📚 Radioactivity: Half-Life and Applications | 放射性：半衰期及应用
Radioactivity is a fundamental concept in physics describing how unstable atomic nuclei release energy and particles. Understanding half-life helps us predict how long a radioactive material remains active, which is crucial for applications in medicine, industry, and energy production.
放射性是物理学中一个基本概念，描述不稳定的原子核如何释放能量和粒子。理解半衰期有助于我们预测放射性物质保持活性的时长，这对医学、工业和能源生产中的应用至关重要。
1. What is Radioactivity? | 什么是放射性？
Radioactivity is the spontaneous emission of radiation from the nucleus of an unstable atom. This occurs because the nucleus contains an imbalance of protons and neutrons, making it energetically favourable to decay into a more stable form.
放射性是不稳定原子核自发发射辐射的现象。这是因为原子核内质子与中子数量不平衡，使得衰变成更稳定形态在能量上更为有利。
The radiation emitted can be in the form of alpha particles, beta particles, or gamma rays. Each type has a different nature, charge, and penetrating ability. The process is random and cannot be influenced by external conditions such as temperature or pressure.
发射的辐射可以是α粒子、β粒子或γ射线。每种类型具有不同的性质、电荷和穿透能力。这一过程是随机的，不受温度或压力等外部条件影响。
| Type | Nature | Charge |
|---|---|---|
| Alpha (α) | Helium nucleus (2p+2n) | +2 |
| Beta (β⁻) | Fast-moving electron | -1 |
| Gamma (γ) | Electromagnetic wave | 0 |
类型 | 本质 | 电荷 (α：氦核, β⁻：快速电子, γ：电磁波)
2. Types of Radiation and Their Properties | 辐射类型及其性质
Alpha particles are heavy and carry a +2 charge. They are strongly ionising because they can easily knock electrons out of atoms, but they have low penetration and can be stopped by a sheet of paper or a few centimetres of air.
α粒子质量大且带+2电荷。它们电离能力强，因为容易将电子从原子中击出，但穿透能力弱，可被一张纸或几厘米空气阻挡。
Beta particles are much lighter and faster than alpha. They are moderately ionising and can penetrate a few millimetres of aluminium. Gamma rays are pure energy with no mass or charge, making them weakly ionising but highly penetrating; they require thick lead or metres of concrete to stop.
β粒子比α粒子轻得多且速度更快。它们电离能力中等，可穿透几毫米的铝。γ射线是纯能量，无质量无电荷，电离能力弱但穿透力极强，需要厚铅板或数米混凝土阻挡。
3. Ionising Power and Penetration | 电离能力和穿透力
The ionising power of radiation is inversely related to its penetrating power. Alpha particles cause the most ionisation per unit length but are easily absorbed. This is why alpha emitters are extremely dangerous if ingested or inhaled, as they can damage living tissue internally.
辐射的电离能力与穿透力呈反比。α粒子在单位长度上产生的电离最多，但容易被吸收。这就是为什么α发射体如果被摄入或吸入会极其危险，它们会在体内损害活体组织。
Beta particles have intermediate ionising and penetrating ability. They pose an external hazard to skin and eyes. Gamma rays, due to their high penetration, can pass through the body and require dense shielding. Understanding these properties is vital for choosing suitable safety measures.
β粒子的电离能力和穿透力居中。它们对皮肤和眼睛构成外部危害。γ射线由于穿透力强，可穿过人体，需要高密度屏蔽。理解这些特性对于选择合适的安全措施至关重要。
4. Nuclear Decay Equations | 核衰变方程
Radioactive decay can be represented by nuclear equations where the total mass number (nucleon number) and atomic number (proton number) are conserved. For alpha decay, a nucleus loses 2 protons and 2 neutrons, so the mass number decreases by 4 and atomic number by 2.
放射性衰变可以用核方程表示，其中总质量数（核子数）和原子序数（质子数）守恒。α衰变中，原子核失去2个质子和2个中子，因此质量数减少4，原子序数减少2。
Example: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
For beta decay, a neutron turns into a proton and emits an electron (beta particle). The mass number stays the same, but the atomic number increases by 1. A beta particle is written as ⁰₋₁e.
β衰变中，一个中子转变为质子并发射一个电子（β粒子）。质量数保持不变，但原子序数增加1。β粒子写作⁰₋₁e。
Example: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e
5. Half-Life Concept | 半衰期概念
Half-life is defined as the time taken for half the nuclei in a radioactive sample to decay, or equivalently, the time for the activity of the sample to halve. It is a constant for a given isotope and cannot be changed by physical or chemical means.
半衰期定义为放射性样品中一半原子核发生衰变所需的时间，或等价地，样品的放射性活度减半所需的时间。对于给定同位素，它是一个常数，无法通过物理或化学手段改变。
Half-life values range from fractions of a second to billions of years. For example, iodine‑131 has a half-life of 8 days, while uranium‑238 has a half-life of about 4.5 billion years. The shorter the half-life, the faster the decay and the more intense the initial activity.
半衰期取值从几分之一秒到数十亿年不等。例如，碘‑131的半衰期为8天，而铀‑238的半衰期约为45亿年。半衰期越短，衰变越快，初始活度越高。
6. Half-Life Calculations | 半衰期计算
)ⁿ, where n is the number of half-lives elapsed. n equals total time divided by half-life.
要计算经过多个半衰期后剩余的质量或活度，使用公式：剩余量 = 初始量 × (½)ⁿ，其中n为经过的半衰期个数。n等于总时间除以半衰期。
For example, a 100 g sample of a radioactive material with a half-life of 5 years will decay to 50 g after 5 years, 25 g after 10 years, and 12.5 g after 15 years. This exponential decay pattern is predictable and used for carbon dating and medical dosimetry.
