📚 761 Radioactivity and Background Radiation | 761 放射性与背景辐射
Understanding radioactivity is fundamental in IGCSE Edexcel Science, as it explains how unstable atomic nuclei release energy and particles. This process not only defines the concept of ionising radiation but also introduces the measurement of activity, the idea of background radiation, and the methods used to detect radiation safely. In this article, we will explore the types of radiation, their properties, how they contribute to background levels, and why half‑life is a key concept for applications ranging from nuclear medicine to geological dating.
理解放射性是 IGCSE Edexcel 科学中的基础内容，因为它解释了不稳定的原子核如何释放能量和粒子。这一过程不仅定义了电离辐射的概念，还引入了活度的测量、背景辐射的思想以及安全探测辐射的方法。本文将探讨辐射的类型、它们的性质、它们如何构成背景辐射水平，以及为什么半衰期是从核医学到地质年代测定等应用中的关键概念。
1. Introduction to Radioactivity | 放射性的介绍
Radioactivity is the spontaneous and random emission of particles or energy from the nucleus of an unstable atom. An atom becomes unstable when there is an imbalance between the number of protons and neutrons in its nucleus. To become more stable, the nucleus releases energy in the form of ionising radiation. The process is entirely independent of any chemical reactions and cannot be sped up or slowed down by changing temperature or pressure.
放射性是不稳定原子核自发且随机地发射粒子或能量的过程。当原子核内质子数与中子数失衡时，原子就会变得不稳定。为了变得更稳定，原子核会以电离辐射的形式释放能量。这一过程完全不依赖于任何化学反应，也不能通过改变温度或压力来加速或减缓。
All radioactive decay is governed by chance – we cannot predict when a particular nucleus will decay, but we can measure the average behaviour of a large number of nuclei. The activity of a radioactive source, measured in becquerels (Bq), tells us the number of decays per second. One becquerel equals one decay per second.
所有放射性衰变都受概率支配——我们无法预测某个特定原子核何时衰变，但可以测量大量原子核的平均行为。放射源的活度以贝克勒尔（Bq）为单位，表示每秒发生衰变的次数。1 贝克勒尔等于每秒 1 次衰变。
2. Types of Nuclear Radiation | 核辐射的类型
IGCSE Edexcel Science introduces three main types of radiation emitted from the nucleus: alpha (α) particles, beta (β) particles and gamma (γ) rays. Alpha particles consist of two protons and two neutrons – essentially a helium‑4 nucleus, represented as ⁴₂He. Beta particles are fast‑moving electrons (β⁻) ejected when a neutron turns into a proton. Gamma rays are electromagnetic waves of very high frequency and energy, emitted after alpha or beta decay to release excess nuclear energy.
IGCSE Edexcel 科学介绍三种来自原子核的主要辐射类型：α 粒子、β 粒子和 γ 射线。α 粒子由两个质子和两个中子组成——本质上是一个氦‑4 原子核，用 ⁴₂He 表示。β 粒子是快速移动的电子（β⁻），当中子转变为质子时被发射出来。γ 射线是频率极高、能量极大的电磁波，在 α 或 β 衰变后发射，以释放多余的核能。
In addition to these, some syllabuses include positron (β⁺) emission, where a proton converts into a neutron while releasing a positively charged electron. However, the core focus for Edexcel IGCSE Science remains the penetrating and ionising properties of α, β and γ radiations and the changes they cause to the parent nucleus.
此外，一些课程还会包含正电子（β⁺）发射，即质子转化为中子并释放一个带正电的电子。然而，Edexcel IGCSE 科学的核心重点仍然是 α、β 和 γ 辐射的穿透力、电离能力以及它们对母核造成的变化。
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus ⁴₂He | Fast electron ⁰₋₁e | EM wave |
| Charge | +2 | -1 | 0 |
| Penetrating power | Stopped by paper or a few cm of air | Stopped by ~3 mm aluminium | Reduced by several cm of lead or metres of concrete |
| Ionising ability | High | Medium | Low |
Table 1: Summary of properties of α, β and γ radiations. Understanding these differences helps explain how each type is used or shielded against in real‑world contexts.
