The Discovery of Radioactivity: History and Experimental Process | 放射性的发现：历史与实验过程

📚 The Discovery of Radioactivity: History and Experimental Process | 放射性的发现：历史与实验过程

The discovery of radioactivity is one of the most remarkable chapters in the history of physics. It began with a chance observation in 1896 and led within two decades to a revolutionary understanding of the atom, the nucleus, and the energy hidden inside matter.

放射性的发现是物理学史上最非凡的篇章之一。它始于1896年的一次偶然观察，并在二十年内引领人们对原子、原子核以及物质内部隐藏的能量产生了革命性的理解。

1. Background: X-rays and Fluorescence | 背景：X射线与荧光

In late 1895, Wilhelm Röntgen discovered X-rays while studying cathode rays in a vacuum tube. He noticed that a fluorescent screen glowed even when shielded from the tube. This sparked intense interest in the relationship between fluorescence and penetrating radiation.

1895年底，威廉·伦琴在研究真空管中的阴极射线时发现了X射线。他注意到荧光屏即使在遮挡下仍会发光。这激发了人们对荧光现象与穿透性辐射之间关系的强烈兴趣。

Henri Becquerel, a French physicist, came from a family that had studied fluorescence and phosphorescence for generations. He hypothesised that fluorescent minerals might also emit X-ray-like radiation after being illuminated by sunlight.

法国物理学家亨利·贝克勒尔出身于一个世代研究荧光和磷光的家族。他假设荧光矿物在阳光照射后也可能发出类似X射线的辐射。

2. Becquerel’s Accidental Discovery | 贝克勒尔的偶然发现

In February 1896, Becquerel placed uranium potassium sulphate crystals on photographic plates wrapped in black paper. He left them on a sunny windowsill, expecting sunlight to excite the crystals and produce penetrating rays.

1896年2月，贝克勒尔将硫酸铀钾晶体放在用黑纸包裹的照相底片上。他将它们放在阳光充足的窗台上，期望阳光激发晶体产生穿透性射线。

One day the weather was cloudy, so he put the wrapped plates and crystals in a dark drawer. Several days later, he developed the plates anyway — and found a strong, sharp image of the crystals. The radiation appeared even without sunlight.

有一天天气阴云密布，于是他将包裹好的底片和晶体放入黑暗的抽屉。几天后，他还是冲洗了底片——结果发现了晶体清晰而强烈的影像。即使没有阳光，辐射仍然出现。

Uranium salts spontaneously emitted penetrating radiation without external excitation.

铀盐在无外部激发的情况下自发发出穿透性辐射。

Becquerel concluded that the uranium itself was the source of the rays. This was the first evidence of what Marie Curie later named “radioactivity”.

贝克勒尔得出结论：铀本身是射线的来源。这是后来居里夫人命名为“放射性”的首个证据。

3. Marie and Pierre Curie: Polonium and Radium | 居里夫妇：钋与镭

Marie Curie, a young Polish physicist working in Paris, decided to investigate Becquerel’s rays for her doctoral thesis. She developed a sensitive electrometer method, based on the ionisation of air, to measure the intensity of radiation quantitatively.

年轻的波兰物理学家玛丽·居里在巴黎工作，决定将贝克勒尔射线作为博士论文课题。她开发了一种基于空气电离的灵敏静电计方法，用于定量测量辐射强度。

She tested many compounds and found that uranium and thorium were both active. More surprisingly, some minerals such as pitchblende were far more radioactive than expected from their uranium content alone.

她测试了许多化合物，发现铀和钍都具有放射性。更令人惊讶的是，某些矿物如沥青铀矿的放射性远高于仅凭铀含量所预期的水平。

Pierre and Marie Curie worked together to purify pitchblende, a tedious chemical process. In 1898 they announced the discovery of polonium (named after Poland), and later the same year they discovered radium, which was over a million times more radioactive than uranium.

皮埃尔和玛丽·居里合作提纯沥青铀矿，过程极其繁琐。1898年他们宣布发现钋（以波兰命名），同年晚些时候又发现了镭，其放射性比铀强一百万倍以上。

The discovery of radium proved that radioactivity was a property of certain atoms, not merely a chemical reaction.

镭的发现证明放射性是某些原子的固有性质，而不仅仅是一种化学反应。

4. Rutherford’s Three Types of Radiation | 卢瑟福的三种辐射

Ernest Rutherford, a New Zealand physicist working at Cambridge and later Manchester, carried out a series of elegant experiments to characterise Becquerel rays. He placed a radioactive source in a lead block with a narrow hole, allowing a beam of radiation to pass between charged plates and into a detector.

