Design and simulations of spin-echo small angle neutron scattering spectrometer at CMRR

Current work presents the design and simulations of the spin-echo small-angle neutron scattering (SESANS) spectrometer at the 20 MW China Mianyang research reactor (CMRR)-based neutron center. Firstly, the components of the SESANS are introduced. Secondly, the Monte Carlo simulations of the key components for determining the technical parameters are presented. The bending radius of the focusing monochromator is set to 2.5 m. The length and radius of curvature of the S-Bender is set to 300 mm and 2.0 m, respectively. Accordingly, simulation results of the virtual spectrometer indicate that the neutron beam transmission is about 0.09 at wavelength of 0.4 nm. Thirdly, the finite element simulation and the testing of the magnetic field suggest that the optimized ampere-turns are 8400 and 8289.9, respectively. Accordingly, the simulation results indicate that the field integral homogeneity of main coil, V-coil, and field stepper are better than 2.91 x 10(-6) T m, 2.3 x 10(-6) T m, and 3.14 x 10(-6) T m, respectively. Last, a multilayer flipper-permalloy with 100 nm FeSi and 10 nm Cr (or Ta) alternately sputtered on a Si wafer was designed and tested. Neutron experiments demonstrate that the maximum flipper efficiency reaches 0.98 and remains stable with increasing the strength of the main fields from 50 Gs to 200 Gs, when the corresponding wavelength is 0.31 nm. Based on the optimized parameters, the SESANS is under construction, and it will be completed at the end of 2021.