Cloud chamber photograph showing the conversion of a gamma ray into an electron-positron pair. Gamma rays do not leave tracks in a cloud chamber because they have no electric charge. But if they have enough energy, they can reveal themselves by converting into matter & antimatter, as in this case. Because they have opposite electric charge, the electron & positron curl away in opposite directions in the cloud chamber's magnetic field. Gamma rays are a very energetic form of light (electromagnetic radiation). They are also one of the 3 types of radiation emitted by radioactive materials. The photo was taken by French physicists Irene Curie & Frederic Joliot.