Optimization of a magnetostatic cavity for a 3He spin analyzer on the CANDOR polychromatic reflectometer

Abstract

A new instrument, the CANDOR (Chromatic Analysis Neutron Diffractometer or Reflectometer) polychromatic beam reflectometer is being built at the National Institute of Standards and Technology Center for Neutron Research. CANDOR will have a capability of polarization analysis for both specular and off-specular scattering, and allow for measuring the magnetic structures of magnetic thin films with a significantly shortened measurement time as compared to a conventional monochromatic beam reflectometer at a continuous neutron source. A critical component of the polarization analysis capability is to develop a 3He neutron spin filter (NSF) for polarization analysis for both specular and off-specular scattering for a continuous wavelength range between 4 Å and 6 Å and a large solid angle coverage for up to 20 energy dispersive detector channels. For ex-situ spin-exchange optical pumping (SEOP) operation, a magnetically shielded solenoid (MSS) will be employed to provide a volume averaged field gradient, better than 5×10−4 cm−1 to maintain 3He polarization relaxation times of at least a few hundred hours on the beam line even when there are significant stray fields from a magnet operating at a field up to 3 T at the sample position. We describe a procedure to design, construct, and optimize a MSS with non-identical compensation coil dimensions on the ends to match the scattered neutron beam rays. The volume averaged field gradient is quantified and optimized over a cell for a MSS using the finite-element software package RADIA and compared with that from the experiment.