表 1：α、β 和 γ 辐射性质总结。理解这些差异有助于解释每种辐射在实际中如何被利用或屏蔽。
3. Nuclear Decay Equations | 核衰变方程
When a nucleus undergoes alpha decay, its atomic number decreases by 2 and its mass number decreases by 4. For example, the decay of uranium‑238 can be written as:
当原子核发生 α 衰变时，其原子序数减少 2，质量数减少 4。例如，铀‑238 的衰变可写为：
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
In beta (β⁻) decay, a neutron turns into a proton, so the atomic number increases by 1 while the mass number remains unchanged. A common example is carbon‑14 decay:
在 β⁻ 衰变中，一个中子转变为质子，因此原子序数增加 1，而质量数保持不变。一个常见例子是碳‑14 的衰变：
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Gamma decay does not change the atomic or mass numbers; the nucleus simply releases excess energy. It is often emitted alongside alpha or beta particles and is written as an equation with the same parent and daughter nuclei plus a gamma ray symbol (γ).
γ 衰变不会改变原子序数或质量数；原子核只是释放多余的能量。它常与 α 或 β 粒子一同发射，在方程式中采用相同的母核与子核，并加上 γ 射线符号 (γ)。
4. Background Radiation | 背景辐射
Background radiation is the low‑level ionising radiation that is always present in the environment. It comes from a variety of natural and artificial sources. In Edexcel IGCSE Science, students are expected to describe the origins of background radiation and explain why counts measured by a detector must have the background level subtracted to find the net count from a source.
背景辐射是环境中始终存在的低强度电离辐射。它来自多种天然和人工辐射源。在 Edexcel IGCSE 科学中，学生需要描述背景辐射的来源，并解释为何探测器测得的计数必须减去本底水平才能获得放射源的净计数。
The average annual background radiation dose for a person in the UK is about 2.5 millisieverts (mSv). This value varies depending on location, altitude and lifestyle factors. Measuring background count rate before an experiment is essential because the natural background adds to any reading taken from a radioactive sample.
英国居民每年平均背景辐射剂量约为 2.5 毫希沃特 (mSv)。该数值因地而异，取决于地点、海拔和生活方式。在进行实验前测量本底计数率至关重要，因为天然本底会叠加到任何来自放射性样品的读数上。
5. Sources of Background Radiation | 背景辐射的来源
Natural sources of background radiation include radon gas, which seeps from rocks and soil, cosmic rays from outer space, and naturally occurring radioactive materials in food and building materials. Radon is the largest contributor, accounting for roughly half of the UK background dose. Cosmic radiation increases with altitude, which is why airline pilots receive a higher annual dose.
背景辐射的天然来源包括从岩石和土壤中渗出的氡气、来自外太空的宇宙射线，以及食品和建筑材料中天然存在的放射性物质。氡气的贡献最大，大约占英国本底剂量的一半。宇宙辐射随海拔升高而增加，这就是为何航空公司飞行员每年接受的剂量更高。
Artificial sources make up a much smaller proportion – typically around 15% – and include medical uses such as X‑rays and nuclear medicine, fallout from previous nuclear weapons tests, and discharges from nuclear power stations. Although the public often worries about artificial radiation, natural sources dominate background exposure.
人工辐射源所占比例小得多——通常约为 15%——包括 X 射线和核医学等医疗用途、以往核武器试验的沉降物以及核电站的排放。尽管公众常对人工辐射感到担忧，但天然来源占据了背景照射的主导地位。
6. Detecting Radiation | 辐射的检测
Radiation cannot be seen, felt or smelled, so we rely on detectors that respond to the ionising effect of the particles or waves. The most common instrument in the school laboratory is the Geiger‑Müller (GM) tube connected to a ratemeter or counter. When radiation enters the tube, it ionises the gas inside, creating a brief pulse of current that the counter registers as a ‘count’.
辐射看不见、摸不着、也闻不到，因此我们依赖能够响应粒子或波的电离效应的探测器。学校实验室中最常用的仪器是盖革‑米勒（GM）计数管，它连接到一个计数率计或计数器上。当辐射进入计数管时，会使管内气体电离，产生一个短暂的电流脉冲，计数器将此记录为一次“计数”。
Photographic film can also detect radiation, as the ionising rays darken the film. This is the basis of personal dosimeters worn by radiographers. A cloud chamber shows the tracks of alpha and beta particles as visible trails of condensation, allowing us to distinguish between the heavily ionising alpha tracks and the thinner, longer beta tracks.
