Alpha-Particle Scattering Experiment (Geiger-Marsden Experiment)

The problem to be solved. By 1910, it was clear an atom contains electrons and an equal amount of positive charge, but the positive charge's arrangement was pure guesswork. J.J. Thomson had proposed a "plum-pudding" model, in which the positive charge was spread evenly throughout the whole volume of the atom, like a diffuse cloud, with electrons studded inside it. Testing this picture required probing the INSIDE of an atom directly -- something no experiment had yet done.

Why alpha particles. Rutherford chose fast, positively charged alpha particles ($\alpha$, essentially bare helium nuclei, charge $+2e$) as the probe, for two reasons: they are emitted with well-defined, reasonably high kinetic energies (a few MeV) by naturally radioactive sources such as radium or polonium, and being positively charged and comparatively massive, any deflection they suffer while passing near an atom's positive charge would be a direct, sensitive measure of how that charge is actually distributed (Exercise 9 examines this choice further).

Experimental arrangement (Figure 1). A narrow, collimated beam of alpha particles from a radioactive source is directed at an extremely thin sheet of gold foil, only about $10^{-7}\ \text{m}$ thick -- gold was chosen because it is exceptionally malleable and can be beaten into foils only a few hundred atoms thick without tearing. Surrounding the foil is a rotatable detector: a circular screen coated with zinc sulphide (ZnS), which produces a tiny flash of light (a scintillation) wherever an alpha particle strikes it; an observer using a low-power microscope attached to the screen counts these flashes as the whole screen-and-microscope assembly is rotated to different angles $\theta$ measured from the original beam direction. The entire apparatus is enclosed in an evacuated chamber, since air molecules would otherwise scatter and absorb the alpha particles before they ever reached the foil. …