📚 Radiation & Half-Life | 辐射与半衰期
Radioactivity involves the spontaneous emission of particles or energy from an unstable atomic nucleus. This random process changes the nucleus into a more stable form, and the concept of half-life provides a measure of how quickly this transformation occurs. In IGCSE Physics, understanding the types of radiation, their properties, nuclear equations, and half-life calculations is essential for explaining both natural and artificial radioactive phenomena.
放射性是指不稳定的原子核自发地发射粒子或能量的过程。这种随机的衰变会使原子核转变为更稳定的形态，而半衰期的概念则用来衡量这种转变的快慢。在IGCSE物理课程中，理解辐射的类型、性质、核方程以及半衰期的计算，是解释自然和人造放射现象的基础。
1. Introduction to Radioactivity | 放射性简介
Radioactivity was first discovered by Henri Becquerel in 1896 when he observed that uranium salts emitted rays that could fog photographic plates. Marie and Pierre Curie later isolated radium and polonium, showing that the emission originates from unstable nuclei. Radioactive decay is a random process: it is impossible to predict exactly when a single nucleus will decay, but for a large number of nuclei, the average rate of decay is constant for a given isotope.
放射性最早由亨利·贝克勒尔于1896年发现，他注意到铀盐会发出能使底片感光的射线。后来，居里夫妇分离出镭和钋，证明了这种辐射来自不稳定的原子核。放射性衰变是一个随机过程：无法准确预言单个原子核在何时衰变，但对于大量原子核，某一特定同位素的平均衰变速率是恒定的。
2. Types of Radiation: Alpha, Beta and Gamma | 辐射类型：α、β 和 γ
There are three main types of nuclear radiation: alpha (α) particles, beta (β) particles and gamma (γ) rays. Each type differs in its nature, charge, penetrating power and ionising ability. Alpha particles are helium nuclei, beta particles are fast-moving electrons or positrons, and gamma rays are high-frequency electromagnetic waves. These radiations can be distinguished by their behaviour in electric and magnetic fields as well as by their ability to pass through materials.
核辐射主要有三种类型：α粒子、β粒子和γ射线。它们的本质、所带电荷、穿透能力和电离能力各不相同。α粒子是氦原子核，β粒子是高速运动的电子或正电子，而γ射线是高频电磁波。通过它们在电场和磁场中的行为以及穿透物质的能力，可以区分这三种辐射。
3. Properties of Alpha Particles | α粒子的性质
An alpha particle consists of two protons and two neutrons, identical to a helium-4 nucleus. It has a charge of +2e and a relatively large mass. Alpha particles travel only a few centimetres in air and can be stopped by a sheet of paper or the outer layer of human skin. However, they are highly ionising because their double positive charge and slow speed allow them to interact strongly with atoms, knocking out electrons and creating many ion pairs per unit length.
α粒子由两个质子和两个中子组成，与氦-4原子核完全相同。它带+2e的正电荷，质量相对较大。α粒子在空气中只能穿行几厘米，一张纸或人体皮肤的表层就能将其拦住。但由于其双正电荷和较低的速度，α粒子能强烈地与原子相互作用，击出电子，单位长度上产生大量的离子对，因此电离能力极强。
4. Properties of Beta Particles | β粒子的性质
A beta particle is a fast-moving electron (β⁻) or positron (β⁺) emitted from the nucleus when a neutron changes into a proton or vice versa. Beta particles carry a single negative or positive charge and have a much smaller mass than alpha particles. They are moderately ionising and can travel up to about one metre in air. Beta radiation can be stopped by a few millimetres of aluminium or other light metals.
β粒子是从原子核中发射出来的高速电子（β⁻）或正电子（β⁺），当中子转变为质子或质子转变为中子时产生。β粒子带一个负电荷或正电荷，质量远小于α粒子。它们具有中等的电离能力，在空气中可穿行约一米。几毫米厚的铝片或其他轻金属就能阻挡β辐射。
5. Properties of Gamma Rays | γ射线的性质
Gamma rays are electromagnetic waves of very high frequency and short wavelength, carrying no charge and having no rest mass. They are emitted from an excited nucleus after an alpha or beta decay. Gamma rays are weakly ionising but extremely penetrating: they can travel through several centimetres of lead or many metres of concrete and require thick dense materials for effective shielding. Unlike alpha and beta particles, gamma rays are not deflected by electric or magnetic fields.
