Scattering of polarized neutrons is a very powerful method for the investigation of complicated magnetic structures. However, classical three-directional polarization analysis copes with the risk of depolarization due to residual magnetic fields. Therefore, the polarization of the neutrons is maintained by the application of guide fields along the flight path of the neutron beam. This setup allows only a determination of the projection of the final polarization vector onto the direction of the guide field. Any component of the polarization turned into a direction perpendicular to the guide field upon the scattering process will be depolarized. Hence, only the three diagonal terms of the polarization matrix (xx, yy, zz) may be determined. Due to the loss of information given by the six off-diagonal (xy, yz, etc.) terms this kind of setup is not suitable for the characterization of complicated magnetic structures.

Alternatively the neutron polarization can be conserved when any residual fields are removed from the sample environment by a zero field chamber. Then, any component of the final polarization vector may be determined and the full polarization matrix can be measured (Fig. 1). This method is known as spherical neutron polarimetry (SNP). Until now CryoPAD (Cryogenic Polarization Analysis Device), presented by Tasset et al. in 1989, was the only device to perform routine SNP measurements at finite scattering angles. It is based on a zero field chamber, realized through a double superconducting Meissner-shield. Because the handling of cryogenic devices is always rather involved we have designed and built (diploma work of Marc Janoschek) an alternative setup based on a zero-field chamber made of highly permeable mu-metal. Advantages of MuPAD over CryoPAD are that it is more cost-efficient and easier to operate because it is a non-cryogenic device.

Publication

Spherical neutron polarimetry with MuPAD
M. Janoschek, S. Klimko, R. Gähler, B. Roessli, P. Böni
Physica B 397 (2007) 125–130

http://dx.doi.org/10.1016/j.physb.2007.02.074