The Rutherford model also called the Rutherford atomic model, nuclear atom, or planetary model of the atom is a description of the structure of atoms proposed (1911) by the New Zealand-born physicist Ernest Rutherford. The model described the atom as a small, dense, charged core called a nucleus, within which nearly all the mass is concentrated, around which the sunshine, negative constituents, called electrons, circulate a long way, very similar to planets revolving around the Sun. The nucleus was postulated as small and dense to account for the scattering of alpha particles from thin foil, as observed in a very series of experiments performed by undergraduate Ernest Marsden under the direction of Rutherford and German physicist Hans Geiger in 1909. A radioactive source emitting alpha particles (i.e., charged particles, a dead ringer for the helium atom nucleus and seven,000 times more massive than electrons) was enclosed within a protective lead shield. The radiation was focused into a narrow beam after passing through a slit in an exceeding lead screen. A skinny section of foil was placed before the slit, and a screen coated with zinc sulfide to render it fluorescent served as a counter to detect alpha particles. As each particle struck the fluorescent screen, it produced a burst of sunshine called scintillation, which was visible through a viewing microscope attached to the rear of the screen. The screen was movable, allowing Rutherford and his associates to see whether the foil deflected any alpha particles.