A neutron interferometric measurement of a phase shift induced by Laue transmission.

The phenomenon of a neutron phase shift due to Laue transmission in a perfectcrystal blade is discussed. Quantitative measurements of this phase shift arepresented in the vicinity of the Bragg condition well in agreement withnumerical calculations. The phase shift shows a strong angular sensitivity andmight constitute an interesting opportunity for precision measurements offundamental quantities like the neutron–electron scattering length or gravita-tional short-range interactions.1. IntroductionLaue diffraction on a perfect crystal slab is the basis forperfect crystal neutron interferometry as it coherently splitsthe incoming neutron wave into a transmitted and a reflectedcomponent. Both wavefunctions can be calculated by dyna-mical diffraction theory. Here we want to focus on the phase ofthe transmitted beam, which shows a very peculiar behavior,different from an ordinary phase shifter described by theindex of refraction (Lemmel, 2007). We refer to this phase asthe ‘Laue phase’ rather than the ‘dynamical phase’ (Ioffe V Wietfeldt et al., 2006) as the latter might bemisleading in the sense of distinguishing dynamical fromtopological phase shifts (Rauch & Werner, 2000). The Lauephase is effective on a much larger angular range than theLaue reflection itself. While the reflected amplitude is essen-tially zero if the beam angle differs more than a few arcseconds from the Bragg condition, the phase of the trans-mitted beam (Laue phase) still shows a remarkable differencefrom the index of refraction phase at angles some tenths ofdegrees off the Bragg condition. First related measurementson this feature have been presented in Graeff etal.(1978). Themotivation for such measurements has additionally grown dueto the fact that the Laue phase depends on the neutron–electron scattering length (Wietfeldt et al., 2006; Ioffe VWerner, 1980). Recently, new approximations smoothlycovering the transition range between the vicinity of the Braggcondition and the case of a simple phase shifter have beenpublished (Lemmel, 2007); these are considered in thefollowing. A neutron wave incident on a perfect crystal slab ofthickness D with an angle close to the Bragg angle