例如，一个100克的放射性样品，半衰期为5年，5年后衰变至50克，10年后25克，15年后12.5克。这种指数衰减规律是可预测的，并用于碳年代测定和医疗剂量测定。
lf-life is 2 hours, after 6 hours (3 half-lives) the activity will be 800 × (½)³ = 100 Bq.
计算也可涉及以贝克勒尔（Bq）为单位的活度。若初始活度为800 Bq，半衰期为2小时，则6小时（3个半衰期）后活度为800 × (½)³ = 100 Bq。
7. Radioactive Contamination vs Irradiation | 放射性污染与辐照
Contamination occurs when unwanted radioactive material deposits on or inside an object or person. This means the source is in direct contact and can continue to expose the object to radiation until removed or decayed.
污染是指不需要的放射性物质附着在物体表面或进入人体内部。这意味着放射源直接接触，并会持续照射该物体直至被清除或衰变完。
Irradiation is the exposure to radiation from a source without direct contact. Once the source is moved away or shielded, the irradiation stops. A person can be irradiated during an X‑ray but not contaminated because no radioactive material enters the body.
辐照是受到放射源的照射而不发生直接接触。一旦移开或屏蔽放射源，辐照即停止。做X光检查时，人体被辐照但未被污染，因为没有放射性物质进入体内。
8. Uses of Radioactivity in Medicine | 放射性在医学中的应用
Radioisotopes are widely used in both diagnosis and therapy. Technetium‑99m, with a half-life of 6 hours, emits gamma rays and is used as a tracer to image organs such as the heart, thyroid, and bones. Its short half-life minimises patient radiation dose.
放射性同位素广泛用于诊断和治疗。锝‑99m半衰期为6小时，发射γ射线，被用作示踪剂对心脏、甲状腺和骨骼等器官成像。其短半衰期最大限度地减少了患者的辐射剂量。
Iodine‑131, a beta and gamma emitter with an 8‑day half-life, is used to treat thyroid cancer. The thyroid gland absorbs iodine, so it concentrates the radiation where it is needed, destroying cancerous cells while sparing surrounding tissue.
碘‑131是一种β和γ发射体，半衰期8天，用于治疗甲状腺癌。甲状腺吸收碘，从而将辐射集中在需要的地方，破坏癌细胞同时保护周围组织。
9. Industrial and Tracer Applications | 工业与示踪应用
In industry, radioactivity is used to monitor thickness of materials such as paper, plastic, and metal sheets. A beta source is placed on one side and a detector on the other; changes in the count rate indicate variations in thickness, allowing automatic control.
在工业中，放射性用于监测纸张、塑料和金属板的厚度。β源置于一侧，探测器置于另一侧；计数率的变化指示厚度变化，从而实现自动控制。
Radioactive tracers can also detect leaks in underground pipelines. A small amount of gamma-emitting isotope is added to the fluid. A detector at the surface pinpoints where radioactivity accumulates, revealing the leak location without excavation.
放射性示踪剂还可检测地下管道的泄漏。将少量γ发射同位素加入流体中，地面探测器定位放射性积聚处，无需开挖即可揭示泄漏位置。
10. Background Radiation and Sources | 背景辐射及其来源
Background radiation is the low-level radiation present in the environment from natural and artificial sources. Natural sources include cosmic rays, radon gas from rocks, and internal radiation from food and drink. Artificial sources include medical X‑rays, nuclear weapons tests, and nuclear power.
背景辐射是环境中存在的低水平辐射，来自天然和人工源。天然源包括宇宙射线、岩石释放的氡气以及食物和饮料带来的内照射。人工源包括医疗X射线、核武器试验和核能。
The typical annual background dose is around 2–3 millisieverts (mSv), but it varies with location and altitude. It is important to consider background count when measuring the activity of a sample in the lab; the true count rate is the measured rate minus the background rate.
典型的年背景剂量约为2–3毫希沃特（mSv），但因地点和海拔而异。在实验室测量样品活度时，必须考虑本底计数；真实计数率等于测量值减去本底率。
11. Safety Precautions and Handling | 安全预防措施与操作
When working with radioactive sources, the three key principles are time, distance, and shielding. Limit exposure time, maximise distance from the source (intensity decreases with the square of distance), and use appropriate shielding depending on the radiation type.
操作放射源时，三个关键原则是时间、距离和屏蔽。限制曝光时间，尽量远离源（强度随距离的平方衰减），并根据辐射类型使用适当的屏蔽。
In schools, only sealed sources of low activity may be used. They must be handled with tongs, pointed away from the body, and stored in lead-lined containers. Never eat, drink, or touch the face when handling sources to prevent contamination.
在学校里，只能使用低活度的密封源。必须用镊子操作，方向远离身体，并存放在带铅衬的容器中。操作时绝对禁止饮食或触摸面部，以防污染。
12. Summary of Key Ideas | 核心概念总结
Radioactivity is the random decay of unstable nuclei, emitting alpha, beta, or gamma radiation. Half-life determines the rate of decay and is essential for practical applications, from medical imaging to industrial monitoring. Safe handling relies on understanding contamination, irradiation, and protective measures.
放射性是不稳定原子核的随机衰变，发射α、β或γ辐射。半衰期决定衰变速率，对于从医学成像到工业监测的实际应用至关重要。安全操作依赖于对污染、辐照和防护措施的理解。
By mastering these concepts, students gain insight into both the risks and benefits of nuclear radiation, a topic that connects physics, chemistry, and biology in the IGCSE Edexcel Science curriculum.
掌握这些概念后，学生将深入了解核辐射的风险与益处，这是Edexcel IGCSE科学课程中联系物理、化学和生物学的一个主题。
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