📚 GCSE Edexcel Physics: Particle Physics Key Points | 粒子物理考点精讲
Welcome to the GCSE Edexcel Physics revision guide for particle physics. This section covers the nuclear model of the atom, radioactivity, half-life, nuclear reactions, and the uses and risks of radiation. Understanding these concepts is essential for the Physics Paper 1 or 2 exam.
欢迎阅读GCSE Edexcel物理粒子物理复习指南。本部分涵盖原子的核模型、放射性、半衰期、核反应以及辐射的应用与风险。理解这些概念对于物理试卷至关重要。
1. Rutherford Scattering and the Nuclear Model | 卢瑟福散射与核模型
The Rutherford scattering experiment fired alpha particles at a thin gold foil. Most particles passed straight through, but a few were deflected at large angles. This led to the nuclear model where an atom has a tiny, dense, positively charged nucleus surrounded by electrons.
卢瑟福散射实验用α粒子轰击薄金箔。大多数粒子径直穿过，但少数粒子以很大角度偏转。由此得出了核模型：原子具有一个微小、致密、带正电的原子核，周围环绕着电子。
Before Rutherford, the plum pudding model suggested that positive charge and mass were spread evenly throughout the atom. Rutherford’s results disproved this and showed that the positive charge is concentrated in the nucleus.
在卢瑟福之前，“葡萄干布丁”模型认为正电荷和质量均匀分布在整个原子中。卢瑟福的结果否定了这个模型，表明正电荷集中在原子核内。
Later, the Bohr model introduced electrons in fixed energy levels, and Chadwick discovered the neutron, completing the nuclear model we use today.
后来，玻尔模型引入了电子处于固定能级的概念，查德威克发现了中子，完善了我们今天使用的核模型。
2. Isotopes and Nuclear Notation | 同位素与核符号
The number of protons in an atom is the atomic number (Z). The total number of protons and neutrons is the mass number (A). Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
原子中的质子数称为原子序数（Z）。质子与中子总数称为质量数（A）。同位素是同种元素的原子，质子数相同但中子数不同。
Nuclear notation shows the element symbol with mass number as a superscript and atomic number as a subscript, e.g. ¹⁴₆C for carbon-14.
核符号用上标表示质量数，下标表示原子序数，例如¹⁴₆C表示碳-14。
Carbon-12 and carbon-14 are both isotopes of carbon, with 6 protons each, but 6 and 8 neutrons respectively. Despite different physical properties, their chemical properties are identical.
碳-12和碳-14都是碳的同位素，各有6个质子，但中子数分别为6和8。尽管物理性质不同，它们的化学性质完全相同。
3. Radioactive Decay: Alpha, Beta and Gamma | 放射性衰变：α、β和γ
Unstable nuclei emit radiation to become more stable. This spontaneous process is called radioactive decay. There are three main types: alpha (α), beta (β), and gamma (γ).
不稳定的原子核会通过发射辐射变得更稳定。这个自发过程称为放射性衰变。主要有三种类型：α、β和γ。
Alpha decay: a heavy nucleus emits an alpha particle (two protons and two neutrons, i.e. a helium nucleus). The mass number decreases by 4, the atomic number decreases by 2. For example, uranium-238 decays to thorium-234: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He.
α衰变：重核发射一个α粒子（两个质子和两个中子，即氦核）。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。
Beta-minus decay: a neutron changes into a proton, emitting an electron and an antineutrino. The mass number stays the same, but the atomic number increases by 1. For example, carbon-14 decays: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e + anti-ν.
β⁻衰变：一个中子转变成质子，并发射一个电子和一个反中微子。质量数不变，原子序数增加1。例如碳-14衰变：¹⁴₆C → ¹⁴₇N + ⁰₋₁e + 反ν。
Gamma radiation is electromagnetic radiation emitted after alpha or beta decay when the nucleus has excess energy. It does not change the mass number or atomic number.
γ辐射是原子核在α或β衰变后仍具有过剩能量时发射的电磁波。它不改变质量数或原子序数。
4. Properties of Alpha, Beta and Gamma Radiation | α、β和γ射线的性质
Alpha particles are heavy, positively charged, and have a short range in air (a few cm). They are highly ionising but can be stopped by a sheet of paper or skin.
α粒子质量大，带正电，在空气中的射程很短（几厘米）。它们电离能力强，但能被一张纸或皮肤阻挡。
Beta particles are fast electrons, much lighter, and negatively charged. They have a medium range in air (around 1 m) and moderate ionising ability. They are stopped by a few mm of aluminium.
