Beryllium bombarded by alpha rays from polonium, c 1937.
© Science Museum / Science & Society Picture Library
A cloud chamber photograph showing a proton ejected from a layer of paraffin wax by neutrons of beryllium bombarded by alpha rays from polonium. 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. This photograph was presented to the museum by Irene Joliot-Curie and her husband Frederic Joliot-Curie.