New capabilities in high-resolution neutron Larmor diffraction at ORNL

Abstract

Using superconducting magnetic Wollaston prisms, high‐resolution neutron Larmor diffraction has been implemented at the High‐Flux Isotope Reactor of Oak Ridge National Laboratory (ORNL), Tennesse, USA. This technique allows the inverse relationship between the achievable diffraction resolution and the usable neutron flux to be overcome. Instead of employing physically tilted radio‐frequency spin flippers, the method uses magnetic Wollaston prisms which are electromagnetically tuned by changing the field configurations in the device. As implemented, this method can be used to measure lattice‐spacing changes induced, for example, by thermal expansion or strain with a resolution of Δd/d ≃ 10−6, and the splitting of sharp Bragg peaks with a resolution of Δd/d = 3 × 10−4. The resolution for discerning a change in the profile of a Bragg peak is Δd/d < 10−5. This is a remarkable degree of precision for a neutron diffractometer as compact as the one used in this implementation. Higher precision could be obtained by implementing this technique in an instrument with a larger footprint. The availability of this technique will provide an alternative when standard neutron diffraction methods fail and will greatly benefit the scientific communities that require high‐resolution diffraction measurements.

High-resolution phonon energy shift measurements with the inelastic neutron spin echo technique

“…The experiment was performed at the HB-1 polarized tripleaxis spectrometer at the High-Flux Isotope Reactor (HFIR) of Oak Ridge National Laboratory, Tennessee, USA. The magnets used were superconducting magnetic Wollaston prisms operated in NSE mode with all the fields in each arm parallel to each other and equal to each other, unlike the special field configurations for diffraction (Li et al, 2017(Li et al, , 2018. The sample measured was pure isotopic Ge 76 of 30 g. The transverse acoustic (TA) phonon at [0, 0, 0.8] was studied in the [2, 2, 0] Brillouin zone, and it was located by conducting constant q scans with the magnetic fields turned off, as shown in Fig.…”

Foreword to the special issue on advanced neutron scattering instrumentation

“…High-resolution neutron Larmor diffraction enabled by superconducting magnetic Wollaston prisms is reported by Li et al (2018) to have been implemented at the High-Flux Isotope Reactor of Oak Ridge National Laboratory (ORNL). As demonstrated elsewhere, higher precision can be obtained by this implementation and it provides an alternative to standard high-resolution diffraction measurements.…”