Radioactivity and Nuclear Radiation | 放射性与核辐射

📚 Radioactivity and Nuclear Radiation | 放射性与核辐射

Radioactivity is one of the most important and fascinating topics in IGCSE Science. It explains how unstable atomic nuclei emit energy and particles, transforming into other elements and giving rise to phenomena that we harness in medicine, industry, and power generation. Understanding the nature of alpha, beta, and gamma radiation, how to represent nuclear changes with equations, and the concept of half-life is essential for Edexcel IGCSE Science students. This article provides a thorough revision of the subject, pairing each key point in English and Chinese to support bilingual learners.

放射性是 IGCSE 科学中最重要且最引人入胜的主题之一。它解释了不稳定的原子核如何放出能量和粒子，变成其他元素，并产生我们在医学、工业和发电中利用的现象。理解 α、β 和 γ 辐射的本质、如何用核方程表示核变化以及半衰期的概念，对于 Edexcel IGCSE 科学的学生来说至关重要。本文对这一主题进行全面复习，每一关键点均以英文和中文配对呈现，以支持双语学习者。

1. What is Radioactivity? | 什么是放射性？

Radioactivity is the spontaneous and random emission of particles or electromagnetic waves from the nucleus of an unstable atom. It was first discovered by Henri Becquerel in 1896 when he noticed that uranium salts fogged a photographic plate even when wrapped. The term "radioactivity" was later coined by Marie Curie, who, together with Pierre Curie, isolated radioactive elements such as polonium and radium.

放射性是不稳定原子核自发、随机地发射粒子或电磁波的现象。它最早由亨利·贝克勒尔于 1896 年发现，当时他注意到铀盐即使在包裹的情况下也会使照相底板感光。“放射性”一词后来由玛丽·居里提出，她与皮埃尔·居里一起分离出了钋和镭等放射性元素。

Radioactive decay is entirely independent of physical conditions such as temperature, pressure, or chemical bonding. The nucleus of an atom becomes unstable if it contains too many protons, too many neutrons, or an unbalanced ratio of the two, and it will decay in an attempt to reach a more stable configuration.

放射性衰变完全不依赖于温度、压力或化学键等物理条件。如果原子核含有过多的质子、过多的中子或二者比例失衡，它就会变得不稳定，并会通过衰变来达到更稳定的结构。

2. Structure of the Atom and Isotopes | 原子结构与同位素

Atoms consist of a small, dense nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge) in energy levels. The atomic number (Z) is the number of protons, which defines the element. The mass number (A) is the total number of protons and neutrons in the nucleus. An isotope is a variant of an element with the same atomic number but a different mass number, meaning it has the same number of protons but a different number of neutrons.

原子由一个包含质子（带正电）和中子（不带电）的小而致密的原子核，以及按能级分布的电子（带负电）组成。原子序数（Z）是质子的数量，它决定了元素的种类。质量数（A）是原子核中质子与中子总数。同位素是同一元素的不同变体，它们具有相同的原子序数但质量数不同，即质子数相同而中子数不同。

For example, carbon has three naturally occurring isotopes: carbon-12 (¹²C, 6 protons, 6 neutrons), carbon-13 (¹³C, 6 protons, 7 neutrons), and carbon-14 (¹⁴C, 6 protons, 8 neutrons). It is the instability of certain isotopes, like carbon-14, that leads to radioactivity.

例如，碳有三种天然同位素：碳‑12（¹²C，6 个质子，6 个中子）、碳‑13（¹³C，6 个质子，7 个中子）和碳‑14（¹⁴C，6 个质子，8 个中子）。正是某些同位素（如碳‑14）的不稳定性导致了放射性。

3. Types of Nuclear Radiation | 核辐射的种类

There are three main types of nuclear radiation emitted during radioactive decay: alpha (α) particles, beta (β) particles, and gamma (γ) rays. They differ in their nature, relative charge, mass, ionising ability, and penetrating power. Alpha and beta are particulate, while gamma is an electromagnetic wave.

放射性衰变中发射的核辐射主要有三种类型：α 粒子、β 粒子和 γ 射线。它们在性质、相对电荷、质量、电离能力和穿透能力上各不相同。α 和 β 是粒子性的，而 γ 是电磁波。

A useful way to recall the properties is that alpha particles are helium nuclei, beta particles are fast-moving electrons (or positrons), and gamma rays are high-energy photons with no mass or charge. In the following sections, each type will be examined in more detail.

