As shown in above figure, they directed a beam of $5.5 \mathrm{MeV} \alpha$-particle emitted from a ${ }_{83}^{214} \mathrm{Bi}$ radioactive source at a thin metal foil made of gold. $\alpha$-particles emitted by a ${ }_{83}^{214} \mathrm{Bi}$ radioactive source were collimated into a narrow beam by their passage through lead bricks. As shown in the figure below, the beam was allowed to fall on a thin foil of gold of thickness $2.1 \times 10^{-7} \mathrm{~m}$.

The scattered $\alpha$-particles on striking the screen produced brief light flashes (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.