What experiment was Chadwick famous for?
According to PhysicsLab Online, James Chadwick was assigned the task of tracking down evidence of Rutherford's tightly bound, but theoretical, "proton-electron pair." Chadwick's experiment showed this was actually a different subatomic particle, now called the neutron.
In 1930 it was discovered that Beryllium, when bombarded by alpha particles, emitted a very energetic stream of radiation. This stream was originally thought to be gamma radiation.
Further investigations into the properties of this radiation revealed contradictory results. Like gamma rays, these rays were extremely penetrating and, since they were not deflected upon passing through a magnetic field, neutral. However, unlike gamma rays, these rays did not discharge charged electroscopes (the photoelectric effect).
In 1932, Chadwick proposed that this particle was Rutherford's neutron. In 1935, he was awarded the Nobel Prize for his discovery. With Chadwick's announcement, Heisenberg then proposed the proton-neutron model for the nucleus.