新西兰物理学家欧内斯特·卢瑟福先后在剑桥和曼彻斯特工作，进行了一系列精巧的实验来表征贝克勒尔射线。他将放射源置于带窄孔的铅块中，使辐射束穿过带电板之间并进入探测器。

He found that the radiation split into three components under an electric field:

他发现辐射在电场中分裂为三个组分：

Alpha (α) rays — positively charged, strongly deflected toward the negative plate, with low penetration.
Alpha (α) 射线 — 带正电，向负极板强烈偏转，穿透力弱。
Beta (β) rays — negatively charged, deflected in the opposite direction, with greater penetration.
Beta (β) 射线 — 带负电，向相反方向偏转，穿透力较强。
Gamma (γ) rays — undeflected by electric or magnetic fields, behaving as high-energy electromagnetic radiation.
Gamma (γ) 射线 — 不受电场或磁场偏转，表现为高能电磁辐射。

By measuring the ratio of charge to mass for beta particles, Rutherford and others showed that beta particles were fast-moving electrons. Alpha particles were later identified as helium nuclei, having charge +2e and mass approximately four atomic mass units.

通过测量β粒子的荷质比，卢瑟福等人证明β粒子是高速运动的电子。α粒子后来被确定为氦原子核，电荷为+2e，质量约为四个原子质量单位。

5. Properties of α, β and γ Radiation | α、β和γ辐射的性质

For CIE A-Level physics, you must master the contrasting properties of the three radiations.

对于CIE A-Level物理，你必须掌握三种辐射的对比性质。

Property	Alpha (α)	Beta (β)	Gamma (γ)
Nature	Helium nucleus ⁴₂He	Fast electron ⁰₋₁e	Electromagnetic wave (photon)
Charge	+2e	−e	0
Speed	≈ 0.05c	up to 0.99c	c
Ionising power	Strongest	Moderate	Weakest
Penetration	Few cm of air; stopped by paper	Few mm of aluminium	Several cm of lead; metres of concrete

The strong ionising power of alpha particles arises from their large charge and mass. They collide with atoms along their path, knocking electrons away and creating a dense track of ions.

α粒子强大的电离能力源于其大电荷和大质量。它们沿途与原子碰撞，撞击出电子，形成致密的离子径迹。

6. Radioactive Decay and the Decay Law | 放射性衰变与衰变定律

Rutherford, Soddy and others established that the emission of radiation is a random, spontaneous and statistical process. For a sample containing N unstable nuclei, the rate of decay is proportional to N.

卢瑟福、索迪等人确立辐射发射是一个随机、自发且统计性的过程。对于含有N个不稳定原子核的样品，衰变率与N成正比。

dN/dt = −λN

Here λ is the decay constant, which is the probability of decay per unit time for a single nucleus. The negative sign indicates that N decreases with time.

其中λ是衰变常数，表示单个原子核单位时间内衰变的概率。负号表示N随时间减少。

Integrating this equation gives the exponential decay law:

积分该方程得到指数衰变定律：

N = N₀e^(−λt)

where N₀ is the initial number of nuclei. The half-life T₁/₂ is related to λ by:

其中N₀是初始原子核数目。半衰期T₁/₂与λ的关系为：

T₁/₂ = ln 2 / λ ≈ 0.693 / λ

The activity A of a sample is the number of decays per second, A = λN, measured in becquerels (Bq). One becquerel is one decay per second.

样品的活度A是每秒衰变数，A = λN，单位为贝克勒尔（Bq）。1贝克勒尔等于每秒一次衰变。

7. Detection: Cloud Chamber and Counter | 探测：云室与计数器

The early experimenters needed tools to visualise or count invisible radiation. The cloud chamber, invented by Charles Wilson in 1911, allowed alpha and beta particles to be photographed as trails of tiny droplets.

早期实验者需要工具来可视化或计数不可见的辐射。查尔斯·威尔逊于1911年发明的云室，能够让α和β粒子以微小液滴的径迹形式被拍摄下来。

Supersaturated alcohol vapour condenses onto ions along the particle’s path, revealing the track.

过饱和酒精蒸气沿粒子路径凝结在离子上，从而显现径迹。

A cloud chamber photograph of alpha particles shows short, thick, straight tracks. Beta particle tracks are longer, thinner and often curved due to collisions with air molecules. Gamma rays produce faint, scattered tracks from secondary electrons.

云室照片中α粒子表现为短、粗、直的径迹。β粒子径迹较长、较细且常因与空气分子碰撞而弯曲。γ射线则产生来自次级电子的模糊、散乱的径迹。

Another key detector is the Geiger-Müller tube. A high voltage between a central wire and the tube wall creates a strong electric field. Each ionising particle triggers an avalanche of charge, producing a voltage pulse that can be counted electronically.

另一种关键探测器是盖革-米勒管。中心丝极与管壁之间的高电压形成强电场。每个电离粒子都会触发电荷雪崩，产生可被电子计数的电压脉冲。

8. Rutherford’s Gold Foil Experiment | 卢瑟福的金箔实验

Between 1908 and 1913, Rutherford and his colleagues Geiger and Marsden performed experiments that revealed the structure of the atom. They directed a beam of alpha particles at a very thin gold foil, about 1000 atoms thick.

1908至1913年间，卢瑟福与助手盖革和马斯登进行了揭示原子结构的实验。他们将α粒子束射向厚度约1000个原子的极薄金箔。

Most alpha particles passed straight through the foil, but a small fraction were deflected through large angles, and one in about 8000 was scattered backwards through more than 90°.

