Abstract
A summary is given of theoretical information on the three processes by which gamma-rays can be elastically scattered by atoms, viz. Rayleigh scattering by bound electrons, Thomson scattering by the nuclear charge and (exceptionally) nuclear resonant scattering.
Interference between the three scattered waves is considered and shown to be of practical importance as between Rayleigh and Thomson scattering of hard gamma-rays at large angles.
The calculated intensities of Rayleigh, Thomson and Compton scattering are plotted against angle of scattering for gamma-ray energies of 2.8 and 0.41 MeV., and for scatterers of Al, Cu and Pb. The graphs illustrate the dominance of Rayleigh scattering at very small angles for all energies, and the comparable intensities of Rayleigh and Thomson scattering at high energies and large angles.
The unidentified hard component found by Pollard and Alburger in the large-angle scattering of 2.8-MeV. gamma-rays by various elements is interpreted as a mixture of Rayleigh and Thomson scattering; theoretical and experimental intensities show reasonable agreement for both light (Al) and heavy (Pb) scatterers.
Experiments on the scattering of 0.41-MeV. gamma-rays at about 115° are reported. In agreement with theory, about 2% of the radiation scattered from lead is found to retain the full energy; for copper and aluminiun, the proportion of elastically scattered photons is much smaller.