📚 Radioactivity and Half-Life | 放射性与半衰期
Radioactivity is a key topic in Edexcel IGCSE Science, linking atomic structure, nuclear decay, half‑life calculations and real‑world applications. Understanding the different types of radiation, how to write balanced decay equations and how half‑life determines the rate of decay will help you succeed in exams and appreciate how radioactivity is used in medicine, industry and research.
放射性是 Edexcel IGCSE 科学科目的一个重要主题，它将原子结构、核衰变、半衰期计算和实际应用紧密联系在一起。理解不同辐射的类型、书写正确的衰变方程以及掌握半衰期如何决定衰变速率，不仅有助于考试拿分，也能帮助你认识放射性在医学、工业和研究中的用途。
1. What is Radioactivity? | 什么是放射性？
Radioactivity is the spontaneous emission of radiation from an unstable atomic nucleus. An unstable nucleus has an imbalance in the number of protons and neutrons, and it becomes more stable by releasing energy or particles. This process is random and cannot be influenced by temperature, pressure or chemical reactions.
放射性是指不稳定的原子核自发地放出辐射。不稳定的原子核内质子和中子的数量不平衡，通过释放能量或粒子来达到更稳定的状态。这个过程是随机的，不受温度、压力或化学反应的影响。
Some atoms are naturally radioactive, such as uranium‑238 and carbon‑14, while others can be made radioactive in a nuclear reactor. Radiation can be detected using a Geiger–Müller tube and counter, or with photographic film.
有些原子是天然放射性的，例如铀‑238 和碳‑14，而另一些则可以在核反应堆中变成放射性核素。辐射可以用盖革‑米勒计数管或照相胶片来探测。
2. Alpha (α) Radiation | α 辐射
Alpha particles are identical to a helium nucleus. They consist of two protons and two neutrons, giving them a charge of +2 and a mass number of 4. Alpha radiation is the most ionising type of radiation because of its large mass and strong positive charge. However, it has very low penetrating power and can be stopped by a sheet of paper, a few centimetres of air, or the outer layer of human skin.
α 粒子与氦原子核完全相同，由两个质子和两个中子组成，因此带 +2 的电荷，质量数为 4。由于质量大且正电荷强，α 辐射是电离能力最强的辐射类型。但它的穿透力非常弱，一张纸、几厘米的空气层或人体皮肤表皮就能将其挡住。
Alpha sources are only hazardous if they enter the body through inhalation or ingestion, because then the highly ionising alpha particles can damage living cells in internal organs.
只有当 α 放射源被吸入或食入体内时才会造成危害，因为高电离能力的 α 粒子会损害内脏器官的活细胞。
3. Beta (β⁻) Radiation | β⁻ 辐射
A beta particle is a high‑speed electron emitted from the nucleus when a neutron turns into a proton. The electron is given the symbol ⁰₋₁e. Beta radiation is moderately ionising and has a greater penetrating power than alpha – it can travel through a few metres of air and is stopped by a few millimetres of aluminium. Beta particles are deflected by electric and magnetic fields because they carry a negative charge.
β 粒子是中子转变为质子时从核内发射出来的高速电子，用符号 ⁰₋₁e 表示。β 辐射的电离能力中等，穿透力比 α 辐射强，可在空气中穿行数米，几毫米厚的铝板就能挡住它。由于带负电荷，β 粒子可在电场和磁场中发生偏转。
In beta decay, the atomic number increases by 1 while the mass number stays the same. The nucleus also emits an electron antineutrino, but this is not required in IGCSE equations.
在 β 衰变中，原子序数增加 1，质量数保持不变。核中还发射一个反电子中微子，但在 IGCSE 层级的方程中不需写出。
4. Gamma (γ) Radiation | γ 辐射
Gamma radiation is electromagnetic radiation of very short wavelength and high energy. It has no mass and no charge. Gamma rays are the most penetrating of the three types – they are only significantly reduced by several centimetres of lead or metres of concrete. They are weakly ionising compared with alpha and beta radiation, but can still damage cells and DNA.
γ 辐射是波长极短、能量很高的电磁辐射，没有质量也不带电荷。γ 射线的穿透力是三种辐射中最强的，只有几厘米厚的铅或几米厚的混凝土才能显著减弱它。与 α 和 β 辐射相比，它的电离能力较弱，但仍能损伤细胞和 DNA。
Gamma emission often occurs after alpha or beta decay when the daughter nucleus is left in an excited state. The nucleus releases the extra energy as a gamma photon without changing its proton or neutron number.
