Alpha backscattering on nucleus
Rutherford observed the backward bounce of some alpha particles as projectiles sent on the atoms of a thin gold foil. He interpreted this rebound as the "backscatter" of a light nucleus (alpha particle) on the heavy nucleus of a gold atom. Because of the mass ratio (A = 4 versus A = 197), the alpha particle bouncing off (on the left figure) at 150 °, loses only a small portion of its energy - that it gives to the gold nucleus - which allow it to emerge from the gold foil. On the contrary, in the case of a collision (on the right) with a lighter nucleus as oxygen (A = 16), the alpha loses most of its energy in aid of oxygen propelled forward.