📚 Atoms & Radioactivity | 原子与放射性
Everything around us is made of atoms, but inside the atom there is a tiny, dense nucleus that holds the secrets of radioactivity. In IGCSE Physics, understanding the structure of the atom, the nature of alpha, beta and gamma radiation, and how unstable nuclei decay is essential. This article covers the Edexcel syllabus points on atomic structure, isotopes, radioactive emissions, half-life, background radiation, and both the uses and dangers of ionising radiation.
我们周围的一切都由原子构成，但原子内部有一个极小却致密的原子核，蕴藏着放射性的秘密。在IGCSE物理中，掌握原子结构、α、β和γ辐射的性质以及不稳定核如何衰变至关重要。本文涵盖Edexcel考纲中关于原子结构、同位素、放射性释放物、半衰期、背景辐射以及电离辐射的应用与危害等内容。
1. Atomic Structure | 原子结构
Atoms consist of a central nucleus surrounded by electrons arranged in energy levels. The nucleus contains positively charged protons and neutral neutrons, collectively called nucleons. Almost all the mass of the atom is concentrated in the nucleus, yet the nucleus occupies only a tiny fraction of the atom's volume. The electrons have very little mass and carry a negative charge equal in magnitude to the proton's positive charge.
原子由一个中心原子核和分层排布的电子组成。原子核包含带正电的质子和不带电的中子，统称为核子。原子几乎全部质量都集中于核内，但原子核只占据原子体积极小的一部分。电子质量极小，带有一个与质子正电荷大小相等的负电荷。
The atomic number (Z) is the number of protons in an atom and determines the element. The mass number (A) is the total number of protons and neutrons. A neutral atom has the same number of electrons as protons. The nuclear model, developed after Rutherford's gold foil experiment, shows that most of the atom is empty space, with a small positive nucleus that can deflect alpha particles backwards.
原子序数（Z）是原子中质子的数目，决定了元素的种类。质量数（A）是质子数与中子数之和。中性原子的电子数与质子数相等。根据卢瑟福金箔实验建立的核模型表明，原子内部大部分是虚空，只有一个很小的带正电的原子核能使α粒子向后偏转。
2. Isotopes | 同位素
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers. For example, carbon-12 (¹² ₆C) and carbon-14 (¹⁴ ₆C) are isotopes of carbon. Both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons.
同位素是质子数相同、中子数不同的同种元素原子。它们的原子序数相同而质量数不同。例如，碳-12（¹² ₆C）和碳-14（¹⁴ ₆C）是碳的同位素。两者都有6个质子，但碳-12有6个中子，碳-14有8个中子。
Because chemical properties are determined by the electron arrangement, and therefore by the number of protons, isotopes of the same element have identical chemical behaviour. However, their nuclear stability may differ greatly; some isotopes are radioactive and will decay spontaneously, while others are stable.
由于化学性质由电子排布决定，从而由质子数决定，同种元素的同位素具有完全相同的化学行为。但它们的核稳定性可能差异很大；一些同位素具有放射性并会自发衰变，而另一些则保持稳定。
3. Radioactive Decay | 放射性衰变
An unstable nucleus releases energy by emitting radiation in a process called radioactive decay. This is a completely random process – we cannot predict which nucleus will decay next, but we can describe the overall behaviour of a large number of nuclei using probability. The nucleus may emit an alpha particle, a beta particle or a gamma ray, often gaining stability by changing the balance of protons and neutrons.
不稳定的原子核通过释放辐射来释放能量，这一过程称为放射性衰变。这是一个完全随机的过程——我们无法预测哪个原子核会接着衰变，但可用概率描述大量原子核的整体行为。原子核可能发射α粒子、β粒子或γ射线，通常通过改变质子与中子的比例而变得更稳定。
Radioactivity was discovered by Henri Becquerel, and further investigated by Marie Curie. The emission of radiation occurs from the nucleus and is unaffected by chemical bonding or physical conditions such as temperature and pressure, because it is a nuclear process. The activity of a radioactive source is measured in becquerels (Bq), where 1 Bq equals one decay per second.
