Ernest Rutherford proved in 1909 that alpha particles are helium nuclei. Working with Thomas Royds, he showed that alpha particles were helium ions—helium atoms stripped of their electrons.
Rutherford and Royds collected alpha particles emitted by radium inside a thin-walled glass tube. After enough particles accumulated, they passed an electrical discharge through the trapped gas and examined its spectrum. The characteristic spectral lines matched helium, demonstrating that alpha radiation and helium were directly connected.
The result built on Rutherford’s earlier work. He had established that alpha particles were positively charged and had measured their charge-to-mass ratio, but the exact identity remained uncertain. The modern description is more precise: an alpha particle is a helium-4 nucleus containing two protons and two neutrons, with a charge of +2.
A common mix-up is 1911, when Rutherford used alpha-particle scattering to propose the nuclear model of the atom. That was a separate milestone; the identification of alpha particles as helium nuclei came in 1909.