ERNST RUTHERFORD ( – ) Fig. . , they directed a beam of . MeV ( -particles emitted from a Bi radioactive source at a thin metal foil made of gold.
Figure . shows a schematic diagram of this experiment. Alpha-particles emitted by a Bi radioactive source were collimated into a narrow beam by their passage through lead bricks. The beam was allowed to fall on a thin foil of gold of thickness .
× – m. The scattered alpha-particles were observed through a rotatable detector consisting of zinc sulphide screen and a microscope. The scattered alpha-particles on striking the screen produced brief light flashes or scintillations. These flashes may be viewed through a microscope and the distribution of the number of scattered particles may be studied as a function of angle of scattering.
FIGURE . Schematic arrangement of the Geiger-Marsden experiment. A typical graph of the total number of ( -particles scattered at different angles, in a given interval of time, is shown in Fig. .
. The dots in this figure represent the data points and the solid curve is the theoretical prediction based on the assumption that the target atom has a small, dense, positively charged nucleus. Many of the ( -particles pass through the foil. It means that they do not suffer any collisions.
Only about . % of the incident ( -particles scatter by more than 1º; and about in deflect by more than 90º. Rutherford argued that, to deflect the ( -particle backwards, it must experience a large repulsive force. This force could FIGURE .
Geiger-Marsden scattering experiment. The entire apparatus is placed in a vacuum chamber (not shown in this figure).