📚 Nuclear Physics | 原子核物理
Nuclear physics is the study of the structure, properties and reactions of atomic nuclei. For CIE A-Level Physics, the topic connects Rutherford scattering, nuclide notation, mass-energy equivalence, radioactive decay and nuclear energy applications.
原子核物理研究原子核的结构、性质与反应。在 CIE A-Level 物理中，该主题串联卢瑟福散射、核素符号、质能等价、放射性衰变以及核能应用。
1. Nuclear Structure and Rutherford Scattering | 原子核结构与卢瑟福散射
Rutherford directed alpha particles at a thin gold foil. Most particles passed almost straight through, but a very small number were deflected through large angles, and a few bounced back towards the source.
卢瑟福用 α 粒子轰击薄金箔。大多数粒子几乎直线穿过，但极少数发生大角度偏转，个别粒子甚至被反弹回来。
These observations showed that the atom is mostly empty space, with a tiny, dense, positively charged nucleus at its centre. The nucleus carries almost all of the mass of the atom.
这些现象说明原子内部绝大部分是空的，中心有一个极小、致密、带正电的原子核，几乎集中了原子的全部质量。
The nucleus consists of protons and neutrons, which are collectively called nucleons. A proton has charge +e and a neutron has zero charge; their masses are almost equal.
原子核由质子和中子组成，二者统称为核子。质子带 +e 电荷，中子不带电；两者的质量几乎相等。
2. Nuclide Notation and Isotopes | 核素符号与同位素
A nuclide is written as A-Z-X notation, where A is the mass number and Z is the proton number. For example, uranium-235 is written as ²³⁵₉₂U, so it has 92 protons and 235 − 92 = 143 neutrons.
核素用 A-Z-X 符号表示，其中 A 是质量数，Z 是质子数。例如铀-235 写作 ²³⁵₉₂U，它有 92 个质子和 235 − 92 = 143 个中子。
Isotopes are atoms of the same element with the same proton number Z but different neutron numbers. They have identical chemical properties because chemical behaviour depends on electrons, but their nuclear properties can be very different.
同位素是同一元素中质子数 Z 相同而中子数不同的原子。由于化学性质取决于电子，所以同位素化学性质相同，但核性质可能差异很大。
The radius of a nucleus is much smaller than the atom. Nuclear radii are of the order of 10⁻¹⁵ m to 10⁻¹⁴ m, while atomic radii are about 10⁻¹⁰ m.
原子核半径远小于原子半径。核半径约为 10⁻¹⁵ m 至 10⁻¹⁴ m，而原子半径约为 10⁻¹⁰ m。
3. The Strong Nuclear Force | 强核力
The strong nuclear force holds nucleons together in the nucleus. It is an attractive force at short range, typically up to about 3 × 10⁻¹⁵ m, and it overcomes the electrostatic repulsion between protons.
强核力将核子束缚在原子核内。它是短程吸引力，作用范围约在 3 × 10⁻¹⁵ m 以内，能够克服质子之间的静电排斥。
At very short separations below about 0.5 × 10⁻¹⁵ m, the strong nuclear force becomes repulsive. This prevents nucleons from collapsing into one another.
在小于约 0.5 × 10⁻¹⁵ m 的极短距离上，强核力表现为排斥力，从而阻止核子相互挤压坍缩。
The strong nuclear force acts between all nucleons and does not depend on electric charge. It is much stronger than the electrostatic force but has a much shorter range.
强核力作用于所有核子之间，与电荷无关。它比静电力强得多，但作用距离很短。
4. Mass Defect and Binding Energy | 质量亏损与结合能
The mass of a nucleus is always slightly less than the total mass of its separated protons and neutrons. This difference is called the mass defect, Δm.
原子核的质量总是略小于其分离的质子和中子质量之和。这个差值称为质量亏损，记为 Δm。
According to Einstein’s mass-energy relation, the missing mass is converted into energy when the nucleus is formed. This energy is the binding energy of the nucleus.
根据爱因斯坦的质能关系，亏损的质量在核形成时转化为能量，这就是原子核的结合能。
E = Δm c²
Binding energy is the energy required to separate a nucleus completely into its individual nucleons. A larger binding energy per nucleon means a more stable nucleus.
结合能是将原子核完全拆散为单个核子所需的能量。每个核子的结合能越大，原子核越稳定。
5. Binding Energy per Nucleon and Nuclear Stability | 比结合能与核稳定性
The binding energy per nucleon is found by dividing the total binding energy by the mass number A. It increases sharply for light nuclei, reaches a maximum near iron-56, and then decreases gradually for heavier nuclei.
比结合能等于总结合能除以质量数 A。轻核的比结合能迅速增大，在铁-56 附近达到最大值，之后随重核逐渐减小。
Nuclei near the peak of the curve are the most stable. Fusion releases energy when light nuclei combine to form a nucleus closer to the peak, while fission releases energy when heavy nuclei split into products closer to the peak.
曲线峰值附近的核最稳定。轻核聚变形成更接近峰值的核时释放能量；重核裂变成更接近峰值的产物时也释放能量。
The greater the binding energy per nucleon, the more energy has been released in forming the nucleus and the more stable the nucleus is.
比结合能越大，核形成时释放的能量越多，原子核也越稳定。
6. Alpha, Beta and Gamma Radiation | α、β、γ 辐射
Unstable nuclei decay by emitting radiation. Alpha particles are helium nuclei, beta particles are fast electrons or positrons, and gamma rays are high-energy electromagnetic photons.
不稳定核通过发射辐射衰变。α 粒子是氦核，β 粒子是高速电子或正电子，γ 射线是高能电磁光子。
| Property | Alpha α | Beta β | Gamma γ |
|---|---|---|---|
| Nature | ⁴₂He nucleus | fast electron/positron | electromagnetic wave |
| Charge | +2e | −e or +e | 0 |
| Ionising power | strong | moderate | weak |
| Penetration | stopped by paper or a few cm of air | stopped by a few mm of aluminium | Published by TutorHao | A-Level Physics Revision Series | aleveler.com
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