PhysicsLAB Resource Lesson
Famous Experiments: The Discovery of the Neutron

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In 1920, Ernest Rutherford postulated that there were neutral, massive particles in the nucleus of atoms. This conclusion arose from the disparity between an element's atomic number (protons = electrons) and its atomic mass (usually in excess of the mass of the known protons present). James Chadwick was assigned the task of tracking down evidence of Rutherford's tightly bound "proton-electron pair" or 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.  However, further investigations into the properties of the 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). Irene Curie and her husband discovered that when a beam of this radiation hit a substance rich in protons, for example paraffin, protons were knocked loose which could be easily detected by a Geiger counter.
In 1932, Chadwick proposed that this particle was Rutherford's neutron. In 1935, he was awarded the Nobel Prize for his discovery. Using kinematics, Chadwick was able to determine the velocity of the protons. Then through conservation of momentum techniques, he was able to determine that the mass of the neutral radiation was almost exactly the same as that of a proton. This is Chadwick's equation:
With Chadwick's announcement, Heisenberg then proposed the proton-neutron model for the nucleus. Rutherford was incorrect in his "proton-electron" pair - there were no "free electrons" in the nucleus. However, once free of the nucleus, evidence was mounting that these neutrons were unstable. By 1932, the products of beta decay had been thoroughly examined. To account for a broad spectrum of electron energies from a typical beta emitter, discussions were taking place in which leading physicists were considering abandoning the concepts of conservation of momentum and conservation of energy in radioactive decays. To bring empirical evidence back into alignment with these fundamental basic principles, Wolfgang Pauli proposed in 1930 the existence of an invisible particle that would carry off the missing energy and momentum. He called this particle the neutrino, or little neutral one.
It wasn't until 1955 that Cowan and Reines, working with discharging radiation from the Savannah River Nuclear Power Plant with its abundant supply of antineutrinos released through the decay of free neutrons, discovered concrete experimental data to support the existence of neutrinos. Forty years later, in 1995, Frederick Reines was awarded the Nobel Prize for his pioneering work.

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