The discovery of the positron

This is a picture of the first positron track discovered by Anderson in 1932, in a cloud chamber, investigating cosmic rays.

A cloud chamber consists of a tank of gas saturated with water vapors. If the piston at the rear of the cloud chamber expands, the gas will cool adiabatically and becomes supersaturated. In the presence of a charged particle the water condenses as droplets which track the path of the particle.

A magnetic field normal to the plane of the chamber will curve the particle path and will let us measure its momentum.

The dark band across the middle of the picture is a lead plate which slows down the particles. As you can see in the picture the curvature of the upper track is smaller than the lower one and so the direction of the particle is upward. Knowing the direction of magnetic field and the direction of the path curvature, this particle should have a positive charge.

It cannot be a proton, since it would have been stopped much earlier in the track and the mass is consistent with the electron mass.

Since the discovery of the positron, many other antiparticles have been observed such as antiproton in 1955 and anti-hydrogen atom in 1995.

-references:

Particle Astrophysics, Donal Perkins.

Picture taken from C.D. Anderson, Physical Review 43, 491 (1933).