γ射线是频率极高、波长极短的电磁波，不带电荷，也没有静质量。通常在α或β衰变后，处于激发态的原子核会发射γ射线。γ射线电离能力很弱，但穿透力极强：它可以穿过数厘米的铅或数米厚的混凝土，需要厚实的致密材料才能有效屏蔽。与α和β粒子不同，γ射线在电场或磁场中不会偏转。
6. Comparing Penetration and Ionisation | 穿透与电离能力的比较
The penetrating power and ionising ability of the three radiations are inversely related. Alpha particles are the most ionising but the least penetrating, while gamma rays are the most penetrating but the least ionising. Beta particles lie between the two. The table below summarises these differences.
三种辐射的穿透能力与电离能力成反比关系。α粒子的电离能力最强但穿透力最弱；γ射线的穿透力最强但电离能力最弱；β粒子则介于二者之间。下表总结了这些差异。
| Radiation | Nature | Charge | Penetrating power | Ionising ability | Stopped by |
|---|---|---|---|---|---|
| Alpha (α) | Helium nucleus | +2 | Low (few cm in air) | High | Paper, skin |
| Beta (β) | Fast electron/positron | −1 or +1 | Medium (∼1 m in air) | Medium | Few mm Al |
| Gamma (γ) | Electromagnetic wave | 0 | High (km in air, cm in lead) | Low | Thick lead/concrete |
Understanding this inverse relationship is crucial for choosing the right radiation source for particular applications. For instance, alpha sources are dangerous if inhaled or ingested because their intense ionisation damages living cells internally, whereas gamma sources are hazardous mainly through external exposure due to their deep penetration.
理解这种反比关系对于根据具体应用选择合适的放射源至关重要。例如，α源如果被吸入或食入会非常危险，因为其强烈的电离作用会在体内损伤活细胞；而γ源则主要通过外照射造成危害，因为它能深深穿透人体。
7. Deflection in Electric and Magnetic Fields | 电场和磁场中的偏转
Charged radiations are deflected when passing through electric or magnetic fields. In a uniform electric field, alpha particles are deflected towards the negative plate and beta particles towards the positive plate, while gamma rays pass through undeflected. Because alpha particles have a much larger mass, their deflection is less than that of beta particles for the same field strength. In a magnetic field, using Fleming's left-hand rule for positive charges, one can predict the direction of deflection: alpha and beta particles curve in opposite directions owing to their opposite charges.
带电辐射通过电场或磁场时会发生偏转。在匀强电场中，α粒子偏向负极板，β粒子偏向正极板，而γ射线则径直穿过不偏转。由于α粒子的质量大得多，在相同场强下其偏转程度小于β粒子。在磁场中，利用正电荷的左手定则可以判断偏转方向：α粒子和β粒子因电荷相反，会向相反方向弯曲。
8. Nuclear Equations and Decay | 核方程与衰变
Radioactive decay can be represented by nuclear equations in which the total mass number (A) and total atomic number (Z) are conserved. In alpha decay, the nucleus loses two protons and two neutrons, so the atomic number decreases by 2 and the mass number by 4. For example, the decay of uranium‑238:
放射性衰变可以用核方程来表示，方程中总质量数（A）和总原子序数（Z）守恒。α衰变时，原子核失去两个质子和两个中子，因此原子序数减少2，质量数减少4。例如铀‑238的衰变：
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
In beta-minus decay, a neutron turns into a proton and an electron is emitted; the atomic number increases by 1 while the mass number stays the same. An example is the decay of carbon‑14:
β⁻衰变时，一个中子转变为质子并放出一个电子；原子序数增加1，质量数保持不变。例如碳‑14的衰变：
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Gamma decay often accompanies alpha or beta decay and is written by adding the gamma symbol (γ) after the daughter nucleus; no change occurs to mass or atomic numbers.
γ衰变通常伴随α或β衰变发生，在子核后面加上γ符号即可；质量和原子序数均不改变。
9. Background Radiation | 背景辐射
We are constantly exposed to low levels of ionising radiation from natural and artificial sources. This background radiation arises from cosmic rays from space, rocks and soil containing radioactive isotopes such as uranium and thorium, radon gas seeping from the ground, and even from the food we eat (e.g., potassium‑40). Artificial sources include medical uses (X‑rays, radiotherapy), nuclear power stations and fallout from past nuclear weapons tests.
我们经常受到来自天然和人工来源的低水平电离辐射照射。这些背景辐射来自宇宙射线、含有铀和钍等放射性同位素的岩石和土壤、从地面渗出的氡气，甚至我们所吃的食物（如钾‑40）。人工来源包括医疗用途（X光、放射治疗）、核电站以及过去核武器试验的沉降物。
- Natural sources: cosmic rays, terrestrial radiation, internal radiation from food and drink.
