📚 371: Radioactivity and Nuclear Particles | 放射性及核粒子
Radioactivity and nuclear particles form one of the most fascinating and essential topics in the Edexcel IGCSE Science syllabus, identified here as topic 371. Understanding the structure of the atom, the nature of radioactive decay and the properties of alpha, beta and gamma radiation is fundamental for explaining phenomena ranging from medical imaging to nuclear power. This article breaks down each key concept with clear English explanations followed by their Chinese equivalents, ensuring bilingual learners can master both the language and the science.
放射性与核粒子是 Edexcel IGCSE 科学教学大纲中最引人入胜且必不可少的话题之一，这里标记为选题 371。理解原子结构、放射性衰变的本质以及 α、β 和 γ 辐射的性质，是解释从医学成像到核能等现象的基础。本文用清晰的英文讲解每个关键概念，紧跟着对应的中文阐述，确保双语学习者能够同时掌握语言与科学内容。
1. Atomic Structure and Isotopes | 原子结构与同位素
All matter is made of atoms, which consist of a tiny central nucleus surrounded by electrons. The nucleus contains protons and neutrons, collectively called nucleons. Protons carry a positive charge, electrons carry a negative charge and neutrons have no charge. An atom is neutral because the number of protons equals the number of electrons. The atomic number (Z) is the number of protons, which determines the element. The mass number (A) is the total number of protons plus neutrons.
所有物质都由原子组成，原子由一个微小的中心原子核以及环绕的电子构成。原子核包含质子和中子，统称为核子。质子带正电荷，电子带负电荷，中子不带电。原子呈中性，因为质子数与电子数相等。原子序数 (Z) 是质子的数目，决定了元素的种类。质量数 (A) 是质子数与中子数的总和。
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. For example, carbon-12 (¹²₆C) has 6 protons and 6 neutrons, while carbon-14 (¹⁴₆C) has 6 protons and 8 neutrons. Isotopes share identical chemical properties because they have the same electron arrangement, but their physical properties, such as mass and stability, can differ. Some isotopes are unstable and decay over time, emitting radiation – these are the radioisotopes at the heart of topic 371.
同位素是同一元素中质子数相同但中子数不同的原子。例如，碳-12 (¹²₆C) 有 6 个质子和 6 个中子，而碳-14 (¹⁴₆C) 有 6 个质子和 8 个中子。同位素具有相同的化学性质，因为它们的电子排布相同，但它们的物理性质（如质量和稳定性）可能不同。一些同位素是不稳定的，会随着时间衰变，放出辐射——这正是选题 371 核心的放射性同位素。
2. Types of Nuclear Radiation | 核辐射的种类
When an unstable nucleus decays, it can emit three main types of radiation: alpha (α) particles, beta (β) particles and gamma (γ) rays. An alpha particle is identical to a helium nucleus, consisting of two protons and two neutrons, and is represented as ⁴₂He or α. A beta particle is a fast-moving electron ejected from the nucleus when a neutron changes into a proton; it is written as ⁰₋₁e or β⁻. Gamma radiation is a form of high-frequency electromagnetic wave, carrying energy away but no mass or charge.
当不稳定的原子核衰变时，会发射出三种主要类型的辐射：α 粒子、β 粒子和 γ 射线。α 粒子等同于一个氦核，由两个质子和两个中子组成，用 ⁴₂He 或 α 表示。β 粒子是一个快电子，是中子变为质子时从核内发射出来的，写作 ⁰₋₁e 或 β⁻。γ 辐射是一种高频电磁波，带走能量但没有质量或电荷。
These emissions differ markedly in their penetrating power, ionising ability and behaviour in electric and magnetic fields. Understanding the differences is critical for selecting appropriate protective measures and for using radioisotopes in practical applications. The table below summarises the key characteristics of each type of radiation.
