Radioactive Decay | 放射性衰变 考点精讲

📚 Radioactive Decay | 放射性衰变 考点精讲

Radioactive decay is a fundamental nuclear process that lies at the heart of IGCSE CCEA Physics. It involves the spontaneous transformation of an unstable atomic nucleus into a more stable one, accompanied by the emission of ionising radiation. Understanding the nature, properties, and effects of alpha, beta, and gamma radiation, as well as concepts like half-life and background radiation, is essential for mastering the topic and applying it to real-world contexts.

放射性衰变是核物理的核心过程，也是IGCSE CCEA物理的重要考点。它涉及不稳定的原子核自发转变为更稳定的核，并放出电离辐射。掌握α、β、γ射线的本质、特性以及半衰期、背景辐射等概念，是学好本单元并将其应用于实际情境的关键。

1. The Atomic Nucleus and Isotopes | 原子核与同位素

All atoms contain a tiny, dense nucleus made of protons and neutrons (collectively called nucleons). The number of protons, Z, defines the element and is called the atomic number. The total number of nucleons, A, is the mass number. Isotopes are atoms of the same element (same Z) that have different numbers of neutrons, hence different mass numbers.

所有原子都包含一个微小的、致密的原子核，由质子和中子（统称核子）组成。质子数 Z 确定了元素种类，称为原子序数。核子总数 A 为质量数。同位素指的是同一元素（质子数相同）但中子数不同，因而质量数不同的原子。

2. What is Radioactive Decay? | 什么是放射性衰变？

Radioactive decay is the random, spontaneous process by which an unstable nucleus emits radiation to become more stable. It cannot be predicted which individual nucleus will decay next; we can only describe the probability of decay for a large number of nuclei. The rate of decay is unaffected by external conditions such as temperature, pressure, or chemical bonding.

放射性衰变是一个随机、自发的过程，不稳定原子核通过放出辐射变得更稳定。无法预知哪个原子核会下一刻衰变，只能描述大量原子核的衰变概率。衰变速率不受温度、压强或化学键等外部条件的影响。

3. Types of Nuclear Radiation | 核辐射的类型

There are three main types of nuclear radiation emitted during decay: alpha (α) particles, beta (β) particles, and gamma (γ) rays. Occasionally, positron (β⁺) emission may be mentioned for completeness, but the CCEA IGCSE specification focuses on alpha, beta-minus, and gamma.

核衰变主要放出三种辐射：α粒子、β粒子和γ射线。为完整起见，有时也会提到正电子发射，但CCEA IGCSE考纲重点在α、β⁻和γ射线。

4. Properties of Alpha, Beta, and Gamma | α、β、γ 的特性

Alpha particle (α): It is a helium nucleus consisting of 2 protons and 2 neutrons. Its symbol is ⁴₂He. It has a charge of +2e and a relative mass of 4 u. Alpha particles travel at about 5–10% of the speed of light.

α粒子：由两个质子和两个中子组成的氦核，符号为 ⁴₂He。它带 +2e 电荷，相对质量约为 4 u，速度约为光速的 5%–10%。

Beta particle (β⁻): It is a fast-moving electron emitted from the nucleus when a neutron decays into a proton. Its symbol is ⁰₋₁e. It carries a charge of –1e and has a very small mass (about 1/1836 of a proton). Beta particles can reach speeds close to the speed of light.

β⁻粒子：核内一个中子衰变成质子时放出的高速电子，符号为 ⁰₋₁e。带 –1e 电荷，质量极小（约为质子的 1/1836），速度可接近光速。

Gamma ray (γ): It is a high-energy electromagnetic wave with no mass and no charge. Gamma photons travel at the speed of light and are often emitted after alpha or beta decay to release excess energy from the nucleus.

γ射线：是一种高能电磁波，无质量、无电荷。γ光子以光速运动，常伴随α或β衰变后释放出原子核的多余能量。

5. Penetrating Power and Ionising Ability | 穿透力与电离能力

The three radiations differ greatly in their ability to penetrate materials and to ionise atoms they encounter. Ionisation refers to the removal of electrons from atoms, creating ions. The table below summarises their properties.

三种辐射在穿透物料和使相遇原子电离的能力上有显著差异。电离指从原子中剥离电子形成离子的过程。下表总结了它们的特性。