High-Field Polarization-Analyzed SANS Setup with Integrated Magnetic Shielding at the China Spallation Neutron Source

摘要: High magnetic fields combined with small-angle neutron scattering (SANS) provide a powerful means of probing nanoscale magnetic inhomogeneities and mesoscale spin textures, and polarization analysis further en ables separation of nuclear and magnetic scattering, also quantitative reconstruction of the three dimensional magnetic moment vector. However, implementing polarization analysis under high magnetic fields remains technically challenging in SANS geometry, where maintaining stable neutron polarization over a wide magnetic field range is a critical requirement. Here we present a high-field-compatible polarization-analyzed SANS sys tem tailored to the Very Small-Angle Neutron Scattering (VSANS) instrument at the Chinese Spallation Neutron Source. Apassive magnetic shielding strategy integrates the neutron spin flipper, high-field sample environment, and polarization analyzer within a compact space while suppressing magnetic interference among them, satisfy ing the stringent requirements for stable in-situ ³He polarization analysis while preserving wide scattering-angle coverage and flexible sample field geometry. Finite-element simulations and three-dimensional field mapping confirm that the residual field at the RF flipper center is reduced from 30 G to 0.93 G, and the field gradient in the ³He neutron spin filter region is suppressed to the order of 10⁻⁴cm⁻¹, which also is confirmed by NMR measurements. McStas simulations agree well with measurements, with polarization about 75% at 4 Å. Applied to Fe₃O₄ nanoparticles at 1.65 T, the system resolves nuclear, parallel magnetic, and perpendicular magnetic scattering components, showing magnetization along the applied field with overall paramagnetic behavior and no detectable perpendicular moment, indicating negligible surface spin canting and no core–shell structure at room temperature. This work establishes a practical technical framework for integrating polarization analysis with high-field sample environments in VSANS.