The validity of the doppler approximation in nuclear elastic scattering of capture gamma rays

Evaluation of resonant scattering of gamma rays from nuclear levels requires the integration of the product of the source line and resonance level curves. This integral has been evaluated for the general case of two Doppler broadened curves, with arbitrary temperatures for source and scatterer nuclei, different natural line widths, and a separation δ eV between the two maxima. It is shown that the shape of a thermal neutron capture gamma ray line can be accurately represented by the pure Doppler shape. Only in special cases, however, can the nuclear level be approximated by the pure Doppler form.