📚 Understanding Radioactive Decay and Half-Life | 理解放射性衰变与半衰期
Radioactivity is the spontaneous emission of ionising radiation from the unstable nucleus of an atom. In the Edexcel IGCSE Combined Science specification, this is often presented as topic 7.40 and focuses on the random nature of nuclear decay, the properties of alpha, beta and gamma radiation, and the meaning of half-life.
放射性是指不稳定原子核自发发射电离辐射的现象。在 Edexcel IGCSE 综合科学课程中，这一主题通常对应 7.40，重点涉及核衰变的随机性、α、β 和 γ 三种辐射的特性以及半衰期的含义。
1. Atomic Structure and Unstable Nuclei | 原子结构与不稳定原子核
Every atom has a central nucleus made of protons and neutrons. Protons have a positive charge and neutrons have no charge. The nucleus is held together by a strong force, but if the balance of protons and neutrons is wrong, the nucleus becomes unstable.
每个原子都有一个由质子和中子组成的原子核。质子带正电荷，中子不带电荷。原子核依靠强力结合在一起，但如果质子和中子的比例失衡，原子核就会变得不稳定。
Unstable nuclei are radioactive. They decay, or change into a more stable nucleus, by emitting particles and energy. This process is completely random: it is impossible to predict which nucleus will decay next.
不稳定的原子核具有放射性。它们通过发射粒子和能量发生衰变，转变为更稳定的原子核。这个过程是完全随机的：无法预测下一个衰变是哪一个原子核。
2. The Three Main Types of Radiation | 三种主要辐射类型
Radioactive materials can emit three types of radiation: alpha (α), beta (β) and gamma (γ). Alpha radiation is a helium nucleus made of 2 protons and 2 neutrons. Beta radiation is a fast-moving electron from the nucleus. Gamma radiation is a high-energy electromagnetic wave.
放射性物质可发射三种辐射：α（阿尔法）、β（贝塔）和 γ（伽马）。α 辐射是包含 2 个质子和 2 个中子的氦核；β 辐射是从原子核中释放的高速电子；γ 辐射是高能电磁波。
| Radiation | Nature | Charge | Mass |
| Alpha (α) | Helium nucleus (⁴₂He) | +2 | 4 u |
| Beta (β) | Fast electron (⁰₋₁e) | −1 | about 1/1836 u |
| Gamma (γ) | Electromagnetic wave | 0 | 0 |
3. Ionising Power and Penetration | 电离能力与穿透力
Ionising radiation removes electrons from atoms, creating ions. Alpha radiation is the most strongly ionising because it is large and slow. Beta is less ionising, and gamma is the least ionising.
电离辐射会从原子中移除电子，从而产生离子。α 辐射由于体积大、速度慢，电离能力最强；β 较弱；γ 最弱。
There is a link between ionising power and penetration. Alpha is stopped by a sheet of paper, beta by a few millimetres of aluminium, and gamma only by thick lead or concrete. The more ionising a radiation is, the more easily it is absorbed.
电离能力与穿透力的存在关联。α 可被一张纸挡住，β 可被几毫米铝片挡住，γ 则需要厚铅块或混凝土才能减弱。电离能力越强，越容易被吸收。
4. Defining Half-Life | 半衰期的定义
The half-life (t½) of a radioactive isotope is the average time taken for half of the unstable nuclei to decay. It is also the time taken for the activity, or rate of decay, to fall to half of its original value.
放射性同位素的半衰期（t½）是指一半不稳定原子核衰变所需的平均时间，也是指放射性活度（即衰变率）降低到原来一半所需的时间。
Because decay is random and spontaneous, half-life is a statistical measure. Different isotopes have different half-lives, ranging from fractions of a second to billions of years.
由于衰变是随机且自发的，半衰期是一种统计量。不同的同位素具有不同的半衰期，从几分之一秒到数十亿年不等。
5. Calculating Half-Life | 半衰期的计算
To calculate how much radioactive material remains, you can use the idea of repeated halving. The amount remaining after n half-lives is:
为了计算剩余放射性物质的量，可以使用反复减半的方法。经过 n 个半衰期后剩余的量是：
Remaining amount = Original amount × (½)ⁿ
剩余量 = 原始量 × (½)ⁿ
Worked example: a sample has an initial activity of 800 Bq. After 6 hours, the activity is 100 Bq. How many half-lives have passed, and what is the half-life?