β粒子是高速电子，质量小，带负电。它们在空气中的中等射程（约1米），电离能力中等。几毫米厚的铝板即可阻挡。
Gamma rays are electromagnetic waves with no mass or charge. They are weakly ionising, have a very long range, and require thick lead or concrete to reduce their intensity.
γ射线是电磁波，无质量、不带电。电离能力很弱，射程很长，需要厚铅板或混凝土来减弱强度。
Comparison table of key properties:
| Property | Alpha | Beta | Gamma |
|---|---|---|---|
| What is it? | Helium nucleus ⁴₂He | Electron ⁰₋₁e | EM wave |
| Charge | +2 | -1 | 0 |
| Penetration | Stopped by paper/skin | Stopped by a few mm Al | Reduced by thick Pb/concrete |
| Ionising power | Very high | Moderate | Weak |
5. Decay Equations | 衰变方程
You must be able to write and balance nuclear decay equations. Mass numbers and atomic numbers must balance on both sides of the equation. In alpha decay, mass number reduces by 4 and atomic number by 2. In beta-minus decay, atomic number increases by 1 while mass number stays the same.
你必须能够书写和配平核衰变方程。方程两边的质量数和原子序数必须守恒。在α衰变中，质量数减4，原子序数减2；在β⁻衰变中，原子序数加1，质量数不变。
Example of alpha decay: radium-226 decaying to radon: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He.
α衰变例子：镭-226衰变成氡：²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He。
Example of beta-minus decay: iodine-131 to xenon: ¹³¹₅₃I → ¹³¹₅₄Xe + ⁰₋₁e + anti-ν.
β⁻衰变例子：碘-131变成氙：¹³¹₅₃I → ¹³¹₅₄Xe + ⁰₋₁e + 反ν。
Gamma emission is often omitted from equations since it does not change the nucleus; it just releases energy.
γ发射通常在方程中省略，因为它不改变原子核，只是释放能量。
6. Half-Life | 半衰期
The half-life of a radioactive isotope is the time taken for the number of undecayed nuclei to halve. It is also the time for the count rate to fall by half. Half-life is constant for a given isotope and is unaffected by temperature or pressure.
放射性同位素的半衰期是指未衰变的原子核数目减半所需的时间，也指计数率减半的时间。对于给定的同位素，半衰期是恒定的，不受温度或压力影响。
We can calculate half-life from a decay graph. For example, if the initial count rate is 120 counts per minute (cpm) and after 30 minutes it becomes 30 cpm, two half-lives have passed (120 → 60 → 30). So the half-life = 30 / 2 = 15 minutes.
我们可以从衰变曲线计算半衰期。例如，初始计数率为120 cpm，30分钟后变为30 cpm，则经历了两个半衰期（120 → 60 → 30），因此半衰期为15分钟。
Key uses of half-life: carbon-14 dating for archaeological samples (half-life 5730 years), medical tracers like technetium-99m (half-life 6 hours) that quickly clear from the body, and safe disposal of nuclear waste by allowing it to decay over many half-lives until activity is negligible.
半衰期的关键应用：考古样品的碳-14测年（半衰期5730年），医用示踪剂如锝-99m（半衰期6小时）能迅速排出体外，以及核废料的安全处置，通过等待多个半衰期直至活度可忽略。
7. Background Radiation | 背景辐射
Background radiation is constantly present and originates from natural and artificial sources. Natural sources include cosmic rays from space, rocks (e.g. granite containing uranium and thorium), radon gas released from the ground, and naturally radioactive isotopes in food (e.g. potassium-40 in bananas).
背景辐射始终存在，来自天然和人工来源。天然来源包括宇宙射线、岩石（如含铀和钍的花岗岩）、从地下释放的氡气，以及食物中的天然放射性同位素（如香蕉中的钾-40）。
Artificial sources include medical procedures such as X-rays and CT scans, nuclear weapons testing, and nuclear power accidents. In the UK, radon gas is the largest single contributor to average annual background radiation dose.
人工来源包括医疗程序如X射线和CT扫描、核武器试验以及核事故。在英国，氡气是平均年背景辐射剂量的最大单一贡献者。
When measuring the count rate from a radioactive source, the background count must be measured separately and subtracted from the reading to give the corrected count rate from the source alone.
测量放射源的计数率时，必须单独测量背景计数并从读数中减去，以得到仅由源引起的校正计数率。
8. Nuclear Fission | 核裂变
Nuclear fission is the splitting of a large, unstable nucleus (e.g. uranium-235 or plutonium-239) after it absorbs a neutron. The nucleus splits into two smaller daughter
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