大多数α粒子径直穿过金箔，但一小部分被大角度偏转，约8000个中约有1个被反向散射超过90°。

Most passed straight through → atoms are mostly empty space.
大多数直接穿过 → 原子内部大部分是空的。
Some deflected at large angles → there is a tiny, dense, positive nucleus.
有些大角度偏转 → 存在微小、致密、带正电的原子核。
Very few bounced back → the nucleus contains almost all the atom’s mass.
极少数反弹回来 → 原子核几乎集中了原子的全部质量。

This experiment replaced the “plum pudding” model of Thomson and established the nuclear model of the atom.

这个实验取代了汤姆孙的“葡萄干布丁”模型，确立了原子的核式模型。

9. Transmutation and the Neutron | 嬗变与中子的发现

In 1919, Rutherford achieved the first artificial nuclear transmutation. He bombarded nitrogen gas with alpha particles and observed that protons were emitted. The reaction can be written as:

1919年，卢瑟福实现了首次人工核嬗变。他用α粒子轰击氮气，观察到质子被释放。反应可写为：

¹⁴₇N + ⁴₂He → ¹⁷₈O + ¹₁H

This experiment also proved that the nucleus contains protons. Later, in 1932, James Chadwick discovered the neutron by bombarding beryllium with alpha particles:

该实验也证明原子核中含有质子。后来在1932年，詹姆斯·查德威克通过用α粒子轰击铍发现了中子：

⁹₄Be + ⁴₂He → ¹²₆C + ¹₀n

The existence of neutrons explained why the mass of a nucleus is larger than the total mass of its protons, and allowed scientists to understand isotopes, nuclear binding energy and the stability of different elements.

中子的存在解释了为什么原子核质量大于其质子总质量，并使科学家得以理解同位素、核结合能以及不同元素的稳定性。

10. Significance for A-Level Physics | 对A-Level物理的意义

At A-Level, you need to connect the discovery of radioactivity to several core topics:

在A-Level中，你需要将放射性的发现与几个核心主题联系起来：

Alpha decay: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He; the mass number decreases by 4 and the atomic number by 2.
Alpha 衰变： ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He；质量数减少4，原子序数减少2。
Beta decay: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e + anti-neutrino; the atomic number increases by 1.
Beta 衰变： ¹⁴₆C → ¹⁴₇N + ⁰₋₁e + 反中微子；原子序数增加1。
Gamma decay: usually follows alpha or beta decay and releases excess energy as a photon.
Gamma 衰变： 通常发生在α或β衰变之后，以光子形式释放多余能量。

You should also be able to use the equations N = N₀e^(−λt) and A = A₀e^(−λt) to calculate half-life, decay constant, and the number of nuclei remaining after a given time.

你还应能够使用方程N = N₀e^(−λt)和A = A₀e^(−λt)来计算半衰期、衰变常数以及给定时间后剩余的原子核数目。

11. Applications and Safety | 应用与安全

Radioactivity is used in medicine (radiotherapy and tracers), industry (thickness gauges), archaeology (carbon dating) and energy production (nuclear reactors). Its detection and safe handling require understanding of penetration, half-life and biological effects.

放射性被用于医学（放疗和示踪剂）、工业（厚度计）、考古（碳定年）和能源生产（核反应堆）。对它的探测和安全处理需要了解穿透力、半衰期和生物效应。

Because alpha particles are the most strongly ionising, they are especially dangerous if ingested or inhaled. Beta particles can cause skin burns, and gamma radiation requires thick shielding such as lead or concrete.

由于α粒子电离能力最强，如果被摄入或吸入则特别危险。β粒子可导致皮肤灼伤，而γ辐射需要铅或混凝土等厚重屏蔽。

The principle of ALARA — As Low As Reasonably Achievable — guides the use of radiation. Time, distance and shielding are the three basic protective measures.

ALARA原则——即“合理可行尽量低”——指导辐射的使用。时间、距离和屏蔽是三种基本防护措施。

12. Conclusion | 结论

From Becquerel’s accidental fogged plates to Rutherford’s nuclear model, the discovery of radioactivity transformed physics. It revealed that atoms are not indivisible, that matter can spontaneously transform, and that enormous energy is locked inside the nucleus.

从贝克勒尔偶然曝光的底片到卢瑟福的核式模型，放射性的发现彻底改变了物理学。它揭示了原子并非不可分割，物质可以自发转化，并且巨大能量被锁在原子核内部。

For A-Level candidates, mastering the experimental history reinforces the concepts of ionising radiation, decay equations, half-life and the structure of the nucleus. Understanding how evidence led to models helps you answer questions that ask “describe”, “explain” and “evaluate” — the very skills examined in CIE Physics Paper 4 and 5.

对于A-Level考生而言，掌握实验历史有助于巩固电离辐射、衰变方程、半衰期和原子核结构等概念。理解证据如何导向模型，能帮助你回答“描述”、“解释”和“评估”类问题——这正是CIE物理卷4和卷5所考查的技能。

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The Discovery of Radioactivity: History and Experimental Process | 放射性的发现：历史与实验过程