The coherence length of a neutron wave packet depends on the spectral width of the distribution of wave vectors that make up the packet. The coherent overlap of the wave packets traversing the two beam paths in a perfect-silicon-crystal neutron interferometer (NI) is altered by placing a material with a neutron-nuclear optical potential in one of the beam paths in the NI. If the optical potential is positive, it causes a delay of the wave packet and a loss of fringe visibility. By use of an analyzer crystal, we narrow the spectral distribution after the mixing and interference has occurred in the last crystal slab of the NI. This increases the coherence length and restores some of the fringe visibility.PACS number(s): 03.65. Bz, 42.50.p

We have demonstrated experimentally that the contrast lost, due to placing slabs of a material having a neutron optical potential in one leg of a neutron interferometer, can be restored by spectral filtering of the beams leaving the interferometer immediately before detection. The non-Gaussian spectral distribution of the neutrons traversing the interferometer in our experiment leads to an unexpected phase reversal of the interferograms. PACS numbers: 42.50.-p, 03.65.BzThe evolution of the wave function y/irj) of a freely propagating neutron in a beam having a Gaussian spectral distribution is described (in part) by the well-known expression for the time-dependent longitudinal spatial length [1]:where m is the neutron mass. The minimum length (7v(0) is related to the spectral width TFor a spectral distribution which is precisely Gaussian, the equality holds. If the wave packet traversing one leg of a neutron interferometer is delayed (displaced in space), say, by passage through a slab of material having an optical potential Fop, relative to the wave packet traversing the other leg of the interferometer, they will no longer spatially overlap in the region of recombination, and the interference contrast will disappear. For this to occur, the relative displacement A/ of the two wave packets need only be a small multiple of the minimum spatial width cJv (O) and not of the much larger width Oxit) [2]. This fact was demonstrated in an experiment by Kaiser, Werner, and George [3] designed to measure the longitudinal coherence length of a neutron beam, and explained in a straightforward way by Klein, Opat, and Hamilton [4]. A detailed treatment of coherence phenomena in neutron interferometry dealing with these matters is given by Petrascheck [5].In an actual neutron interferometry experiment there is no a priori reason to believe that the spectral distribution g(k) = \a(k)\^ is precisely Gaussian, since it depends in detail on the mosaic spread of the monochromator, the collimators before and after the monochromator, and the Bragg angles of the interferometer and the monochromator. We show here that the non-Gaussian nature of a real neutron beam gives rise to some interesting and (initially) unexpected effects in neutron interferometry. Further-more, we have demonstrated experimentally, for the first time, that the interference contrast lost due to relative spatial displacement of the waves traversing the two legs of the interferometer can be partially restored by spectral filtering of the beams immediately before detection.The experiment was carried out using the skewsymmetric interferometer at beam port C at the University of Missouri Research Reactor. A schematic diagram of the setup is shown in Fig. 1. Neutrons of mean wavelength X =2.349 A from a pyrolytic graphite (002) monochromator (2^A/=41°) are incident upon the perfect silicon crystal interferometer. The (220) lattice planes are used in the interferometer (2^^...

Neutron-scattering studies of a single crystal of sodium have been carried out at large and small scattering angles. Near the sodium 110 Bragg reflection, we find features which are similar to those which have been observed previously in potassium. Using additional data gathered at small scattering angles, we show that the features near 110 can be quantitatively assigned to an experimental double-scattering artifact, in terms of peak intensities as well as positions. Overall, we find no neutron-scattering evidence of charge-density waves in sodium.