Finite element analysis of zero magnetic field shielding for polarized neutron scattering

Abstract

<sec>Polarized neutron scattering, as one of the experimental techniques of neutron scattering, is a powerful tool for exploring the microstructure of matter. In polarized neutron scattering experiments, magnetic field maintains and guides the neutron polarization, and determines the sample magnetic environment. For complex magnetic sample, it is often necessary to apply zero-field environment at the sample position, so that general polarization analysis becomes feasible. To achieve effective zero-field environment for polarized neutron experiment, carefully designed magnetic field is required.</sec><sec>In this work, we demonstrate a zero-field sample chamber designed for polarized neutron experiment by utilizing both permalloy material and high-<i>T</i>_C superconducting films. This design adopts a simple and low-maintenance ‘deep-well’ shape to achieve effective shielding. The study uses finite element simulation method to analyze the effect of dimensions on the magnetic field shielding performance of the device of the model, including height, arm length, opening radius, and superconductor distance. At optimal dimensions, the designed zero field chamber achieves an internal magnetic field integral of 0.67 G·cm along the neutron path under the geomagnetic field condition. The maximum neutron depolarization for 0.4 nm neutrons is 0.76%, which sufficient for general polarization analysis application. The finite element method simulation results are examined by neutron Bloch equation dynamics simulations and in-lab measurement . Based on the established effective zero-field shielding design, we further discuss the relationship between magnetic field shielding and the dimensions of the device. The application of the device to spectrometers and the future improvement in the device structure are also discussed.</sec>