Nondispersive sample arrangement in neutron interferometry

Abstract

The visibility of the interference at high order decreases due to the longitudinal coherence length of the beam which is determined by its wavelength spread. Much higher interference orders can be observed if the surfaces of the phase shifting sample is placed parallel to the reflecting planes of the interferometer crystal. In this case the contrast of the interference pattern is determined by the transverse coherence length which is orders of magnitudes larger than the longitudinal coherence length. Moreover the pattern is nearly independent from the wavelength spread of the beam. This feature has been shown by experiments performed at the high flux reactor at Grenoble. This method permits higher intensities and therefore higher accuracies in neutron interferometer measurements. An accuracy of Δ _χ/χ =2.2.10⁻⁵ has been achieved for the phase shift of a nondispersively cut Bismuth sample whose coherent scattering length has been determined asb _c =8.521(4) fm.