放射性由贝克勒尔发现，并由居里夫人进一步研究。辐射从原子核发出，不受化学键或温度、压力等物理条件的影响，因为它属于核过程。放射源的活度以贝可勒尔（Bq）为单位，1 Bq相当于每秒衰变一次。
4. Alpha Particles | α粒子
An alpha particle is a helium nucleus, consisting of two protons and two neutrons. It is represented by the symbol ⁴₂He or the Greek letter α. Because it has a charge of +2 and a relatively large mass, alpha radiation interacts strongly with matter, causing intense ionisation. It can knock electrons out of atoms in its path, producing many ion pairs over a short distance.
α粒子是一个氦原子核，由两个质子和两个中子组成。它用符号 ⁴₂He 或希腊字母 α 表示。由于α粒子带有+2的电荷且质量相对较大，它与物质发生强烈的相互作用，造成强烈的电离。它能在短距离内从路径上的原子中撞出大量电子，产生许多离子对。
Alpha particles have a very short range in air – only a few centimetres – and can be stopped by a sheet of paper or the outer layer of human skin. Their great ionising ability makes them especially dangerous if an alpha-emitting source is ingested or inhaled, because then they can damage delicate internal tissues.
α粒子在空气中的射程非常短——仅几厘米——并且能被一张纸或人体皮肤外层阻挡。它们极强的电离能力意味着，一旦吸入或食入α放射源，就会变得极为危险，因为它们能损伤脆弱的内部组织。
5. Beta Particles | β粒子
A beta particle is a fast-moving electron emitted from the nucleus when a neutron turns into a proton. It is represented by the symbol ⁰₋₁e or β⁻. Beta particles have a charge of −1 and a much smaller mass than alpha particles. They are moderately ionising: they can still remove electrons from atoms, but they produce fewer ion pairs per millimetre of travel than alpha radiation.
β粒子是原子核内一个中子转变为质子时释放的高速电子，用符号 ⁰₋₁e 或 β⁻ 表示。β粒子带−1电荷，质量比α粒子小得多。它们具有中等的电离能力：虽然仍能从原子中撞出电子，但每毫米行程产生的离子对比α辐射少。
Beta particles can travel several metres in air and can penetrate paper or the outer skin. They are stopped by a few millimetres of aluminium. Because their ionising power is lower, they are less damaging per unit path length than alpha particles but can penetrate deeper into the body, potentially harming internal organs if the source is outside the body.
β粒子在空气中能传播数米，并可穿透纸张或皮肤表层。它们能被几毫米厚的铝板阻挡。由于其电离能力较弱，单位路径损伤比α粒子小，但穿透力更强，若放射源位于体外，仍可能深入体内伤害内脏。
6. Gamma Rays | γ射线
Gamma rays are electromagnetic waves of very high frequency and short wavelength. They carry no charge and have no mass. The symbol γ is used. Being uncharged, gamma rays lose energy much more slowly than alpha or beta particles and are far more penetrating. They can travel through many centimetres of lead or even metres of concrete, and only a dense, thick material can significantly reduce their intensity.
γ射线是频率极高、波长极短的电磁波，不带电，也没有质量，用符号 γ 表示。由于不带电，γ射线能量损失比α或β粒子慢得多，穿透力远胜前两者。它们可穿透数厘米的铅甚至数米厚的混凝土，只有致密厚实的材料才能显著减弱其强度。
Gamma rays are the least ionising of the three types of radiation. They tend to interact by transferring energy to an entire atom, rather than directly removing an electron. However, because of their deep penetration, gamma rays can cause widespread damage to tissues and are particularly hazardous for external exposure.
γ射线是三种辐射中电离能力最弱的。它们往往通过与整个原子交换能量来发生相互作用，而非直接撞出电子。然而，由于其穿透深度大，γ射线可对组织造成广泛损伤，在外部照射时尤其危险。
7. Comparison of Penetration and Ionisation | 穿透力与电离能力比较
The table below summarises the penetrating and ionising abilities of the three types of radiation, along with the materials that stop them and their ranges in air.
