Alpha-particle scattering

Geiger and Marsden aimed a beam of 5.5 MeV alpha-particles from a radioactive ²¹⁴₈₃Bi source at a thin gold foil. Lead bricks collimated the particles into a narrow beam, and the whole apparatus sat in a vacuum chamber.

The gold foil was 2.1 × 10⁻⁷ m thick. A rotatable detector, a zinc sulphide screen viewed through a microscope, caught the scattered particles: each one made a brief flash of light (a scintillation), so the number scattered could be counted at each angle.

Most alpha-particles went straight through the foil without any collision. Only about 0.14% were scattered by more than 1°, and about 1 in 8000 were deflected by more than 90°.

To turn an alpha-particle back, a very large repulsive force is needed. Rutherford argued that this is possible only if most of the atom's mass and all its positive charge are packed tightly at the centre, so an alpha-particle can come very close to that charge without entering it. The data agreed, so Rutherford is credited with discovering the nucleus.

In Rutherford's model the nucleus holds all the positive charge and most of the mass, and the electrons move in orbits some distance away. The experiments put the nuclear size at about 10⁻¹⁵ m to 10⁻¹⁴ m, while kinetic theory gave the atom a size of about 10⁻¹⁰ m: 10,000 to 100,000 times larger. Most of an atom is empty space.

That is why most alpha-particles pass straight through. Only one that comes near a nucleus feels the intense field there and is scattered through a large angle. Electrons, being so light, hardly affect the alpha-particles.

Example 12.1: the electron's orbit (about 10⁻¹⁰ m) is 10⁵ times the nuclear radius (about 10⁻¹⁵ m). A solar system in the same proportion would put the earth at 10⁵ × 7 × 10⁸ m = 7 × 10¹³ m from the sun, more than 100 times its real orbital radius of 1.5 × 10¹¹ m. An atom is far emptier than the solar system.