Wavelength selecting asymmetric Bragg diffraction

In the case of dynamical diffraction the exact angular position of a Bragg reflected X-ray or neutron beam is shifted by some arc sec with respect to the angle calculated by the bragg equation. This shift effect can be enhanced in the case of asymmetric diffraction and be taken, e.g. to select or suppress harmonic wavelengths, or possibly to investigate epitaxic layers. In an experiment it is shown, that with a double crystal diffractometer and with an even coarse divergence different neutron wavelengths can be separated. From this experiment the wavelength spectrum could be evaluated which differs from the one that was expected from the moderation temperature of the reactor. Ideas for further applications are given.