γ 放射通常发生在 α 或 β 衰变之后，子核处于激发态，释放多余能量成为 γ 光子，其质子数和中子数都不发生变化。
5. Nuclear Decay Equations | 核衰变方程
You need to be able to write and balance equations for alpha and beta decay. In both types, the total mass number and total atomic number must be equal on both sides. For alpha decay, a nucleus loses 2 protons and 2 neutrons. A typical example is:
你需要能够书写并配平 α 衰变和 β 衰变的方程。在两种衰变中，两侧的总质量数和总原子序数都必须相等。在 α 衰变中，一个核失去 2 个质子和 2 个中子。典型例子如下：
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
For beta decay, a neutron changes into a proton and emits an electron. The mass number stays the same but the atomic number goes up by 1:
在 β 衰变中，一个中子变为质子并发射一个电子。质量数不变，原子序数增加 1：
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
In IGCSE Science, you do not need to include the antineutrino, but you should always check that the top numbers (mass numbers) and bottom numbers (atomic numbers) balance.
在 IGCSE 科学中无需写出反中微子，但必须始终检查上标数字（质量数）和下标数字（原子序数）是否平衡。
6. Understanding Half‑Life | 理解半衰期
Half‑life (t₁/₂) is the time taken for half of the unstable nuclei in a radioactive sample to decay. It is a characteristic property of each radioactive isotope and cannot be changed by physical or chemical conditions. Half‑life values range from fractions of a second to billions of years.
半衰期 (t₁/₂) 是指放射性样品中一半不稳定原子核发生衰变所需的时间。它是每种放射性同位素的特征性质，不受物理或化学条件的影响。半衰期的数值可以从不足一秒到数十亿年不等。
Because radioactive decay is a random process, half‑life is a statistical measure – it tells us about the average behaviour of a very large number of nuclei. The activity of a sample (measured in becquerels, Bq) decreases as the number of unstable nuclei falls.
放射性衰变是随机过程，因此半衰期是一个统计量度，它反映的是大量原子核的平均行为。样品的活度（单位是贝克勒尔 Bq）随不稳定核数量的减少而降低。
7. Half‑Life Calculations | 半衰期计算
You should be able to calculate the remaining mass or number of nuclei after a given number of half‑lives. The fraction remaining after n half‑lives is (½)ⁿ. The general formula linking remaining nuclei N, original number N₀, time t and half‑life t₁/₂ is:
你应当能够计算经过给定半衰期次数后剩余的原子核质量或数量。经过 n 个半衰期后剩余的比例为 (½)ⁿ。联系剩余核数量 N、初始数量 N₀、时间 t 和半衰期 t₁/₂ 的通用公式是：
N = N₀ × (½)t / t₁/₂
For example, if a sample initially contains 80 g of a radioisotope with a half‑life of 5 days, after 15 days (3 half‑lives) the remaining mass is 80 × (½)³ = 10 g.
例如，某样品最初含有 80 g 半衰期为 5 天的放射性同位素，经过 15 天（3 个半衰期）后，剩余质量为 80 × (½)³ = 10 g。
You may also be asked to find the half‑life given the initial and remaining masses over a known time. Divide the time by the number of half‑lives needed to go from the initial mass to the remaining mass.
考题也可能给出初始质量和一定时间后的剩余质量，要求你求出半衰期。只需用时间除以从初始质量减少到剩余质量所经过的半衰期次数。
8. Reading Half‑Life from Graphs | 从图像读取半衰期
IGCSE questions often provide a graph of activity (or number of undecayed nuclei) against time. To find the half‑life, choose a starting value on the curve, note the time, then find the time taken for the activity to drop to half of that value. Repeat the measurement at different points on the curve to confirm the half‑life is constant.
IGCSE 考题经常会提供活度（或未衰变核数量）对时间的图像。要找出半衰期，先在曲线上选择一个起始值，记录对应的时间，然后找到活度下降到该数值一半所用的时间。还可以在曲线上不同位置重复测量，以验证半衰期是否保持不变。
A smooth, decreasing exponential curve is typical. If the graph is plotted with a logarithmic y‑axis, the decay appears as a straight line, but this is beyond the IGCSE scope. You should simply focus on the time interval for a halving of activity.
