Who proved that alpha particles are helium nuclei?

Ernest Rutherford proved that alpha particles are helium nuclei.

Rutherford had already identified alpha radiation as a positively charged, relatively weakly penetrating component of radioactive emissions. By the early 1900s, he suspected that alpha particles might be helium atoms missing electrons, but the evidence was not yet conclusive. The difficulty was that the particles were tiny and emitted amid other radioactive products.

In 1909, Rutherford and chemist Thomas Royds carried out a decisive experiment. They allowed alpha particles from a radium source to pass through a very thin window into an evacuated tube. As the particles accumulated, they produced a gas. An electrical discharge through that gas generated a spectrum matching helium, demonstrating that alpha particles were helium ions.

Later developments in atomic physics clarified the modern description: an alpha particle is a helium-4 nucleus containing two protons and two neutrons, with a charge of +2. Once it captures two electrons from its surroundings, it becomes an ordinary neutral helium atom. Marie Curie and Henri Becquerel were central to radioactivity research, but they did not perform this proof.