这些辐射在穿透能力、电离能力以及在电场和磁场中的行为上差异显著。理解这些差异对于选择合适的防护措施以及将放射性同位素用于实际应用至关重要。下表总结了每种辐射的关键特性。
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus (⁴₂He) | Fast electron (⁰₋₁e) | Electromagnetic wave |
| Charge | +2 | -1 | 0 |
| Mass (relative) | 4 | 1/1840 (negligible) | 0 |
| Penetration | Stopped by paper or few cm of air | Stopped by ~3 mm aluminium | Reduced by thick lead or concrete |
| Ionising power | Very high | Moderate | Very low |
English translation of row labels for clarity: Nature, Charge, Mass (relative), Penetration, Ionising power. Note that alpha particles are the most ionising but least penetrating, while gamma rays are the most penetrating but least ionising. This trade-off is central to their safe usage.
表格行标签中文对应：本质、电荷、质量(相对)、穿透力、电离能力。注意 α 粒子电离能力最强但穿透力最弱，而 γ 射线穿透力最强但电离能力最弱。这种取舍是它们安全使用的核心。
3. Properties in Electric and Magnetic Fields | 在电场与磁场中的行为
Because alpha and beta particles carry electric charge, they are deflected by electric and magnetic fields, whereas gamma rays, having no charge, travel straight on. In an electric field, the positive alpha particle is attracted towards the negative plate and undergoes a small deflection due to its relatively large mass. The negative beta particle is attracted towards the positive plate and is deflected much more because its mass is far smaller.
由于 α 和 β 粒子带有电荷，它们会在电场和磁场中发生偏转；而 γ 射线因为没有电荷，沿直线传播。在电场中，带正电的 α 粒子被吸引向负极板，偏转较小，因为它的质量相对较大。带负电的 β 粒子被吸引向正极板，偏转程度大得多，因为它的质量极小。
In a magnetic field, the direction of deflection is given by Fleming's left-hand rule or simply by considering the interaction of moving charges. Alpha and beta particles are deflected in opposite directions, helping to separate and identify the three types of radiation in simple cloud chamber experiments. Gamma rays show no deflection, confirming their electromagnetic character.
在磁场中，偏转方向可以通过弗莱明左手定则或简单地考虑运动电荷与磁场的相互作用来确定。α 和 β 粒子向相反方向偏转，这有助于在简单的云室实验中分离和识别这三种辐射。γ 射线不显示任何偏转，证实了其电磁特性。
4. Nuclear Decay Equations | 核衰变方程
Radioactive decay can be represented by balanced nuclear equations in which both the total mass number and the total atomic number are conserved. For alpha decay, the mass number decreases by 4 and the atomic number decreases by 2. For example, the decay of uranium-238 to thorium-234 is written as:
放射性衰变可以用平衡核方程表示，其中总质量数和总原子序数守恒。对于 α 衰变，质量数减少 4，原子序数减少 2。例如，铀-238 衰变为钍-234 可表示为：
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
For beta decay, a neutron turns into a proton, so the mass number stays the same while the atomic number increases by 1. An antineutrino is also emitted, but it is often omitted at IGCSE level. The decay of carbon-14 to nitrogen-14 is:
对于 β 衰变，中子转变成质子，因此质量数保持不变，原子序数增加 1。还会发射一个反中微子，但在 IGCSE 阶段通常省略。碳-14 衰变为氮-14 的方程为：
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Gamma emission frequently accompanies alpha or beta decay to release excess energy from the nucleus. It does not change the mass number or atomic number, so the nuclide symbol remains the same, often with an asterisk or 'm' to indicate a metastable state. For instance, technetium-99m decays to technetium-99 by emitting a gamma photon.
γ 发射通常伴随着 α 或 β 衰变，以释放原子核中多余的能量。它不会改变质量数或原子序数，因此核素符号保持不变，通常用星号或 ‘m’ 表示亚稳态。例如，锝-99m 衰变成锝-99，放出一个 γ 光子。
5. Half-Life Concept and Calculations | 半衰期概念与计算
The half-life of a radioactive isotope is the time taken for half of the unstable nuclei in a sample to decay. This is a constant for a given isotope and is unaffected by temperature, pressure or chemical bonding. Half-lives can range from fractions of a second to billions of years. On a graph of activity against time, the time for the count rate to halve can be read directly.
