WHAT ARE ALPHA PARTICLES?

In 1909, Rutherford and his colleague Thomas Royds really wanted to know what made up alpha particles. They carried out many experiments with alpha particles emitted from radon—a gas that occurs naturally as the decay product of radium, and which often accumulates in basements to a point where it may pose health risks. The radon samples often contained helium, making Rutherford and Royds suspect that alpha particles were “naked” helium ions, that is, helium atoms that had lost two electrons. They obtained definitive proof after conducting a very elegant experiment (Figure 68) in which they placed a large quantity of purified radon gas into a glass tube with an extremely thin wall. An evacuated collecting tube surrounded the thin-walled, radon-containing tube. After 2 days, they passed high voltage between two electrodes to cause any collected gas to glow—just like what happens inside a modern neon tube. Indeed, the contents produced a telltale yellow glow indicative of pure helium.

Figure 68 This is a highly simplified view of the experiment performed by Rutherford and Royds to demonstrate that alpha particles are helium ions. They placed radon gas—an alpha emitter—inside a glass tube with very thin walls. This tube was itself inside an evacuated collecting tube. After 2 days, they could detect helium gas in the collection tube by watching for the telltale yellow glow that helium produces when excited by high voltage.

As a control, Rutherford and Royds also ran the experiment with pure helium gas inside the thin-walled tube. They left it for a few days and attempted again to detect helium gas that would have diffused through the thin glass wall into the outer collection vessel. However, they could not detect any helium, meaning that for some reason an alpha particle can penetrate the thin glass wall of the tube, but a helium atom cannot. Rutherford thus concluded that the “naked” helium ions were much, much smaller than regular helium atoms. This hinted that the atom was not as homogeneous as the “plum-pudding” model would have it, and that the mass might be more concentrated, leaving quite a bit of empty space within the atom’s boundaries.