I. CURIE & F. JOLIOT / SCIENCE PHOTO LIBRARY I. CURIE & F. JOLIOT / SCIENCE PHOTO LIBRARY
Taken in 1932, this was one of the first pictures to show the presence of the neutron, newly discovered by James Chadwick at the Cavendish Laboratory in Cambridge. The cloud chamber photo shows the track of a proton recoiling after being struck by a neutron; because the neutron has no electric charge, it does not itself produce a track. The neutron enters the cloud chamber from below and strikes a sheet of paraffin wax (white horizontal line). The proton is knocked from the paraffin; the small gap before its track is seen is because drops in the cloud chamber did not condense on ions near the paraffin. The photo was taken by French physicists Irene Curie and Frederic Joliot.
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