照相胶片也能探测辐射，因为电离射线会使胶片变黑。这就是放射技师佩戴的个人剂量计的基本原理。云室可以以可见的冷凝径迹显示 α 和 β 粒子的轨迹，使我们能够区分电离能力强的 α 径迹与较细、较长的 β 径迹。
7. Corrected Count Rate and Experimental Procedure | 校正计数率与实验步骤
To accurately measure the activity of a radioactive sample, you must first record the background count rate over a period of time (e.g., 5 minutes) and calculate the average count per second or per minute. Then place your source a fixed distance from the GM tube and record the total count rate. The corrected count rate is:
为了准确测量放射性样品的活度，你必须先记录一段时间的本底计数率（例如 5 分钟），并计算每秒或每分钟的平均计数。然后将放射源置于距 GM 计数管固定距离处，记录总计数率。校正后的计数率为：
Corrected count rate = Total count rate − Background count rate
This corrected value is the radiation caused only by the source. Edexcel IGCSE practical assessments often ask students to describe how to use this method to investigate the penetrating power of different types of radiation by placing absorbers such as paper, aluminium and lead between the source and the detector.
校正后的数值就是仅由放射源引起的辐射。Edexcel IGCSE 实践考试常要求学生描述如何利用该方法，在源和探测器之间放置纸、铝和铅等吸收材料来研究不同类型辐射的穿透能力。
8. Half‑Life and Its Importance | 半衰期及其重要性
The half‑life of a radioactive isotope is the time taken for half of the nuclei in a sample to decay, or equivalently, the time for the activity to fall to half its initial value. Half‑life is constant for a given isotope and is unaffected by physical conditions. For example, iodine‑131 has a half‑life of 8 days, while uranium‑238 has a half‑life of 4.5 billion years.
放射性同位素的半衰期是指样品中一半原子核发生衰变所需的时间，或者说活度降至初始值一半所需的时间。对于给定的同位素，半衰期是恒定的，不受物理条件影响。例如，碘‑131 的半衰期为 8 天，而铀‑238 的半衰期为 45 亿年。
Half‑life is crucial for applications such as radioactive dating (e.g., carbon‑14 dating of once‑living materials), medical tracer selection (isotopes with short half‑lives minimise patient exposure while providing clear images), and nuclear waste management, where long half‑lives demand secure, long‑term storage.
半衰期在放射性年代测定（如用碳‑14 测定曾经有生命的物质）、医用示踪剂的选择（半衰期短的同位素可最大限度地减少患者受照并提供清晰图像）以及核废料管理（长半衰期要求安全的长期储存）等应用中至关重要。
9. Decay Curves and Graphical Analysis | 衰变曲线与图形分析
A radioactive decay curve plots the activity or number of parent nuclei against time. It always shows an exponential decrease. From the graph, you can read the half‑life by finding the time at which the activity drops to half of any chosen starting value. Edexcel IGCSE questions often present a graph and ask you to determine the half‑life and to predict the activity after a certain number of half‑lives.
放射性衰变曲线绘制活度或母核数量随时间的变化。它总是呈现指数衰减。从图形中，你可以通过找出活度降至任一选定初始值一半所对应的时间来读取出半衰期。Edexcel IGCSE 题目常给出图形，要求你确定半衰期，并预测经过若干半衰期之后的活度。
After one half‑life the activity is ½ of the original, after two half‑lives it is (½)² = ¼, after three half‑ ⅛, and so on. This pattern allows straightforward predictions without a graph: if the half‑life is 6 hours and the initial activity is 800 Bq, after 18 hours (3 half‑lives) the activity will be 800 × ½ × ½ × ½ = 100 Bq.
