物理学 1911

α 与 β 粒子被物质的散射，及原子的结构

欧内斯特·卢瑟福

原子几乎空无一物，质量与电荷全挤在一个微小的中央核里。

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In depth · the introduction

向一片薄薄的金箔发射极小的「子弹」，几乎全部穿了过去——可偏偏有顽固的少数被原路弹了回来，我们就是这样找到了原子的核。

核心想法

二十世纪初，人人都把原子想象成一团软软的、正电荷均匀分布的球，电子像葡萄干一样点缀其中——一种「葡萄干布丁」。卢瑟福的团队这样去检验它：把 α 粒子（又快、又重、带正电的小颗粒）射向一片只有几千个原子那么厚的金箔。

若原子真像布丁，那每一颗粒子都该至多被轻轻一推、径直穿过。然而，竟有少数被大幅偏折——有些几乎原路返回。要把一颗又快又重的粒子弹回去，唯一的办法，就是它撞上了某个又小、又硬、又带强电的东西。卢瑟福由此断定：原子几乎全部的质量、以及全部的正电荷，都坐落在中央一个微小的核里，电子则远在其外，而这之间几乎全是空荡荡的空间。

它是如何诞生的

在曼彻斯特大学，卢瑟福让汉斯·盖革和一位年轻的学生欧内斯特·马斯登去数那些微弱的闪光——α 粒子击中屏幕时一闪而过，这是在暗室里进行的、耐心而磨眼的活儿。1909 年，马斯登几乎是顺带地查看：有没有粒子朝着源的方向折返回来。令所有人震惊的是，真有几颗。

卢瑟福把这份惊讶反复掂量了一年多。他后来说，这「大概是我这辈子遇到过的最不可思议的事——几乎就像你朝一张薄纸射出一发 15 英寸的炮弹，它却弹回来打中了你」。到 1911 年，他备齐了数学：与一个集中的中央电荷之间的单次近距离擦过，便能把一颗 α 粒子甩向后方，而各个角度上的粒子数目，遵循着他能写下来的一条精确法则。

它为何重要

这是原子拥有「中心」的时刻。有核图像取代了那团毫无特征的布丁，成为其后一切物质物理的根基——它直接通向两年后玻尔的模型，并最终通向质子、中子的发现与核能。它还确立了一种至今仍在推动发现的方法：想知道某样东西由什么构成，就向它发射粒子，再研究它们如何被散射。

一个可以想象的画面

想象你在一间黑屋里，朝一个看不见的物体一遍遍滚动玻璃弹珠，记录每一颗弹向哪个方向。多数都径直滚了过去，告诉你屋里大半是空的。但每隔一阵，总有一颗猛地反弹回来——根据这种事发生的频率，以及反弹的角度，你就能推断出：屋子正中某处，有一根又小、又硬、又重的桩，哪怕你从未直接看见它。α 粒子就是那些弹珠，原子核就是那根桩。

它的位置

一个世纪以前，道尔顿重新提出物质由原子构成，而 J·J·汤姆孙又刚刚发现了电子——但原子仍被想象成一团软软的、没有结构的东西。卢瑟福给了它「建筑」：一个坚硬的中心，与一片广袤的空荡。然而，他的有核原子按经典物理本该顷刻坍塌；是尼尔斯·玻尔在 1913 年，用崭新的量子观念，让环绕的电子稳定下来。从卢瑟福的核出发，一条条线通向质子、中子，以及那套日后揭开内部夸克的整套散射方法。

The original document

Original source text

E. Rutherford · Philosophical Magazine, Series 6, 21 (1911): 669–688

§ 1 — The problem

It is well known that the α and the β particles suffer deflexions from their rectilinear paths by encounters with atoms of matter.

Rutherford opens by separating two ideas. A particle may be turned a little by each of many atoms it passes — he calls this “compound” scattering — or it may be turned through a considerable angle by a single close encounter, which he calls “single” scattering. The accepted Thomson atom, with its positive charge spread thinly through the whole sphere, could only ever give the gentle, compound kind.

But Geiger and Marsden had just found something the spread-out atom could not explain — a few α particles turned right around. Rutherford quotes their result:

If the high velocity and mass of the α-particle be taken into account, it seems surprising that some of the α-particles, as the experiment shows, can be turned within a layer of 6 × 10⁻⁵ cm. of gold through an angle of 90°, and even more.

§ 2 — A single encounter

It seems reasonable to suppose that the deflexion through a large angle is due to a single atomic encounter, for the chance of a second encounter of a kind to produce a large deflexion must in most cases be exceedingly small.

A simple calculation shows that the atom must be a seat of an intense electric field in order to produce such a large deflexion at a single encounter.

Rutherford then treats the α particle and the atom's charge as points obeying the inverse-square law, and works out the hyperbolic orbit. The geometry ties the deflexion angle to how nearly head-on the shot is, and predicts that the number of particles scattered at an angle φ should vary as cosec⁴(φ/2) — falling off steeply, but never to zero, so that very large angles, though rare, are not forbidden.

[ … ]

§ 4 — The structure of the atom

Considering the evidence as a whole, it seems simplest to suppose that the atom contains a central charge distributed through a very small volume, and that the large single deflexions are due to the central charge as a whole, and not to its constituents.

He is careful about what the experiment can and cannot show: the sign of the central charge is left open (a positive or a negative core of the same magnitude would scatter the α the same way), and its magnitude comes out roughly proportional to the atomic weight. The word “nucleus” does not yet appear — that name, and the identification of the central charge with the atomic number, came in the two years that followed.

University of Manchester · April 1911