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“Keeps out the alpha rays, Max! You don’t get old.”

Eventually alpha rays were identified as helium atoms stripped of their electrons—that is, helium nuclei-—and beta rays as energetic electrons.

He had named them alpha rays, but as he found he could not get them to penetrate even a thin sheet of paper, he was confident that they must be atoms of matter.

If trapped inside the body, though, alpha rays can cause cancer.

The alpha rays are projected particles which lose energy in penetrating matter.