Chadwick's neutron chamber (replica) used in his experiment to discover the neutron.
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 force of this impact, neutrons were given off, and could only be detected when they dislodged protons from a piece of paraffin wax. Chadwick's theories and experiments helped to rewrite existing beliefs concerning what was contained in the nucleus of an atom.