Measuring Silicon and Germanium Structure Factors with Pendellösung Interferometry*

Dynamical diffraction describes waves inside perfect crystals when an incident neutron wave nearly satisfies the Bragg condition. The interference of these waves, called pendellösung, can be observed by the intensity modulation of the forward diffracted or reflected beams. Pendellösung can be used to determine structure factors (describing the interaction of the neutron with the unit cell) with relative uncertainties of 10^-5, to investigate interactions Beyond the Standard Model, measure the internal structure of the neutron, and provide information on thermal motion of the atoms in a lattice. While neutron-silicon structure factors have been measured for the (111), (220), and (400) reflections, quality data do not exist for high-order reflections and no data exists for germanium. Progress in measuring additional structure factors in silicon, measuring germanium structure factors, and systematic improvements are discussed. The pulsed neutron source at the VULCAN beamline located at ORNL’s SNS was used to pursue higher order reflections. Leveraging VULCAN to measure structure factors simultaneously will reduce systematic uncertainties associated with the previous experiment. Using the BT-8 Diffractometer at the NCNR to measure high-order reflections in silicon will also be discussed. These measurements will allow for the study of anharmonic contributions, increase the precision of the determined neutron charge radius, and provide further constraints on an atomic length scale fifth force.