一个有用的记忆方法是：α 粒子是氦核，β 粒子是快速移动的电子（或正电子），γ 射线是没有质量和电荷的高能光子。以下各节将更详细地逐一介绍。

4. Alpha Radiation | α 辐射

An alpha particle (α) consists of two protons and two neutrons, identical to a helium-4 nucleus. It is represented as ⁴₂He or α. Because it carries a double positive charge (+2e), it is strongly ionising. Alpha particles can knock electrons out of atoms as they pass through matter, producing many ions over a very short distance.

α 粒子由两个质子和两个中子组成，与氦‑4 原子核完全相同，用 ⁴₂He 或 α 表示。由于它带有两个正电荷（+2e），电离能力很强。当 α 粒子穿过物质时，它能够撞击出原子的电子，在很短的距离内产生大量离子。

Alpha radiation has the lowest penetrating power: it can be stopped by a few centimetres of air or a thin sheet of paper. It cannot penetrate human skin but can be extremely dangerous if a source is inhaled or ingested, as the highly ionising radiation damages living tissue from inside the body.

α 辐射的穿透力最弱：几厘米的空气或一张薄纸就可以阻挡它。它不能穿透人体皮肤，但如果其放射源被吸入或吞入，则会非常危险，因为高电离辐射会从体内损伤活组织。

5. Beta Radiation | β 辐射

A beta particle (β⁻) is a fast-moving electron ejected from the nucleus when a neutron transforms into a proton. This process increases the atomic number by 1 while leaving the mass number unchanged. Beta particles carry a single negative charge (−1e) and have a negligible mass compared with protons or neutrons.

β⁻ 粒子是从原子核中射出的快速移动的电子，它是在一个中子转变成质子时产生的。这一过程使原子序数增加 1，而质量数保持不变。β 粒子带有一个负电荷（−1e），其质量与质子或中子相比可以忽略不计。

Beta radiation is moderately ionising — less ionising than alpha radiation but more penetrating. It can travel about 1 metre through air and can pass through paper, but it is stopped by a few millimetres of aluminium. There is also a rarer form called positron emission (β⁺), where a proton turns into a neutron and emits a positive electron, but this is not always required at IGCSE level.

β 辐射的电离能力中等——弱于 α 辐射但比 α 辐射更具穿透力。它可以在空气中行进大约 1 米，能穿透纸张，但会被几毫米厚的铝阻挡。还有一种较罕见的形式称为正电子发射（β⁺），即一个质子转变成中子并发射一个正电子，不过这在 IGCSE 层次并不总是要求掌握。

6. Gamma Radiation | γ 辐射

Gamma rays (γ) are electromagnetic waves with extremely short wavelengths and very high frequencies. They are not particles and therefore carry no charge and have no mass. Gamma radiation is usually emitted after an alpha or beta decay event, when the daughter nucleus remains in an excited state and loses excess energy by emitting a gamma photon.

γ 射线是波长极短、频率极高的电磁波。它们不是粒子，因此不带电荷，也没有质量。γ 辐射通常在 α 或 β 衰变后发射，此时子核仍处于激发态，通过发射一个 γ 光子来释放多余的能量。

Gamma radiation is the most penetrating of the three types. It can travel through several centimetres of lead or many metres of concrete and is only partially attenuated by thick shielding. It is the least ionising, which means it passes through matter without causing as much direct ionisation, but its high energy makes it a serious external hazard.

γ 辐射是三种辐射中穿透力最强的。它能穿透数厘米厚的铅或数米厚的混凝土，只能通过厚屏蔽层部分衰减。它的电离能力最弱，这意味着它穿过物质时不会引起那么多的直接电离，但其高能量使其成为一种严重的外照射危害。

7. Penetration and Ionisation | 穿透力与电离能力

The ability of radiation to penetrate materials is inversely related to its ionising power. Alpha particles are highly ionising but poorly penetrating; gamma rays are weakly ionising but highly penetrating; beta particles lie in between. This trade-off can be summarised in a table and is central to understanding how we use and protect against each type of radiation.

辐射穿透物质的能力与其电离能力成反比。α 粒子电离能力很强但穿透力弱；γ 射线电离能力弱但穿透力强；β 粒子则居于两者之间。这一权衡关系可以用表格总结，对于理解如何利用和防护各类辐射至关重要。

Type	Nature	Charge	Ionising Power	Penetration	Stopped by
Alpha (α)	Helium nucleus	+2	Very high	Few cm of air	Paper, skin
Beta (β⁻)	Fast electron	-1	Moderate	~1 m of air	3–5 mm Al
Gamma (γ)	EM wave	0	Low	Very far	Thick lead/concrete

Experiments with Geiger–Müller (GM) tubes and absorbers of different thicknesses allow students to investigate these properties in the laboratory. When a piece of paper is inserted between an alpha source and the detector, the count rate drops dramatically, demonstrating the low penetration. With a gamma source, even thick aluminium only causes a small reduction, while a thick lead block is required to significantly lower the count rate.

