Gamma rays from thorium C11, c 1937.
© Science Museum / Science & Society Picture Library
Cloud chamber photograph showing a pair of electrons produced in the chamber gas by a quantum of gamma rays from thorium C11. A circular chamber is filled with a water and alcohol solution which is vapourised. When particles with an electric charge are pased through this chamber, droplets (of water and alcohol vapour) are formed at the points where the particle pases. A light source at the side of the chamber illuminates the resulting trail of droplets (the particle track), which is then photographed.