Alpha particles:

He nuclei:

2 protons, 2 neutrons;
charge: 2e; mass: 4u

Rutherford could determine the charge to mass ratio of alpha particle by deflecting them in electric and magnetic fields.
To verify that alpha particles were doubly charge He ions (He nuclei), alpha particles were allowed to penetrate a very thin glass wall of an evacuated tube and captured inside the tube. A spark  provided a shower of electrons,  Some electrons were captured by the ions to form neutral atoms.  The spectrum of the light emitted by the resulting gas showed that it was helium.