Alpha particle

In 1909, Hans Geiger and Ernest Marsden, working under Ernest Rutherford, directed a beam of positively charged alpha particles at a very thin sheet of gold foil. The beam entered the detecting enclosure through an opening, so it encountered the foil before the fluorescent screen. The screen produced tiny flashes where outgoing alpha particles struck it, allowing their scattering directions to be recorded.

Ernest Rutherford and his colleagues Geiger and Marsden tested the plum pudding model by firing positively charged alpha particles at thin gold foil. Most passed through, while a small number were deflected sharply. If positive charge had been spread throughout each atom, such large deflections would have been difficult to explain.