下表总结了三种辐射的穿透力、电离能力、阻挡材料及在空气中的射程。
| Radiation | Ionising power | Penetration | Stopped by | Range in air |
|---|---|---|---|---|
| Alpha (α) | Very high | Very low | Paper, skin | ~5 cm |
| Beta (β) | Medium | Medium | Few mm Al | ~1 m |
| Gamma (γ) | Low | Very high | Thick Pb, concrete | Infinite (inverse square) |
A common experiment to demonstrate these properties involves placing a detector such as a Geiger-Muller tube at different distances and interposing sheets of paper, aluminium or lead. The count rate drops sharply when the right absorber is inserted, confirming the identity of the radiation.
展示这些性质的常见实验是，在不同距离处放置盖革-米勒管探测器，并在中间插入纸张、铝板或铅片。当插入适当的吸收体时，计数率会急剧下降，从而确定辐射的类型。
8. Nuclear Decay Equations | 核衰变方程
In any nuclear decay equation, both the total mass number and the total atomic number are conserved. For alpha decay, the mass number decreases by 4 and the atomic number decreases by 2. An example is the decay of uranium-238 to thorium-234:
在任何核衰变方程中，总质量数和总原子序数都是守恒的。α衰变中，质量数减少4，原子序数减少2。例如铀-238衰变为钍-234：
²³⁸ ₉₂U → ²³⁴ ₉₀Th + ⁴₂He
For beta decay, a neutron in the nucleus changes into a proton, so the atomic number increases by 1 while the mass number stays the same. An electron antineutrino is also emitted, but the IGCSE equation commonly shows only the beta particle. Carbon-14 decay is a typical case:
β衰变中，原子核内一个中子转变为质子，因此原子序数增加1，质量数保持不变。同时放出一个反电子中微子，但IGCSE方程通常只给出β粒子。碳-14衰变是典型例子：
¹⁴ ₆C → ¹⁴ ₇N + ⁰₋₁e
Gamma radiation is often emitted alongside alpha or beta decay if the daughter nucleus is left in an excited state. Gamma emission does not change the mass number or atomic number, so the equation simply shows the nucleus dropping to a lower energy level, often indicated by the symbol γ.
γ辐射往往伴随着α或β衰变产生，如果子核处于激发态。γ发射不改变质量数和原子序数，因此方程只表示核跃迁至较低能级，通常用符号 γ 标示。
9. Half-Life | 半衰期
The half-life of a radioactive isotope is the time taken for half of the unstable nuclei in a sample to decay, or equivalently, the time taken for the count rate from the sample to fall to half its initial value. It is a statistical measure that applies to a large number of nuclei; the decay of an individual nucleus is unpredictable.
放射性同位素的半衰期是指样品中一半的不稳定原子核发生衰变所需的时间，或等价地，样品的计数率降至初始值一半所需的时间。半衰期是针对大量原子核的统计量度，单个核的衰变不可预测。
Half-life values vary enormously among isotopes: uranium-238 has a half-life of about 4.5 billion years, while radon-222 has a half-life of only 3.8 days. Using the half-life, we can calculate the remaining mass or activity after a given time. For example, if the half-life is 10 minutes and we start with 80 g, after 20 minutes (two half-lives) there will be 20 g left.
不同同位素的半衰期差异巨大：铀-238的半衰期约为45亿年，而氡-222的半衰期只有3.8天。利用半衰期可以计算给定时间后剩余的质量或活度。例如，若半衰期为10分钟，初始质量为80克，经过20分钟（两个半衰期）后，将剩余20克。
Half-life graphs plot activity or number of undecayed nuclei against time. Such graphs always show the same characteristic exponential decay shape. From the graph, the half-life can be read directly as the time interval for the count rate to halve.
半衰期图绘出了活度或未衰变核数目随时间的变化。此类图总是呈现相同的指数衰减形状。从图上可直接读出计数率减半所对应的时间间隔，即半衰期。
10. Background Radiation | 背景辐射
We are constantly exposed to low levels of ionising radiation from natural and artificial sources, called background radiation. Natural sources include cosmic rays from space, radioactive gases such as radon from the ground, and naturally occurring radioactive materials in rocks, soil, food and even the human body. Artificial sources include medical X-rays, nuclear power and fallout from past nuclear weapons testing, though these contribute a smaller proportion.
