How was beta decay discovered?
It's a fascinating story of scientific discovery, and Henri Becquerel gets the credit.
In 1896, Becquerel found that a crystal of a uranium salt made an image on a photographic plate. He had discovered natural radioactivity.
In 1898, Pierre and Marie Curie discovered two intensely radioactive sources, polonium and radium. This made it much easier to study "Becquerel rays".
In the same year, Ernest Rutherford found that Becquerel rays consisted of both positive and negative particles. He named them alpha and beta particles.
In 1899, two Vienna physicists named Stefan Meyer and Egon Ritter von Schweidler studied the magnetic deflection of Becquerel rays. They concluded that the rays were negatively charged particles.
In 1900, Becquerel showed that the beta rays from radium had the same charge-to-mass ratio as the electrons in J.J. Thomson's cathode ray particles. They were electrons. But they had much greater energies than the electrons in cathode ray tubes.
By the middle of 1900, most scientists agreed that beta particles were high-speed electrons.