Chadwick's neutron detector, c 1932.
© Science Museum / Science & Society Picture Library
This detector was used by James Chadwick (1891-1974) in his discovery of the neutron. Inside the detector, particles from a radioactive source hit a beryllium target. From the resulting nuclear reactions neutrons were given off, detected when they dislodged protons from a piece of paraffin wax (see 1980-1048). Chadwick's discovery helped to rewrite existing beliefs concerning what was contained in the nucleus of an atom.