📚 Rutherford Scattering | 卢瑟福散射
At the beginning of the 20th century, the microscopic structure of matter was still an enigma. The discovery of the electron by J.J. Thomson had shattered the idea of the indivisible atom, yet the internal arrangement of charge and mass remained unclear. The Geiger–Marsden experiment, performed under Ernest Rutherford's direction, provided the first unequivocal evidence that atoms possess a tiny, massive, positively charged nucleus — entirely transforming our model of the atom. For IB Physics students, Rutherford scattering is a mandatory case study that illustrates how experimental evidence can overthrow a scientific paradigm and lay the foundations for nuclear physics.
20世纪初，物质的微观结构仍然是一个谜。J.J.汤姆逊发现电子打破了原子不可分割的观念，但电荷和质量在原子内部如何排布仍不清楚。在欧内斯特·卢瑟福指导下进行的盖革–马斯登实验首次提供了确凿证据，证明原子具有一个极小、高密度、带正电的原子核 —— 彻底改变了人们对原子的认识。对于IB物理学生而言，卢瑟福散射是一个必学案例，它展示了实验证据如何推翻旧有科学范式，并为核物理奠定基础。
1. Historical Background: Thomson's Plum Pudding Model | 历史背景：汤姆逊的梅子布丁模型
Before Rutherford's intervention, the accepted picture of the atom was J.J. Thomson's 'plum pudding' model. In this model, the atom consisted of a uniform sphere of positive charge roughly 10⁻¹⁰ m in radius, with negatively charged electrons embedded inside it like plums in a pudding. The positive charge was thought to be spread over the entire atomic volume, producing only a weak electric field. Consequently, any alpha particle passing through such an atom would experience only small, random deflections — typically less than 1° — because the positive charge was too dilute to exert a strong Coulomb repulsion. Large-angle scattering would be practically impossible, as it would require the alpha particle to encounter a strong localised field, which the Thomson model did not provide.
在卢瑟福之前，公认的原子图像是J.J.汤姆逊的“梅子布丁”模型。该模型中，原子由一个半径约10⁻¹⁰米的均匀正电荷球体组成，带负电的电子像布丁中的梅子一样镶嵌其中。正电荷被认为遍布整个原子体积，只产生微弱的电场。因此，任何穿过这种原子的α粒子只会受到随机的小角度偏转 —— 通常小于1° —— 因为正电荷过于分散，无法产生强库仑斥力。大角度散射几乎不可能发生，因为那需要α粒子遇到一个强局域电场，而汤姆逊模型并不提供这种可能性。
2. The Geiger–Marsden Experimental Apparatus | 盖革–马斯登实验装置
The experiment designed by Hans Geiger and Ernest Marsden under Rutherford' travelling at about 1.5×10⁷ m s⁻¹. The beam was directed onto an extremely thin gold foil — only a few hundred atoms thick — e foil, allowing the observers to count the number of scattered particles at any angle θ relative to the original beam direction. The entire experiment was enclosed in a vacuum chamber to avoid unwanted scattering by air molecules.
在卢瑟福指导下，汉
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