In high school we were told that the idea "alpha particles are actually helium nuclei" came from observing the right-angled fork track, produced by placing an alpha source in a diffusion cloud chamber filled with helium.
From the point of view of the Geiger-Marsden Scattering Experiment, an alpha particle hitting a helium nucleus is an extremely rare event. Besides, there is no guarantee at all that the alpha particles would not hit the nuclei of other gas molecules in the chamber.
So my question is: How did people discover this extremely rare event (sit there and stare at the chamber for hours?), and why were they so sure that it was a helium nucleus that the alpha particle hit?
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