放射性同位素的半衰期是样品中一半不稳定原子核发生衰变所需的时间。这是一个给定同位素的常数，不受温度、压力或化学键的影响。半衰期可以从几分之一秒到数十亿年不等。在活度-时间图上，可以直接读出具计数率减半所需的时间。
If you know the half-life, you can calculate the fraction of a sample remaining after a given number of half-lives using the formula:
如果知道半衰期，你可以使用下面的公式计算经过一定数量的半衰期后样品剩余的份额：
Activity after n half-lives = Initial activity × (½)ⁿ
time ÷ half-life). Similarly, the mass of undecayed isotope follows the same exponential pattern. For example, if an isotope has a half-life of 2 years, after 6 years (3 half-lives) only one-eighth of the original mass remains undecayed. Being comfortable with these calculations is essential for exam success in topic 371.
其中 n = 半衰期个数 (总时间 ÷ 半衰期)。同样，未衰变同位素的质量遵循相同的指数规律。例如，如果某同位素的半衰期为 2 年，那么 6 年 (3 个半衰期) 后，只有原始质量的八分之一尚未衰变。熟练掌握这些计算对选题 371 的考试成功至关重要。
6. Detection and Measurement of Radiation | 辐射的探测与测量
Radiation cannot be detected by human senses, so instruments are used. The Geiger–Müller (GM) tube connected to a counter or ratemeter is the most common device. When radiation enters the tube, it ionises the gas inside, causing a pulse of current registered as a count. The count rate is measured in counts per second or counts per minute.
辐射无法被人的感官察觉，因此需要使用仪器。最常用的装置是连接计数器或率表的盖革-米勒 (GM) 管。当辐射进入管内，会使内部的惰性气体电离，产生一个电流脉冲，被计为一个计数。计数率以每秒计数或每分钟计数来衡量。
Background radiation must always be subtracted from measurements to obtain the net count rate from a source. Background radiation comes from cosmic rays, rocks, radon gas and even human bodies. Photographic film can also detect radiation; film badges are worn by workers in nuclear facilities to monitor cumulative exposure. Cloud chambers allow us to visualise tracks: thick, short tracks for alpha; thin, wispy tracks for beta; and faint, straight lines for gamma.
在测量中必须减去本底辐射，才能得到放射源的净计数率。本底辐射来自宇宙射线、岩石、氡气甚至人体。照相胶片也能探测辐射；核设施工作人员佩戴的胶片徽章可以监测累积照射量。云室使我们能够看到径迹：α 粒子留下粗短的径迹；β 粒子留下细而弯曲的径迹；γ 射线留下微弱的直线。
7. Uses of Radioactive Isotopes | 放射性同位素的用途
Radioisotopes have numerous beneficial applications across medicine, industry and agriculture. In medicine, technetium-99m is used as a tracer to image organs because its short half-life (6 hours) and gamma emission minimise patient dose while allowing external detection. Iodine-131 is taken up by the thyroid gland and can be used both for imaging and for treating hyperthyroidism.
放射性同位素在医学、工业和农业中有着众多有益的应用。在医学上，锝-99m 被用作示踪剂对器官成像，因为它半衰期短 (6 小时) 且发射 γ 射线，既降低患者剂量又便于外部探测。碘-131 会被甲状腺吸收，可用于成像和治疗甲亢。
In industry, gamma-emitting sources such as cobalt-60 are used to inspect welds and detect cracks in metal structures through non-destructive testing. The thickness of paper or plastic can be controlled by monitoring beta radiation passing through the material; when thickness varies, the count rate changes, triggering automatic adjustments. Carbon-14 dating enables archaeologists to estimate the age of ancient organic materials by measuring the remaining ¹⁴C content.