典型的图像是一条平滑下降的指数曲线。如果纵坐标采用对数刻度，衰变会呈现为一条直线，但这超出了 IGCSE 的范围。你只须关注活度减半所对应的时间间隔即可。
9. Uses of Radiation in Medicine and Industry | 放射性在医学与工业中的应用
Radioisotopes have many practical uses. Gamma‑emitting isotopes such as technetium‑99m are used as medical tracers because gamma rays can be detected outside the body and the short half‑life (about 6 hours) limits the patient’s radiation dose. Tracers are injected or swallowed and their path through the body is monitored with a gamma camera.
放射性同位素有许多实际应用。释放 γ 射线的同位素如锝‑99m被用作医学示踪剂，因为 γ 射线可在体外被探测到，而且较短的半衰期（约 6 小时）能限制病人所受的辐射剂量。示踪剂通过注射或吞服进入人体，其路径由伽马相机监视。
In radiotherapy, cobalt‑60 or iodine‑131 can be used to destroy cancer cells. The gamma radiation is focused on the tumour to kill malignant cells while minimising harm to surrounding healthy tissue.
在放射治疗中，钴‑60 或碘‑131 可用于杀灭癌细胞。将 γ 辐射聚焦于肿瘤上，摧毁恶性细胞，同时尽量减少对周围健康组织的伤害。
In industry, beta emitters such as strontium‑90 can be used to monitor the thickness of paper or metal foil. A detector measures how much radiation passes through the material; if the thickness changes, the reading changes and rollers can be adjusted automatically.
在工业上，β 放射源如锶‑90 可用于监测纸张或金属箔的厚度。探测器测量穿透材料的辐射量；如果厚度发生变化，读数也随之变化，滚压机便可自动调整。
Gamma sources are also used to sterilise medical equipment and to detect leaks in underground pipes without digging up the pipeline.
γ 源还用于对医疗设备进行消毒，以及在不挖掘管道的情况下检测地下管道的泄漏。
10. Background Radiation and Safety Precautions | 背景辐射与安全预防
We are all exposed to background radiation from both natural and artificial sources. Natural sources include cosmic rays from space, radioactive gases such as radon from rocks, and radioisotopes in food and building materials. Artificial sources include medical X‑rays, nuclear power stations and fallout from nuclear weapons testing.
我们都会受到来自天然和人造源的背景辐射。天然源包括来自太空的宇宙射线、岩石释放的放射性气体如氡气，以及食物和建筑材料中的放射性同位素。人造源包括医疗 X 射线、核电站和核武器试验的放射性沉降物。
The level of background radiation varies with location, altitude and geology. Some areas have high radon gas concentrations; high‑altitude flights increase cosmic ray exposure.
背景辐射水平因地点、海拔和地质条件而异。部分地区氡气浓度高；高海拔飞行会增加宇宙射线的照射。
To use radioactive sources safely, the three key principles are: minimise exposure time, maximise distance from the source, and use appropriate shielding – thick lead for gamma, aluminium for beta, and minimal shielding for alpha. Sources should never be handled directly; tongs or robotic arms must be used.
安全使用放射源要遵循三个关键原则：尽量缩短受照时间、尽可能远离放射源，并使用合适的屏蔽——γ 用厚铅板，β 用铝板，α 需很少屏蔽但应注意防吸入。源绝不能直接用手触碰，必须使用钳子或机械臂操作。
11. Summary of Key Points | 要点总结
Radioactivity results from unstable nuclei emitting alpha, beta or gamma radiation. Alpha particles are helium nuclei with high ionising power but low penetration; beta particles are fast electrons with moderate ionising ability and penetration; gamma rays are electromagnetic waves with low ionisation but very high penetration. Nuclear equations must balance mass and atomic numbers. Half‑life is the time taken for half the unstable nuclei to decay, and it is used in calculations, graph reading and to choose appropriate isotopes for specific applications. Safe handling of sources relies on time, distance and shielding.
放射性源于不稳定的原子核发射 α、β 或 γ 辐射。α 粒子是氦核，电离能力强但穿透力弱；β 粒子是高速电子，电离能力和穿透力中等；γ 射线是电磁波，电离能力弱但穿透性极强。核方程必须满足质量数和原子序数平衡。半衰期是一半不稳定核衰变所需的时间，用于计算、图像解读以及为特定用途选择合适的同位素。放射源的安全使用依赖于时间、距离和屏蔽三大原则。
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