Spin echo scattering angle measurement at a pulsed neutron source

Two experiments were performed to adapt spin echo scattering angle measurement (SESAME) to pulsed neutron sources. SESAME is an interferometric method that provides enhanced resolution of neutron scattering angles without the loss of neutron intensity that results when collimation is used to improve angular resolution. The method uses the neutron equivalent of optical wave plates to produce a phase difference between the two neutron spin components of a polarized neutron beam. Because the wave plate is inclined to the neutron beam, this phase difference depends sensitively on the trajectory of the neutron. In the absence of a sample, a second wave plate, which is parallel to the first, undoes the phase difference introduced by the first wave plate, producing a polarization identical to that of the incident neutron beam. When a scattering sample is placed between the two neutron wave plates, the cancellation of the phase difference between the neutron spin states is not perfect and the resulting neutron-beam polarization is a measure of the distribution of scattering angles. In the first experiment, thin (30 and 60 µm-thick) magnetized Permalloy films were used as neutron wave plates. In a second experiment, current-carrying solenoids with triangular cross sections were used as birefringent prisms for neutrons. The arrangement of these prisms was such that they mimicked the effect of the neutron wave plates in the first experiment. In both experiments, correlation lengths in the scattering sample of about 1000 Å were probed using very simple and inexpensive equipment. These experiments brought to light a number of advantages and disadvantages of implementing SESAME at pulsed neutron sources and provided insights into the relative merits of SESAME and traditional small-angle neutron scattering.