At the time, they didn’t know what these rays were.
They didn’t know that alpha rays are helium-4 nuclei;
they didn’t know that beta rays are electrons;
they didn’t know that gamma rays are, like X-rays, photons.
X-rays are emitted by excited electrons;
gamma rays were emitted by the decay of nuclei or other particles.
See also in The book of science:
Readings on wikipedia: