Incoming links: Becquerel's rays
These must have been gamma rays.
Just before he left Cambridge for Montreal in 1898, Rutherford conducted a simple, systematic experiment to study the absorption of rays from uranium. [...] In 1901 he determined that Becquerel's rays are indeed electromagnetic rays. He called them γ (gamma) rays.
This terminology is used e.g. in Marie Curie's Polonium paper:
Some minerals containing uranium and thorium (pitchblende, chalcolite, uranite) are very active from the point of view of the emission of Becquerel rays.