在工业上，钴-60 等发射 γ 射线的源被用于检测焊缝和金属结构中的裂纹，实现无损检测。纸张或塑料的厚度可以通过监测穿过材料的 β 辐射来控制：当厚度变化时，计数率改变，触发自动调整。碳-14 定年法让考古学家能够通过测量古代有机材料中剩余的 ¹⁴C 含量来估算其年代。
8. Dangers and Safety Precautions | 危害与安全预防
Ionising radiation can damage living cells and cause mutations or cancer. Alpha particles are particularly hazardous if inhaled or ingested, because their high ionising power can severely damage internal tissues despite their low penetration. Beta and gamma radiation can penetrate the skin and damage deeper organs. Therefore, strict safety rules apply when handling radioactive sources.
电离辐射会损伤活细胞，引起突变或癌症。α 粒子如果被吸入或摄入则特别危险，因为它们虽然穿透力低，但高电离能力会严重损伤内部组织。β 和 γ 辐射能够穿透皮肤，损伤深层器官。因此，在处理放射源时必须遵守严格的安全规定。
Key precautions include: minimising exposure time, maximising distance from the source (intensity decreases with the square of the distance), using appropriate shielding (lead for gamma, aluminium for beta), and never pointing a source at anyone. Radioactive materials must be stored in lead-lined containers and handled with tongs. Washing hands and not eating or drinking in the lab are essential routines. The ALARA principle – As Low As Reasonably Achievable – guides radiation protection in all contexts involving topic 371.
关键的预防措施包括：尽量减少照射时间，尽量增大与源的距离 (强度与距离的平方成反比)，使用适当的防护屏蔽 (铅用于 γ 射线，铝用于 β 射线)，以及绝不将源指向任何人。放射性物质必须存放在内衬铅的容器中，并用夹钳操作。实验室内要洗手、禁止饮食，这些都是基本规程。ALARA 原则——即“合理可行尽量低”——指导着所有涉及选题 371 的辐射防护。
9. Background Radiation | 背景辐射
Background radiation is the low-level radiation that we are all exposed to naturally. Its sources include cosmic rays from outer space, radioactive rocks such as granite, radon gas seeping from the ground, and even naturally occurring radioisotopes in food and the human body. A small portion also comes from medical uses and nuclear fallout.
本底辐射是我们都自然暴露于其中的低水平辐射。其来源包括来自外层空间的宇宙射线、花岗岩等放射性岩石、从地面渗出的氡气，甚至食物和人体中天然存在的放射性同位素。一小部分也来自医学用途和核沉降物。
The level of background radiation varies by location: some regions have granite bedrock that emits more radon, while air travel increases cosmic ray exposure. When scientists perform experiments with radioactive sources, they must always record the background count and subtract it from readings to obtain a true value for the source under investigation. This avoids misinterpretation of results in half-life and penetration experiments.
本底辐射水平因地而异：一些地区有花岗岩基岩，氡气释放更多；而航空旅行会增加宇宙射线的暴露。当科学家使用放射源进行实验时，必须始终记录本底计数并从读数中将其减去，以获得所研究源的真实数值。这可以避免在半衰期和穿透实验中误读结果。
10. Nuclear Fission and Fusion (Introduction) | 核裂变与核聚变（简介）
As an extension often touched upon in IGCSE science, nuclear fission is the splitting of a heavy nucleus such as uranium-235 or plutonium-239 after absorbing a neutron. This releases a huge amount of energy plus more neutrons, which can trigger a chain reaction. Nuclear power stations use controlled fission to generate electricity without emitting greenhouse gases.
作为 IGCSE 科学中常涉及的拓展内容，核裂变是指重核（如铀-235 或钚-239）吸收一个中子后分裂成两个较小的核，释放出巨大能量和更多的中子，从而可能引发链式反应。核电站利用可控的裂变来发电，且不排放温室气体。
fission. Fusion powers the Sun and other stars but has not yet been sustained on Earth for power generation due to the extremely high temperatures and pressures required. Both fission and fusion illustrate the conversion of mass to energy according to Einstein's famous equation E = mc².
核聚变是两个轻核（如氢同位素氘和氚）结合成一个氦核的过程，释放的能量比裂变还要大。聚变是太阳及其他恒星的能量来源，但由于需要极高的温度和压力，尚未在地球上实现持续发电。裂变和聚变都体现了根据爱因斯坦著名方程 E = mc² 的质量-能量转化。
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