Short-ranged Force Limits from Pendellosung at the Spallation Neutron Source

Pendellosung is an interferometry technique based on dynamical diffraction in "perfect crystals." It has been successfully used with neutron beams to produce the most stringent limits to date for an extension of gravity with a Yukawa-like potential and a length scale near atomic dimensions. These limits were placed using the NIOF and NIOFa facilities at the NCNR using a 1 cm thick Si crystal (with the edges machined to permit measurements with several different orientations). For these measurements, three Bragg conditions were measured (<111>, <220> and <400), with each Bragg condition requiring a repositioning of the crystal. This approach also required a high precision measurement of the effective crystal thickness for each Bragg condition using a separate interferometric measurement. An alternative is to use a pulsed beam, where a number of the harmonics of each Bragg condition can be measured in the same crystal orientation, as the wavelength varies continuously over each pulse of the Spallation source. A strong advantage of this method is that it is then possible to use ratios of the pendellosung phases measured for each Bragg condition, avoiding the need for separate effective thickness measurements. A proof of principle of this method for a single Bragg condition was performed at J-PARC by Itoh et al (Nucl. Instrum. and Meth. A 908, p. 78-81, 2018), but pendellosung for multiple harmonics has not yet been demonstrated. We are developing a technique to utilize the pulsed beam at the SNS to measure multiple harmonics of the <111> condition. Proof-of-concept runs were performed in 2022 on the VULCAN instrument, and recently moved to a new set of runs on SNAP. We will present the status of these runs and prospects for new short-ranged force limits. This method also permits high precision measurement of higher order structure factors which are sensitive to anharmonic terms in the crystal potential, possibly shedding light on poorly understood thermal properties of Si.