我们持续暴露在低水平的电离辐射中，这些来自天然和人工源的辐射叫作背景辐射。天然源包括来自太空的宇宙射线、由地面释放的氡等放射性气体，以及岩石、土壤、食物甚至人体内天然存在的放射性物质。人工源有医用X射线、核能以及过去核武器试验的沉降物，不过它们所占的比例较小。
Background radiation varies with location: areas rich in granite often have higher levels due to uranium and thorium in the rock. Jobs such as airline crew receive higher cosmic ray doses at altitude. When measuring the activity of a radioactive source in the laboratory, the background count rate must be subtracted from the total count rate to obtain the true count rate of the source.
背景辐射因地点而异：花岗岩丰富的地区因岩石中含有铀和钍，辐射水平往往较高。空乘等职业由于在高海拔飞行，接受的宇宙射线剂量更大。在实验室测量放射源的活度时，必须从总计数率中减去背景计数率，才能得到放射源的真实计数率。
11. Uses of Radiation | 放射性的用途
Radiation has many beneficial applications in medicine, industry and research. Alpha sources such as americium-241 are used in smoke detectors: the alpha particles ionise air between two electrodes, producing a small current. Smoke particles absorb the alpha particles, reducing the current and triggering the alarm.
放射在医学、工业和科研领域有许多有益的应用。α源（如镅-241）用于烟雾探测器：α粒子使两电极之间的空气电离，产生微小电流。烟雾颗粒吸收α粒子，电流减小，从而触发警报。
Beta emitters are ideal for monitoring the thickness of paper, plastic or metal foil during production. A beta source and detector are placed on opposite sides of the material; if the thickness changes, the amount of beta radiation reaching the detector changes, allowing automatic adjustment of rollers. Gamma rays are widely used to sterilise medical equipment by killing bacteria, and in radiotherapy to treat cancerous tumours by targeting the cancer cells with precise beams.
β放射源非常适合在生产过程中监测纸张、塑料或金属箔的厚度。将β源和探测器分别置于材料两侧；若厚度变化，到达探测器的β辐射量随之改变，从而自动调节轧辊。γ射线广泛用于对医疗设备进行灭菌（杀灭细菌），以及通过精确束流照射癌细胞来治疗癌症的放疗。
Tracers are another important application: a radioactive isotope is introduced into a system, and its radiation is detected to follow the movement of fluids, detect leaks or study processes in medicine, such as examining thyroid function with iodine-131. The isotope chosen must have a half-life long enough to complete the study but short enough to minimise long-term radiation dose.
示踪物是另一重要应用：将放射性同位素引入体系中，通过探测其辐射来追踪液体流动、检测泄漏或进行医学研究，例如用碘-131检查甲状腺功能。所选同位素的半衰期必须足够长以完成研究，同时又要足够短以尽量减少长期辐射剂量。
12. Dangers and Safety Precautions | 危险与安全措施
Ionising radiation can damage cells by breaking chemical bonds and altering DNA, which may lead to cell death, mutations or cancer. The severity of the damage depends on the type and energy of radiation, the dose received, and whether the exposure is acute or prolonged. Alpha sources are most dangerous inside the body, while gamma and high-energy beta sources present risks from external exposure.
电离辐射会通过破坏化学键和改变DNA来损伤细胞，可能导致细胞死亡、突变或癌症。损伤的严重程度取决于辐射的类型和能量、接受的剂量以及是急性还是长期暴露。α放射源在体内最为危险，而γ源和高能β源则带来外部照射风险。
To minimise risk, strict safety precautions must be followed when handling radioactive sources. Sources should never be touched with bare hands; forceps or tongs should be used and the source kept at arm's length. When not in use, sources must be stored in lead-lined containers. Exposure time should be kept as short as possible, and shielding appropriate to the radiation type must be in place. In school laboratories, sealed sources with low activity are used, and a badge monitor may measure cumulative dose.
为了将风险降至最低，操作放射源时必须严格遵守安全预防措施。绝对不可用手直接触碰，必须使用镊子或钳子，并将放射源保持在臂长距离。不使用时，应将源存放在衬铅的容器中。照射时间应尽可能短，并根据辐射类型设置适当的屏蔽。学校实验室使用低活度密封源，并可佩带个人剂量计监测累积剂量。
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