IB Physics: Radioactive Decay and Nuclear Energy | IB物理：核物理中的衰变与核能

📚 IB Physics: Radioactive Decay and Nuclear Energy | IB物理：核物理中的衰变与核能

Nuclear physics is one of the most rewarding topics in IB Physics. It links the micro-world of quarks and gluons to the macroscopic energy that powers stars and nuclear reactors. In this revision guide, you will learn how to write decay equations, apply the exponential decay law, calculate binding energy, and evaluate the energy released in fission and fusion. We will also highlight common exam traps and provide practice questions.

核物理是IB物理中最具价值的话题之一。它将夸克与胶子的微观世界与恒星和核反应堆中的宏观能量联系起来。在这篇复习指南中，你将学会书写衰变方程、应用指数衰变定律、计算结合能，并评估裂变与聚变释放的能量。我们还会指出常见考试陷阱并提供练习题。

1. Atomic Structure and Nuclide Notation | 原子结构与核素符号

The nucleus is composed of protons and neutrons, collectively called nucleons. The proton number Z defines the element, while the nucleon number A = Z + N gives the total number of nucleons. In standard notation, a nuclide is written as ^A_ZX, for example ²³⁸₉₂U for uranium-238.

原子核由质子和中子组成，统称为核子。质子数Z决定元素种类，核子数A = Z + N表示核子总数。标准核素符号写作^A_ZX，例如铀-238写作²³⁸₉₂U。

Isotopes have the same Z but different N. They have identical chemical behaviour, but their nuclear stability and mass differ. Understanding this distinction is essential for balancing nuclear equations.

同位素具有相同的Z但不同的N，它们的化学性质相同，但核稳定性和质量不同。理解这一区别对配平核反应方程至关重要。

2. Types of Radioactive Decay | 放射性衰变的类型

Alpha decay occurs when an unstable nucleus emits an alpha particle, which is a helium nucleus ⁴₂He²⁺. The mass number decreases by 4 and the atomic number by 2. A typical example is:

α衰变发生于不稳定核发射一个α粒子时，α粒子即氦核⁴₂He²⁺。质量数减少4，原子序数减少2。一个典型例子是：

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He²⁺

Beta-minus decay occurs when a neutron converts into a proton, emitting an electron (β⁻) and an antineutrino. In this process the mass number is unchanged, but the atomic number increases by 1. Example:

β⁻衰变发生在一个中子转化为一个质子时，同时发射一个电子（β⁻）和一个反中微子。该过程中质量数不变，原子序数增加1。例如：

¹⁴₆C → ¹⁴₇N + e⁻ + ν̄ₑ

Beta-plus decay occurs when a proton converts into a neutron, emitting a positron (β⁺) and a neutrino. The atomic number decreases by 1, while the mass number stays constant. Example:

β⁺衰变发生在一个质子转化为一个中子时，同时发射一个正电子（β⁺）和一个中微子。原子序数减少1，质量数保持不变。例如：

¹¹₆C → ¹¹₅B + e⁺ + νₑ

Gamma decay follows alpha or beta decay when the daughter nucleus is left in an excited state. It releases a high-energy photon without changing A or Z.

γ衰变通常发生在α或β衰变之后，此时子核处于激发态。它发射高能光子，但A和Z均不改变。

3. Decay Law and Half-Life | 衰变定律与半衰期

The decay constant λ is the probability of decay per unit time. The activity A is the rate at which nuclei decay, given by A = λN, where N is the number of undecayed nuclei. The number of nuclei decreases exponentially with time:

衰变常数λ是单位时间内发生衰变的概率。活度A是原子核衰变的速率，满足A = λN，其中N是未衰变的原子核数。原子核数随时间指数减少：

N = N₀e^−λt, A = A₀e^−λt

The half-life T_1/2 is the time taken for half the nuclei to decay. It is related to λ by:

半衰期T_1/2是指一半原子核发生衰变所需的时间，它与λ的关系为：

T_1/2 = ln2 / λ

These equations are central to IB data-analysis questions. Always check whether a question gives λ or T_1/2 before substituting values.

这些方程是IB数据分析题的核心。代入数值前，一定要确认题目给出的是λ还是T_1/2。

4. Decay Chains and Radio

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