例题：某样品初始活度为 800 Bq。6 小时后活度变为 100 Bq。问经过了多少个半衰期？半衰期是多少？
| Time (hours) | 0 | 2 | 4 | 6 |
| Activity (Bq) | 800 | 400 | 200 | 100 |
200 → 100. Therefore the number of half-lives is 3, and the half-life is 6 ÷ 3 = 2 hours.
6 小时内活度减半了三次，即 800 → 400 → 200 → 100。因此半衰期个数为 3，所以半衰期为 6 ÷ 3 = 2 小时。
6. Carbon Dating and Other Uses | 碳定年与其他应用
Carbon-14 has a half-life of 5730 years and is used to date ancient organic materials. While an organism is alive, it absorbs carbon-14 from the environment; after death the carbon-14 decays without being replaced.
碳-14 的半衰期为 5730 年，可用于古代有机物质的年代测定。生物体活着时从环境中吸收碳-14；死亡后碳-14 不断衰变而不再得到补充。
By comparing the remaining carbon-14 activity with that of a living sample, scientists can estimate the age of the object. Other uses of radioactivity include medical tracers, cancer treatment and checking the thickness of metal sheets in industry.
通过比较剩余碳-14 的活度与活体样品的活度，科学家可以估算物体的年龄。放射性的其他用途包括医用示踪剂、癌症治疗以及工业中检测金属薄片的厚度。
7. Dangers of Radiation and Safety Measures | 辐射的危害与安全措施
Ionising radiation can damage living cells by changing the chemical structure of DNA. This can cause cancer, cell mutation or, with very high doses, death. The risk depends on the type of radiation, the distance from the source and the exposure time.
电离辐射会通过改变 DNA 的化学结构来破坏活细胞，从而可能引发癌症、基因突变，或在极高剂量下导致死亡。风险取决于辐射类型、与放射源的距离以及暴露时间。
To stay safe, workers should use tongs to keep a distance, place sources in lead-lined containers, keep exposure times as short as possible, and use shielding such as lead aprons or concrete walls.
为保障安全，操作人员应使用长柄夹以保持距离，将放射源放在铅衬容器中，尽量缩短暴露时间，并使用铅围裙或混凝土墙等屏蔽措施。
8. Nuclear Fission and Fusion | 核裂变与核聚变
Nuclear fission is the splitting of a large, unstable nucleus, such as uranium-235, into two smaller nuclei when it absorbs a neutron. This releases a large amount of energy and more neutrons, which can cause a chain reaction.
核裂变是指大型不稳定原子核（如铀-235）在吸收一个中子后分裂成两个较小原子核的过程，释放大量能量并产生更多中子，从而可能引发链式反应。
Nuclear fusion is the joining of two small nuclei to form a larger nucleus, for example hydrogen isotopes fusing to form helium. Fusion releases even more energy than fission and powers the Sun and other stars.
核聚变是两个小原子核结合形成一个较大原子核的过程，例如氢同位素聚变生成氦。聚变释放的能量比裂变更多，太阳和其他恒星的能源来自核聚变。
9. Summary and Exam Tips | 总结与考试提示
In the Edexcel IGCSE exam, you may be asked to compare alpha, beta and gamma radiation, to calculate half-life from data, or to evaluate the risks and benefits of radioactivity. Always state whether a radiation is strongly or weakly ionising and how it is absorbed.
在 Edexcel IGCSE 考试中，你可能会被要求比较 α、β 和 γ 辐射，根据数据计算半衰期，或评估放射性的风险与收益。回答时要明确指出某种辐射电离能力强弱以及它如何被吸收。
-life equation is simple halving, and the uses of radiation must always be balanced against the dangers.
请记住这些关键事实：α 是氦原子核，β 是高速电子，γ 是电磁波。半衰期的计算就是反复“减半”，同时放射性的应用必须与其风险进行权衡。
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