通过使用盖革‑米勒（GM）管和不同厚度的吸收体进行的实验，学生可以在实验室中研究这些性质。当在 α 源和探测器之间插入一张纸时，计数率会急剧下降，这证明了其低穿透力。对于 γ 源，即使厚铝也只能引起很小的减少，而需要厚铅块才能明显降低计数率。

8. Radioactive Decay and Half-Life | 放射性衰变与半衰期

Radioactive decay is a random process: you cannot predict exactly when a particular nucleus will decay, but you can describe the behaviour of a large number of nuclei statistically. Each radioactive isotope has a characteristic half-life, which is the time taken for half the nuclei present in a sample to decay. The half-life is constant for a given isotope and is unaffected by physical or chemical changes.

放射性衰变是一个随机过程：你无法准确预测某个特定原子核何时会衰变，但能从统计上描述大量原子核的行为。每一种放射性同位素都有一个特征半衰期，即样品中一半原子核发生衰变所需的时间。半衰期对于给定的同位素是恒定的，不受物理或化学变化的影响。

The number of undecayed nuclei N remaining after a certain time t can be calculated using the equation:

N = N₀ × (½)^{t / T₁/₂}

where N₀ is the initial number of undecayed nuclei and T₁/₂ is the half-life. A typical IGCSE question might provide a graph of activity against time and ask students to read the half-life from the graph or to calculate the amount of a radioactive material remaining after several half-lives.

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经过时间 t 后剩余的未衰变原子核数 N 可用以下公式计算：

N = N₀ × (½)^{t / T₁/₂}

其中 N₀ 是初始未衰变原子核数，T₁/₂ 是半衰期。典型的 IGCSE 题目可能给出活度随时间变化的曲线图，要求学生从图上读取半衰期，或计算经历几个半衰期后剩余放射性物质的数量。

For example, if a sample of iodine-131 has a half-life of 8 days and initially contains 1000 undecayed nuclei, after 24 days (three half-lives), the number remaining will be:

1000 × (½)³ = 125 nuclei.

This exponential decay is crucial for applications as varied as radioactive dating, medical tracers, and safely handling radioactive waste.

例如，若一份碘‑131 样品的半衰期为 8 天，且初始含有 1000 个未衰变原子核，则 24 天（三个半衰期）后，剩余数目为：

1000 × (½)³ = 125 个原子核。

这种指数衰减对于放射性定年、医学示踪剂以及安全处理放射性废物等多种应用至关重要。

9. Nuclear Equations | 核方程

Nuclear equations show the changes that take place in the nucleus during radioactive decay. In any nuclear equation, the total mass number (top number) and the total atomic number (bottom number) must be conserved. This allows us to identify the unknown product of a decay process.

核方程展示了放射性衰变过程中原子核发生的变化。在任何核方程中，总质量数（上标数字）和总原子序数（下标数字）必须守恒。这使我们能够确定衰变过程的未知产物。

For alpha decay, the nucleus loses 2 protons and 2 neutrons, so the mass number decreases by 4 and the atomic number decreases by 2:

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

对于 α 衰变，原子核失去 2 个质子和 2 个中子，因此质量数减少 4，原子序数减少 2：

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

For beta-minus decay, a neutron turns into a proton, emitting an electron (β⁻ particle) and an antineutrino (often omitted in IGCSE equations). The mass number stays the same, while the atomic number increases by 1:

¹⁴₆C → ¹⁴₇N + ⁰₋₁e

对于 β⁻ 衰变，一个中子转变成一个质子，同时发射出一个电子（β⁻ 粒子）和一个反中微子（在 IGCSE 方程中常被省略）。质量数保持不变，而原子序数增加 1：

¹⁴₆C → ¹⁴₇N + ⁰₋₁e

ecay, you can deduce the daughter nucleus is protactinium-234, because ₉₀Th → ₉₁Pa + ⁰₋₁e.

平衡核方程是一项关键技能。例如，如果已知钍‑234 发生 β 衰变，你可以推断出子核是镤‑234，因为 ₉₀Th → ₉₁Pa + ⁰₋₁e。

10. Background Radiation | 背景辐射

We are constantly exposed to low levels of radiation from both natural and artificial sources. This is known as background radiation. Natural sources include cosmic rays from space, radon gas from the ground, and radioactive isotopes in rocks, soil, and even our own bodies (e.g., carbon-14, potassium-40). Artificial sources include medical X‑rays, nuclear weapons testing fallout, and emissions from nuclear power plants under normal operation.