- Artificial sources: medical imaging, nuclear medicine, industrial sources, nuclear accidents.
天然源：宇宙射线、大地辐射、饮食中的内照射。人工源：医学影像、核医学、工业源、核事故。
10. Half-Life: Definition and Meaning | 半衰期：定义与含义
The half-life of a radioactive isotope is the average time taken for half the nuclei in a sample to decay, or equivalently for the count rate to fall to half its initial value. It is a constant characteristic of each isotope and is unaffected by physical conditions such as temperature and pressure. Half-lives vary greatly, from fractions of a second to billions of years. Because radioactive decay is random, the half-life is a statistical measure; it does not mean that exactly half the nuclei decay in one half-life, but rather that there is a 50% chance for any given nucleus to decay within that time.
放射性同位素的半衰期是指样品中半数原子核发生衰变，或计数率下降至初始值一半所需的平均时间。它是每种同位素固有的特征常数，不受温度和压力等物理条件的影响。半衰期的长短差异极大，从不到一秒到数十亿年不等。由于放射性衰变是随机的，半衰期是一种统计量度；这并不意味着在一段半衰期内恰好有半数的原子核衰变，而是指任何一个原子核在该时间内有50%的几率发生衰变。
11. Half-Life Calculations and Graphs | 半衰期计算与图表
The decay of a radioactive sample follows an exponential pattern. If N₀ is the initial number of undecayed nuclei, after n half-lives the number remaining N is given by:
放射性样品的衰变遵循指数规律。若N₀为未衰变原子核的初始数目，经过n个半衰期后剩余的数目N为：
N = N₀ × (½)ⁿ
The same formula applies to mass and activity (count rate). For instance, a 100 g sample with a half-life of 5 years will be reduced to 50 g after 5 years, 25 g after 10 years, and so on. On a graph of activity against time, the half-life can be read directly as the time interval over which the activity halves. A smooth curve through the points approaches the time axis asymptotically but never reaches zero.
该公式同样适用于质量和活度（计数率）。例如，一个100克的样品半衰期为5年，5年后剩50克，10年后剩25克，依此类推。在活度‑时间图上，可以直接读出活度减半所需的时间间隔，这就是半衰期。通过各点画出的平滑曲线渐近地贴近时间轴，但永远不会达到零。
fallen to 20 counts per second. How many half-lives have passed, and what is the half-life? per second. That is 4 half-lives in 45 minutes, so one half-life = 45 ÷ 4 = 11.25 minutes.
计算示例：某放射源初始计数率为每秒320次。45分钟后，计数率降至每秒20次。经历了多少个半衰期？半衰期是多少？320→160→80→40→20，经过了4个半衰期，所以一个半衰期 = 45 ÷ 4 = 11.25分钟。
12. Applications and Safety | 应用与安全
Radioactive isotopes are used widely: medical tracers (e.g., technetium‑99m with a half-life of about 6 hours) allow doctors to image organs; gamma sources are employed in radiotherapy to treat cancer; industrial thickness gauges use beta radiation to monitor paper or metal foil production; americium‑241 in smoke detectors emits alpha particles to detect smoke; and carbon‑14 dating determines the age of ancient organic materials. In all applications, the choice of isotope depends on the type of radiation, its half-life and its chemical properties.
放射性同位素应用广泛：医学示踪剂（如半衰期约为6小时的锝‑99m）可帮助医生进行器官成像；γ射线源用于放射治疗癌症；工业测厚仪利用β辐射监测纸张或金属箔的生产厚度；烟雾探测器中的镅‑241发射α粒子以检测烟雾；碳‑14定年法可以测定古代有机物质的年龄。所有应用都需根据辐射类型、半衰期和化学性质选择合适的同位素。
Safety precautions when handling radioactive materials are based on three key principles: minimise exposure time, maximise distance from the source (intensity follows the inverse‑square law), and use appropriate shielding (e.g., lead for gamma, aluminium for beta). Gloves and tongs should be used to avoid direct contact, and sources must be stored in labelled lead‑lined containers. Never point a source at anyone and wash hands after use.
处理放射性物质时的安全预防基于三大原则：尽可能缩短照射时间，尽量增大与源的距离（强度遵循平方反比定律），并使用适当的屏蔽（如用铅屏蔽γ射线，用铝屏蔽β射线）。应使用手套和钳子避免直接接触，放射源必须存放在贴有标签的铅衬容器中。切勿将放射源对准任何人，使用后要洗手。
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