For example, the decay of uranium-238 to thorium-234 is written:
在任何核衰变中，质量数 A 和质子数 Z 都守恒。α 衰变使 A 减少 4、Z 减少 2。例如，铀-238 衰变为钍-234 可写作：
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
In beta-minus decay, a neutron changes into a proton, increasing Z by 1 while A stays the same, accompanied by the emission of an electron and an antineutrino. For carbon-14:
在 β⁻ 衰变中，一个中子变为质子，Z 增加 1，A 不变，同时发射一个电子和一个反中微子。以碳-14 为例：
¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̅
Gamma emission does not change A or Z; it simply releases excess energy from the nucleus. Strictly speaking, it often accompanies another decay, such as the gamma emission from cobalt-60.
γ 辐射不改变 A 或 Z；它只是释放原子核的多余能量。严格说来，它通常伴随其他衰变，例如钴-60 的 γ 辐射。
These equations demonstrate the conservation of mass–energy, charge, and nucleon number, fundamental principles tested in A-Level examinations.
这些方程体现了质能、电荷和核子数的守恒，是 A-Level 考试中考查的基本原理。
11. Background Radiation and Safety | 背景辐射与安全
Background radiation is always present, originating from cosmic rays, terrestrial rocks (e.g., granite containing uranium), radon gas, and even food. Typical background count rates are around 0.5 Bq per litre of air or 20–50 counts per minute for a standard GM tube. When measuring a radioactive source, the background count must always be subtracted.
本底辐射无处不在，来源于宇宙射线、地球岩石（如含铀花岗岩）、氡气甚至食物。典型的本底计数率约为每升空气 0.5 Bq，对于标准 GM 管则为每分钟 20–50 次计数。测量放射源时，必须扣除本底计数。
Safety precautions follow the three basic rules: distance, shielding, and time. Increasing distance from a source reduces exposure according to the inverse-square law. Shielding using appropriate materials (paper for alpha, perspex or aluminium for beta, lead for gamma) stops or reduces radiation. Minimising exposure
Published by TutorHao | A-Level Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图