Becquerel, studying fluorescence, discovered that some substances spontaneously emit radiation, with no prior stimulus and in a continuous manner.
As X rays had just been discovered, he thought that this was the nature of what radioactive substances emitted, as it seemed to be a similar radiation. Today we know that this radiation includes gamma rays and two types of particles which are emitted at very high speed: alpha and beta.
Gamma radiation (g) is electromagnetic waves, with more energy than X rays. In reality, even luminous radiation, contains photons which act like particles.