Zero-Field Polarimeters

Following the pioneering experiment of Alperin [HA], who demonstrated that the generalized or three-dimensional polarization analysis could be performed by connecting two different guide-field directions onto a zero-field sample chamber, F. Tasset [FT] built in 1989 an apparatus at the ILL to determine the direction of the scattered polarization vector for any given incident polarization and any scattering angle. Using his expertise with superconducting screens developed when building the Cryoflipper, he constructed a compact Cryogenic Polarization Analysis Device (Cryopad) which takes advantage of the Meissner shields to properly define the magnetic field and zero-field regions crossed by the incident and scattered neutron beams. Thanks to this device, all the components of the complicated expression of the final polarization vector can be measured, which provides unique information on magnetic structures and nuclear/magnetic interferences occurring in the scattering process.

Since then, a second and third generations have been designed and constructed. Four copies of the third generation have been built: one for IN22 - CEA Grenoble, one for TAS-1 at JAEA, one for HeiDi at FRM-2 and one used on several instruments at ILL. Cryopad was initially used to investigate magnetic ground states on single crystal diffractometers. Today, it is also used to investigate magnetic excitations on three-axis and spin-echo spectrometers and to measure the neutron electric dipole moment on specific beamlines.