Beta decay

Uranium emits them, you can see their mass to charge ratio under magnetic field and so deduce that they are electrons.

Caused by weak interaction TODO why/how.

The emitted electron kinetic energy is random from zero to a maximum value. The rest goes into a neutrino. This is how the neutrino was first discovered/observed indirectly. This is well illustrated in a decay scheme such as Figure "caesium-137 decay scheme".