Mass-Energy and the Neutron in the Early Thirties

Abstract

The ArgumentEinstein's mass-energy relationship was not confirmed experimentally until 1933 when Bainbridge showed that the Cockcroft-Walton experiment afforded a test of it. Earlier, however, it had been used constantly in the analysis of nuclear reactions, as can be seen in those involved in the determination of the mass of the neutron. Chadwick in 1932 was convinced that the neutron mass was about 1.0067 amu (atomic mass units), indicating that the neutron was a proton-electron compound, since that figure was less than the sum of the proton and electron masses. Chadwick's value was challenged in 1933 by Lawrence, who proposed a much lower value of 1.0006 amu, and by Curie and Joliot, who proposed a much higher value of 1.011 amu.Much controversy ensued; eventually, Chadwick and Goldhaber showed in 1934 that the neutron mass was about 1.0080 amu, greater than the sum of the proton and electron masses, proving that the neutron was a new elementary particle (which could decay spontaneously), and providing conclusive experimental support for excluding electrons from the nucleus. These results remained unchanged with further refinements in the last decimal place, the entire pursuit of which provided still further vindication of Einstein's massenergy relationship.

Historical Surprises1

“…(Stuewer 1993). Ernest O. Lawrence in Berkeley, on the basis of his experiments, calculated a much lower value than Chadwick's in early 1933, indicating a much higher electron-proton binding energy, while Ir ene Curie and Fre´de´ric Joliot in Paris, on the basis of their experiments, calculated a higher value than Chadwick's a few months later, indicating that the electron and proton were not bound together at all.…”