GCSE OCR Physics: Radioactive Decay | 放射性衰变考点精讲

📚 GCSE OCR Physics: Radioactive Decay | 放射性衰变考点精讲

Radioactive decay is a spontaneous and random process by which an unstable atomic nucleus loses energy by emitting radiation. In OCR GCSE Physics, this topic covers the nature of alpha, beta, and gamma radiation, their properties, nuclear equations, half-life, and the uses and dangers of radioactive materials. Mastering these concepts is essential for the exam.

放射性衰变是一种自发的、随机的过程，不稳定的原子核通过释放辐射来损失能量。在 OCR GCSE 物理中，本专题涵盖 α、β 和 γ 辐射的本质、性质、核方程、半衰期以及放射性物质的应用与危害。掌握这些概念对考试至关重要。

1. Atomic Structure and Isotopes | 原子结构与同位素

Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons. The atomic number (Z) is the number of protons, which determines the element. The mass number (A) is the total number of protons and neutrons. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. For example, carbon-12 (¹²₆C) and carbon-14 (¹⁴₆C) are isotopes; both have 6 protons, but carbon-14 has 8 neutrons instead of 6.

原子由包含质子和中子的原子核以及绕核运动的电子组成。原子序数 (Z) 就是质子数，决定了元素的种类。质量数 (A) 是质子数与中子数之和。同位素是同一种元素中质子数相同但中子数不同的原子。例如碳-12 (¹²₆C) 和碳-14 (¹⁴₆C) 互为同位素，两者均有6个质子，但碳-14有8个中子，而碳-12只有6个中子。

2. Unstable Nuclei and Radioactive Decay | 不稳定原子核与放射性衰变

Some isotopes are unstable because their nuclei have an imbalance of protons and neutrons. This instability causes the nucleus to spontaneously emit radiation in order to become more stable — a process called radioactive decay. The decay is completely random and unaffected by external conditions such as temperature, pressure, or chemical bonding. We cannot predict which individual nucleus will decay next.

一些同位素不稳定，是因为原子核内的质子和中子比例失衡。这种不稳定性使原子核自发地释放辐射，以变得更稳定——这一过程称为放射性衰变。衰变完全是随机的，不受温度、压力或化学键等外部条件的影响。我们无法预知具体哪个原子核会在下一刻发生衰变。

3. Alpha, Beta and Gamma Radiation | α、β、γ 辐射

There are three main types of nuclear radiation: alpha (α), beta (β) and gamma (γ). An alpha particle is a helium nucleus, consisting of 2 protons and 2 neutrons, symbolised as ⁴₂He. A beta particle (specifically β⁻) is a fast-moving electron emitted when a neutron in the nucleus changes into a proton, written as ⁰₋₁e. Gamma radiation is not a particle but an electromagnetic wave of very high frequency and energy, having no mass and no charge, represented by ⁰₀γ.

核辐射主要有三种类型：α (alpha)、β (beta) 和 γ (gamma)。α 粒子是氦原子核，由2个质子和2个中子组成，符号为 ⁴₂He。β 粒子（特指 β⁻）是原子核内一个中子转变为质子时释放出的高速电子，写作 ⁰₋₁e。γ 辐射不是粒子，而是频率极高、能量极大的电磁波，既无质量也不带电荷，符号为 ⁰₀γ。

4. Properties and Comparisons | 特性对比

Alpha radiation is the most ionising (it easily knocks electrons off atoms) but the least penetrating; it can be stopped by a sheet of paper or a few centimetres of air. Beta radiation is moderately ionising and moderately penetrating; it passes through paper but is absorbed by a few millimetres of aluminium. Gamma radiation is the least ionising but the most penetrating; thick lead or several metres of concrete are needed to reduce its intensity significantly. Both alpha and beta are charged, so they are deflected by electric and magnetic fields, whereas gamma carries no charge and travels in a straight line.

α 辐射的电离能力最强（很容易击出原子的电子），但穿透力最弱；一张纸或几厘米空气就能将其阻挡。β 辐射的电离能力和穿透力均为中等；它能穿透纸张，但会被几毫米厚的铝片吸收。γ 辐射的电离能力最弱，但穿透力最强；需要厚铅板或数米混凝土才能显著降低其强度。α 和 β 都带电，因此在电场和磁场中会发生偏转，而 γ 不带电荷，沿直线传播。

The table below summarises the key properties of alpha, beta, and gamma radiation:

下表总结了 α、β 和 γ 辐射的主要特性：

5. Nuclear Decay Equations | 核衰变方程

In nuclear equations, the total mass number and total atomic number must balance. In alpha decay, the nucleus loses 2 protons and 2 neutrons, so the mass number decreases by 4 and the atomic number decreases by 2. For example, uranium-238 decays to thorium-234: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. In beta-minus decay, a neutron transforms into a proton, emitting an electron and an antineutrino. The mass number remains the same, while the atomic number increases by 1. An example is carbon-14 decaying to nitrogen-14: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e.

在核方程中，总质量数和总原子序数必须守恒。在 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。例如铀-238衰变为钍-234：²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He。在 β⁻ 衰变中，一个中子转变为质子，释放出一个电子和一个反中微子。质量数保持不变，而原子序数增加1。例如碳-14衰变为氮-14：¹⁴₆C → ¹⁴₇N + ⁰₋₁e。

6. 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 for the count rate (activity) to fall by half. Importantly, half-life is a constant for a particular isotope — it cannot be changed by physical or chemical means. A short half-life means the source is highly active and decays quickly, while a long half-life indicates low activity but prolonged emission. The random nature of decay means that exactly half the nuclei decay only on average over a large number of nuclei.

放射性同位素的半衰期是指样品中一半不稳定原子核发生衰变，或计数率（活度）降至一半所需的时间。重要的是，半衰期对于特定同位素是恒定的 —— 无法通过物理或化学方法改变。半衰期短意味着放射源活度高、衰变迅速，而半衰期长则表示活度低但辐射持续很久。衰变的随机性意味着 “准确一半的核衰变” 只是大量原子核的平均结果。

7. Half-life Calculations | 半衰期计算

If a sample initially contains N₀ unstable

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