The use of symmetry to correct Larmor phase aberrations in spin echo scattering angle measurement

Abstract

Spin echo scattering angle measurement ͑SESAME͒ is a sensitive interference technique for measuring neutron diffraction. The method uses waveplates or birefringent prisms to produce a phase separation ͑the Larmor phase͒ between the "up" and "down" spin components of a neutron wavefunction that is initially prepared in a state that is a linear combination of in-phase up and down components. For neutrons, uniformly birefringent optical elements can be constructed from closed solenoids with appropriately shaped cross sections. Such elements are inconvenient in practice, however, both because of the precision they demand in the control of magnetic fields outside the elements and because of the amount of material required in the neutron beam. In this paper, we explore a different option in which triangular-cross-section solenoids used to create magnetic fields for SESAME have gaps in one face, allowing the lines of magnetic flux to "leak out" of the solenoid. Although the resulting field inhomogeneity produces aberrations in the Larmor phase, the symmetry of the solenoid gaps causes the aberrations produced by neighboring pairs of triangular solenoids to cancel to a significant extent. The overall symmetry of the SESAME apparatus leads to further cancellations of aberrations, providing an architecture that is easy to construct and robust in performance.

TOF-SEMSANS—Time-of-flight spin-echo modulated small-angle neutron scattering

“…The precession regions are similar to the coils developed by Pynn and co-workers. 8 The current sheets have been supplied with 8.5 A and 18 A (scan 1) current corresponding to magnetic fields of B 1 % 2.3 mT and B 2 % 4.52 mT, respectively, and with 3.7 A and 8.7 A (scan 2), i.e., 1.15 mT and 2.26 mT, and the corresponding distances to the detector were L 1 ¼ 2.635 m and L 2 ¼ 1.365 m. The beam was polarized and analyzed by means of multi channel supermirror devices. For detection a micro-channel plate (MCP) based detector with fast Medipix readout electronics was used in order to achieve sufficient time and spatial resolution.…”

Spin echo small angle neutron scattering using a continuously pumped 3He neutron polarisation analyser

“…In earlier work it was shown how symmetry across the various components serves to cancel out most of the magnetic field aberrations [21,28,41]. Despite this symmetry allowing most aberrations to be cancelled, the total polarisation can still be low due to low flipping efficiency at the various current sheet interfaces between field regions [19].…”

“…The coils are π flippers, π 2 flippers and prisms. The π 2 flippers stop and start the precession; a π flipper at the centre of the precession region flips the neutron spins; whilst four prism coils with inclined current sheets serve to encode the scattering angle [21]. There are two prism coils placed before the sample and two after.…”

“…Measurements of this wire show a transmission of 97.5%/layer. To increase transmission and prevent neutron depolarisation due to uncontrolled field transitions [21], the wires of the upstream and downstream faces of each device are folded away from the beam. The various types of coils are shown along with the magnetic field orientations in figure 2.…”

Spin echo modulated small-angle neutron scattering using superconducting magnetic Wollaston prisms

“…Instead of directly measuring the neutron scattering angles, several alternative approaches encode the scattering angle using the Larmor precession of the neutron polarization through a series of well defined magnetic fields (Krouglov et al, 2003;Bouwman et al, 1999;Parnell et al, 2015;Strobl, Tremsin et al, 2012;Strobl, Weider et al, 2012). These Larmor methods include spin echo small-angle neutron scattering (Bouwman et al, 1999;Pynn et al, 2008;Rekveldt, 1996;Bouwman et al, 2000;Major et al, 2009) (SESANS) and spin echo modulated small-angle neutron scattering (Bouwman et al, 1999;Strobl, Tremsin et al, 2012;Strobl, Weider et al, 2012;Gä hler, 2006) (SEMSANS). Even though these two methods are both implemented using Larmor encoding and both measure the same correlation function for the sample, they differ in concept.…”