In rutherford's gold foil experiment what kind of particles in dense atomic nuclei caused some alpha particles to bounce straight back from the gold foil

In Rutherford's gold foil experiment, Nuclei in the gold atom caused some of the alpha particles to bounce straight back from the gold foil Alpha particles are positively charged as is the nucleus of atoms therefor repelling each other. Because the large spacing of the nucleus in the gold atoms only a small amount of alpha particles will collide directly with the nucleus, therefor only a small amount of alpha particles will be reflected back.