我们持续受到来自天然和人工源的低水平辐射照射，这称为背景辐射。天然源包括来自太空的宇宙射线、地面释放的氡气以及岩石、土壤甚至我们身体中的放射性同位素（如碳‑14、钾‑40）。人工源包括医疗 X 射线、核武器试验沉降物以及核电站正常运行时的排放。

The total background radiation dose varies according to geographical location and occupation. When performing experiments on radioactivity in the school laboratory, it is essential to measure and subtract the background count from readings to obtain the true count due to the radioactive source alone.

总背景辐射剂量因地理位置和职业而异。在学校实验室进行放射性实验时，必须测量并减去背景计数，才能得到仅由放射源引起的真实计数。

11. Uses of Radiation | 辐射的应用

Radioactive isotopes have a wide range of practical applications that exploit the properties of alpha, beta, and gamma radiation.

放射性同位素有广泛的实际应用，利用了 α、β 和 γ 辐射的性质。

• Smoke detectors: Alpha emitters like americium-241 ionise the air between two electrodes, creating a small current. When smoke particles enter, they reduce the ionisation, the current drops, and the alarm is triggered.
• 烟雾探测器：像镅‑241 这样的 α 发射体使两个电极间的空气发生电离，产生微小电流。当烟雾颗粒进入时，会降低电离程度，电流下降，从而触发警报。

• Thickness monitoring: Beta emitters are used in industry to measure and control the thickness of materials such as paper, plastic, and metal foils. The absorption of beta particles depends on the thickness of the material; any variation triggers a feedback mechanism to adjust rollers.
• 厚度监测：β 发射体在工业上用于测量和控制纸、塑料和金属箔等材料的厚度。β 粒子被吸收的量取决于材料的厚度；任何变化都会触发反馈机构来调整轧辊。

• Medical tracers: Short-lived gamma emitters like technetium-99m are injected into the body to image organs and detect abnormalities, as gamma rays can escape the body and be detected externally while causing minimal internal ionisation.
• 医学示踪剂：像锝‑99m 这样的短寿命 γ 发射体被注入体内，用于对器官进行成像并检测异常，因为 γ 射线能穿出体外并被外部探测到，同时引起的内部电离极小。

• Radiotherapy: Gamma rays from cobalt-60 are directed at cancerous tumours to destroy malignant cells, often with careful rotation of the source to minimise damage to surrounding healthy tissue.
• 放射治疗：来自钴‑60 的 γ 射线被导向癌性肿瘤，以摧毁恶性细胞，通常还会小心地旋转放射源，以尽量减少对周围健康组织的损伤。

• Sterilisation: Gamma rays are used to sterilise medical equipment and to preserve food by killing bacteria and insects without using chemicals or high temperatures.
• 灭菌：γ 射线用于对医疗器械进行灭菌，并通过杀死细菌和昆虫来保存食品，无需使用化学品或高温。

12. Safety and Hazards | 安全与危害

Ionising radiation is hazardous because it can damage cells and the DNA inside them, potentially leading to cell death, mutations, or cancer. The hazards depend on the type of radiation and whether the source is outside or inside the body. Alpha sources are most dangerous if ingested or inhaled; gamma rays pose the greatest external threat because of their high penetration.

电离辐射是危险的，因为它会损伤细胞以及细胞内的 DNA，可能导致细胞死亡、突变或癌症。危害程度取决于辐射类型以及放射源是在体外还是体内。若被摄入或吸入，α 放射源最为危险；由于穿透力强，γ 射线造成外部威胁最大。

Precautions when handling radioactive materials include:

• Using forceps or tongs to keep distance between the body and the source.
• Pointing sources away from people and minimising exposure time.
• Storing sources in lead-lined containers and using shielding where appropriate.
• Wearing protective clothing and film badges to monitor accumulated dose.
• Ensuring that all radioisotopes are clearly labelled and that spillages are promptly dealt with according to local rules.

处理放射性材料时的预防措施包括：

• 使用镊子或钳子，使身体与放射源保持距离。
• 将放射源指向远离人的方向，并尽量减少照射时间。
• 将放射源存放在铅衬容器中，并在适当情况下使用屏蔽物。
• 穿戴防护服并佩戴胶片剂量计以监测累积剂量。
• 确保所有放射性同位素都有清晰标签，并且任何溢漏都依据当地规则立即处理。

In an IGCSE context, you should be able to explain why a particular type of radiation is suitable for a specific use based on its ionising power and penetration, and also justify safety precautions using the same principles. Understanding the balance between the benefits and risks of ionising radiation is a key part of scientific literacy.

在 IGCSE 语境中，你应该能够根据辐射的电离能力和穿透力解释为何某类辐射适用于特定用途，并同样运用这些原理论证安全预防措施的合理性。理解电离辐射利弊之间的平衡，是科学素养的关键一环。

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