Ultra-high Resolution Larmor Diffraction in Backscattering Geometry

Neutron Larmor diffraction (LD) is a new technique developed by T. Rekveldt to increase the resolution of conventional neutron diffractometers by fully taking advantage of the additional spin freedom of the neutron. Inside a magnetic field, the neutron spin will execute a motion called Larmor precession around the magnetic field vectors, and the accumulated Larmor phase (ɸ) is proportional to the magnetic field intensity (B), neutron wavelength (λ), and the path length (L) through the magnets. By tilting the field boundaries of the magnets before and after the sample to be parallel to the crystal plane of interest, the measurement of the lattice spacing (d) can be linearly translated into the measurement of the Larmor phase as: ɸ ∝ BLd, the value of which is independent of the beam divergence, or the mosaic spread of the sample. With a high resolution in the Larmor phase measurement of ∆ɸ/ɸ ~ 10-6, the same resolution could be achieved for the measurement of the change in the lattice constant (∆d/d=∆ɸ/ɸ). We propose to deploy LD in the back scattering mode so that the diffracted neutrons are scattered back along the incident beam. With this, the LD setup could be simplified, where only one magnet is required before the sample and its field boundaries are naturally parallel to the crystal plane making LD easily hosted on a conventional diffractometer.