📚 Radiation & Half-Life | 辐射与半衰期
In IGCSE Physics, radiation and half-life form a fundamental topic that connects atomic structure to practical applications. This article explores the nature of radioactive decay, the properties of different types of radiation, and the concept of half-life—a crucial idea for understanding how radioactive materials behave over time.
在IGCSE物理学习中，辐射与半衰期是将原子结构与实际应用联系起来的基础课题。本文将探讨放射性衰变的本质、不同类型辐射的性质，以及半衰期这一理解放射性物质随时间变化规律的关键概念。
1. What is Radioactivity? | 什么是放射性？
Radioactivity is the spontaneous emission of particles or electromagnetic radiation from the nucleus of an unstable atom. This process is entirely random and cannot be influenced by physical or chemical conditions such as temperature, pressure, or chemical bonding. When an unstable nucleus decays, it transforms into a more stable configuration, often becoming a different element in the process.
放射性是不稳定原子核自发发射粒子或电磁辐射的过程。这一过程完全是随机的，不受温度、压力或化学键合等物理或化学条件的影响。当不稳定的原子核发生衰变时，它会转变为更稳定的结构，在此过程中通常变成不同的元素。
The term "radiation" in this context refers to the energy or particles released during decay. There are three main types of radiation emitted by radioactive substances: alpha (α), beta (β), and gamma (γ) radiation. Each type has distinct properties that determine how it interacts with matter, as we will explore in the following sections.
在此语境中，“辐射”指的是衰变过程中释放的能量或粒子。放射性物质发射的辐射主要有三种类型：α（阿尔法）、β（贝塔）和γ（伽马）辐射。每种类型都有独特的性质，决定了它们与物质的相互作用方式，我们将在以下各节中探讨。
2. Types of Radiation | 辐射的类型
Alpha radiation consists of alpha particles, which are identical to helium nuclei. Each alpha particle contains 2 protons and 2 neutrons, giving it a charge of +2 and a mass number of 4. Alpha particles can be written as ⁴₂He or ⁴₂α. They are relatively large and slow-moving compared with other types of radiation.
α辐射由α粒子组成，α粒子与氦原子核完全相同。每个α粒子含有2个质子和2个中子，使其带+2电荷，质量数为4。α粒子可写作⁴₂He或⁴₂α。与其他类型的辐射相比，它们体积较大且运动速度较慢。
Beta radiation consists of fast-moving electrons emitted from the nucleus. When a neutron in the nucleus transforms into a proton, an electron is ejected at high speed. Beta particles can be written as ⁰₋₁e or ⁰₋₁β. They carry a charge of -1 and have negligible mass compared with alpha particles.
β辐射由原子核发射的高速电子组成。当原子核中的中子转变为质子时，会高速弹出一个电子。β粒子可写作⁰₋₁e或⁰₋₁β。它们带有-1电荷，与α粒子相比质量可忽略不计。
Gamma radiation is electromagnetic radiation of very high frequency and energy, similar to X-rays but even more energetic. It has no mass and no charge. Gamma rays are often emitted alongside alpha or beta decay to release excess energy from the newly formed nucleus.
γ辐射是频率极高、能量极大的电磁辐射，与X射线相似但能量更高。它没有质量和电荷。γ射线通常在α或β衰变过程中伴随发射，以释放新生原子核的多余能量。
3. Properties of Radiation | 辐射的性质
The three types of radiation differ significantly in their ionising power, penetration power, and behaviour in electric and magnetic fields. Understanding these differences is essential for determining which type of radiation is suitable for a given application.
三种辐射在电离能力、穿透能力以及电场和磁场中的行为方面存在显著差异。理解这些差异对于确定哪种辐射适合特定应用至关重要。
Alpha particles are strongly ionising because they are large and carry a double positive charge. They collide with atoms along their path, knocking electrons off and creating ions. However, due to their size, they lose energy quickly and have low penetration power—they can be stopped by a sheet of paper or a few centimetres of air. In an electric field, alpha particles are deflected towards the negative plate, though only slightly because of their relatively large mass.
α粒子体积大且带双正电荷，因此电离能力很强。它们沿途与原子碰撞，将电子击出并产生离子。然而，由于体积较大，它们很快损失能量，穿透能力低——一张纸或几厘米厚的空气就能将其阻挡。在电场中，α粒子向负极板偏转，但由于质量相对较大，偏转程度很小。
Beta particles have moderate ionising power—approximately one-tenth that of alpha particles. They are smaller and faster, so they can penetrate further, passing through paper but being stopped by a few millimetres of aluminium. In electric and magnetic fields, beta particles are deflected much more than alpha particles because of their tiny mass, and they bend towards the positive plate.
β粒子电离能力中等——约为α粒子的十分之一。它们更小更快，因此穿透力更强，能穿过纸张，但会被几毫米厚的铝板阻挡。在电场和磁场中，由于质量极小，β粒子的偏转程度远大于α粒子，且向正极板弯曲。
Gamma radiation has the weakest ionising power but the greatest penetration power. It can pass through human tissue and requires several centimetres of lead or several metres of concrete to be significantly absorbed. Gamma rays are not deflected by electric or magnetic fields since they carry no charge.
