The Nuclear Photoeffect

Abstract

The first photonuclear reaction was demonstrated by Chaswick and Goldhaber in 1934 [1], who irradiated an ionization chamber filled with deuterium with the 2.62 Mev γ-rays emitted by ThC”. They detected the photoprotons by their ionization in the deuterium gas and measured their energy. By performing a similar irradiation with the γ-rays of 1.8 Mev energy of RaC, and observing almost no ionization due to photoprotons, they were able to deduce that the threshold of the reaction lay between 1.8 and 2.3 Mev. Apart from the intrinsic interest of the new phenomenon of the photodisintegration of a nucleus, they were thus enabled to estimate the mass of the neutron, combining the binding energy with the spectroscopic masses of hydrogen and of deuterium. The measurements indicated that the neutron mass was greater than that of the proton, and in fact that it was unstable energetically against decay by negative electron emission to a proton. Many years later the radioactive decay of the neutron was in fact observed. At the same time Chadwick and Goldhaber measured the cross-section for photodisintegration of the deuteron at an energy of 2.62 Mev, and found a value in encouraging agreement with the theoretical one of Bethe and Peierls [2].