Chadwick (1891-1974) used these pieces of paraffin wax in his neutron detector. Inside the detector, particles from a radioactive source hit a beryllium target. From the force of this impact, neutrons were given off, and could only be detected when they dislodged protons from a piece of the wax. Chadwick's discovery of the neutron helped to rewrite existing beliefs concerning what was contained in the nucleus of an atom.
© Science Museum / Science & Society Picture Library