γ辐射的电离能力最弱，但穿透能力最强。它能穿透人体组织，需要几厘米厚的铅或几米厚的混凝土才能有效吸收。γ射线不带电荷，因此在电场和磁场中不发生偏转。
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus (2p + 2n) | Fast-moving electron | Electromagnetic wave |
| Charge | +2 | -1 | 0 |
| Mass | 4 u | ~0 | 0 |
| Ionising power | Strong | Moderate | Weak |
| Stopped by | Paper / few cm of air | Few mm of aluminium | Several cm of lead |
| Deflection in E-field | Slight, toward negative | Large, toward positive | None |
4. Decay Equations | 衰变方程式
Nuclear decay can be represented by balanced equations that show the transformation of one nucleus into another. In these equations, both mass number (the top number) and charge/atomic number (the bottom number) must be conserved on both sides of the equation.
核衰变可以用配平的方程式来表示，显示一个原子核转变为另一个原子核的过程。在这些方程中，质量数（上方数字）和电荷/原子序数（下方数字）必须在方程两侧都守恒。
For alpha decay, the parent nucleus loses 2 protons and 2 neutrons. The mass number decreases by 4 and the atomic number decreases by 2. For example, uranium-238 decays to thorium-234:
对于α衰变，母核失去2个质子和2个中子。质量数减少4，原子序数减少2。例如，铀-238衰变为钍-234：
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
For beta decay, a neutron changes into a proton. The mass number stays the same, but the atomic number increases by 1. For example, carbon-14 decays to nitrogen-14:
对于β衰变，中子转变为质子。质量数保持不变，但原子序数增加1。例如，碳-14衰变为氮-14：
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
nd can be written as part of a combined equation, for example: ⁶⁰₂₇Co → ⁶⁰₂₈Ni + ⁰₋₁e + γ.
对于γ发射，原子核仅释放多余能量。质量数和原子序数保持不变。γ发射通常伴随α或β衰变发生，可写在组合方程中，例如：⁶⁰₂₇Co → ⁶⁰₂₈Ni + ⁰₋₁e + γ。
5. Half-Life Definition | 半衰期的定义
The half-life of a radioactive isotope is defined as the time taken for half of the unstable nuclei in a sample to decay. Equivalently, it is the time taken for the activity—or count rate—of a source to fall to half of its initial value.
放射性同位素的半衰期定义为样品中一半不稳定核发生衰变所需的时间。等价地，它也是辐射源的活度（即计数率）降至初始值一半所需的时间。
Half-life is a characteristic property of each radioactive isotope. It cannot be altered by any physical or chemical means, including temperature changes, pressure variations, or chemical state. Some isotopes have half-lives of fractions of a second, while others—such as uranium-238—have half-lives of billions of years.
半衰期是每种放射性同位素的特征性质。任何物理或化学手段都无法改变它，包括温度变化、压力变化或化学状态。有些同位素的半衰期不到一秒，而另一些——如铀-238——半衰期长达数十亿年。
The decay of individual nuclei in a sample is completely random; we cannot predict which specific nucleus will decay next. However, for a sufficiently large number of nuclei, the overall decay pattern follows a predictable exponential curve, and the half-life provides a quantitative measure of how quickly the sample decays.
样品中单个原子核的衰变是完全随机的；我们无法预测下一个衰变的会是哪个原子核。然而，对于足够大量的原子核，整体衰变模式遵循可预测的指数曲线，半衰期为样品衰变的快慢提供了定量度量。
6. Calculating Half-Life | 计算半衰期
To calculate the amount of a radioactive substance remaining after a certain number of half-lives, we can use a simple halving approach. After one half-life, ½ of the original remains; after two half-lives, ¼ remains; after three half-lives, ⅛ remains; and so on.
要计算经过若干半衰期后剩余的放射性物质数量，我们可以使用简单的减半方法。经过一个半衰期，剩余原来的½；经过两个半衰期，剩余¼；经过三个半衰期，剩余⅛；依此类推。
ber of half-lives that have elapsed. If the initial activity is A₀ and the final activity is A, then:
一般关系式为：剩余比例 = (½)ⁿ，其中n为经过的半衰期个数。若初始活度为A₀，最终活度为A，则：
A = A₀ × (½)ⁿ
as an initial activity of 800 counts per second and a half-life of 6 hours, after 24 hours—which is 4 half-lives— ounts per second. This shows how the activity decreases by half in every half-life period.
例如，如果样品初始活度为每秒800次计数，半衰期为6小时，那么24小时后——即4个半衰期——活度将为800 × (½)⁴ = 800 ÷ 16 = 每秒50次计数。这显示了活度在每个半衰期期间如何减半。
When solving half-life problems in exams, follow these steps: first, identify the initial quantity and the half-life; second, calculate the number of half-lives n by dividing elapsed time by the half-life; finally, apply the formula above or perform repeated halving. Always remember to subtract background radiation from measured values
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