经过一个半衰期后，活度为初始值的 ½；两个半衰期后为 (½)² = ¼；三个半衰期后为 (½)³ = ⅛，以此类推。这种模式允许在不使用图形的情况下直接进行预测：如果半衰期为 6 小时，初始活度为 800 Bq，那么 18 小时（3 个半衰期）后活度将为 800 × ½ × ½ × ½ = 100 Bq。
10. Safety Precautions When Handling Radioactive Sources | 处理放射源的安全防护措施
Working with radioactive sources demands strict safety protocols to minimise exposure to ionising radiation. The key principles are: keep the source as far away from the body as possible (distance), use shielding appropriate to the radiation type, limit the time of exposure, and never point the source directly at anyone. In school laboratories, sealed sources kept in lead‑lined containers should always be handled with tongs.
使用放射源需要严格的安全规程，以尽量减少电离辐射的暴露。关键原则是：使放射源尽可能远离身体（距离）、使用与辐射类型相应的屏蔽、限制暴露时间，并切勿将源直接对准任何人。在学校实验室中，应始终用镊子夹取保存在铅衬容器内的密封源。
Contamination occurs when radioactive material gets onto skin, clothing or equipment, potentially being ingested or inhaled; irradiation is simply being exposed to the radiation itself without direct contact. IGCSE candidates should distinguish between these two and understand that contamination implies the material remains radioactive, whereas irradiation stops when the source is removed.
当放射性物质附着在皮肤、衣物或设备上，可能被摄入或吸入时，便发生沾染；而照射仅仅是指受到辐射本身的暴露，并无直接接触。IGCSE 学生应能区分这两者，并理解沾染意味着物质仍具放射性，而一旦移走放射源，照射便停止。
11. Uses of Radioactivity in Medicine and Industry | 放射性在医学和工业中的应用
Medical tracers rely on gamma‑emitting isotopes with short half‑lives. For instance, technetium‑99m is injected into the patient and its path through the body is tracked with a gamma camera. Because gamma rays penetrate soft tissue and can be detected outside the body, they allow doctors to image organs and detect blockages or tumours without surgery.
医用示踪剂依赖发射 γ 射线且半衰期短的同位素。例如，将锝‑99m 注射到患者体内，用伽马相机追踪其在体内的路径。由于 γ 射线能够穿透软组织并可在体外被探测，医生借此无需手术即可对器官成像，并发现堵塞或肿瘤。
In industry, alpha sources are used in smoke alarms: the alpha particles ionise air between two plates, creating a tiny current. When smoke enters, it absorbs the alpha particles, reducing the current and triggering the alarm. Beta sources are used to monitor the thickness of paper or foil; a drop in the detected count indicates the material is too thick, and a rise shows it is too thin.
在工业上，α 源用于烟雾报警器中：α 粒子将两片平板之间的空气电离，产生微小电流。当烟雾进入时，会吸收 α 粒子，从而减少电流并触发警报。β 源用于监测纸张或金属箔的厚度；探测到的计数下降表明材料过厚，计数上升则表明过薄。
12. Linking Concepts: The Big Picture of Radioactivity | 概念关联：放射性的整体图景
The topic of radioactivity weaves together atomic structure, the strong nuclear force, energy transformations and probability. By understanding the properties of α, β and γ radiation, students can explain not only laboratory observations but also the underlying mechanisms that power nuclear reactors and enable the dating of ancient artefacts. Background radiation provides a real‑world context for detecting ionising events, reinforcing the importance of control experiments in science.
放射性这一主题将原子结构、强核力、能量转化以及概率联系在一起。通过理解 α、β 和 γ 辐射的性质，学生不仅能解释实验室观察到的现象，还能阐明驱动核反应堆以及使古代文物年代测定成为可能的深层机制。背景辐射为探测电离事件提供了现实背景，从而强化了科学中对照实验的重要性。
The Edexcel IGCSE examination expects you to be able to write balanced nuclear equations, interpret half‑life graphs, compare radiation types in a table, and apply your knowledge to safety scenarios and practical methods. Reviewing these concepts regularly, especially through mathematical practice with half‑life calculations, will build confidence for both the written paper and practical assessments.
Edexcel IGCSE 考试要求你能够书写配平的核方程、解读半衰期图形、在表格中比较辐射类型，并将知识应用于安全情境和实践方法中。经常复习这些概念，尤其是通过半衰期计算的数学练习，将为笔试和实践评估建立信心。
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