Internal Scattering of Gamma-Rays

Abstract

If an element is irradiated with gamma-rays of sufficiently high energy, the upper limit of the Compton recoils is less than the minimum energy of photo- or conversion electrons from any shell. But electrons of energy between these two limits may be ejected in processes in which the momentum condition can be relaxed. Such processes are (1) the scattering of an external gamma-ray by a bound electron, where momentum can be taken up by the nucleus, (2) the internal scattering by the electrons of the radioactive atom itself, where the radiation field of the near-by nucleus can fulfill momentum conditions impossible for a plane wave. We consider the second case, for scattering of an electric dipole gamma-ray by $s$ electrons. We use Dirac electron theory with Born approximation. The process is of order $\alpha$ compared to the internal conversion, as expected. Our small result indicates that most of the electrons observed in such a region—for instance from the 2.62-Mev gamma-ray of Th C″—are of instrumental origin. This is in agreement with the results of the latest experiments.