The diffractometer POLI is dedicated to the investigation of single cristalline samples with complex magnetic structures using neutron spin polarization.
Neutron beam polarization can be treated as a classical vector. Zero-field spherical neutron polarimetry (SNP) allows to measure all components of the scattered polarization vector. Determining the relationship between the directions of incident and scattered polarizations gives access to the 16 independent correlation functions involved in the most general nuclear and magnetic scattering process. Generally this leads to the determination of the direction of the magnetic interaction vectors of magnetic structures. For those structures, in which nuclear and magnetic reflections coincide in reciprocal space, SNP leads to the determination of the amplitude of the magnetic interaction vectors, and hence to the magnetisation distribution.
Currently the instrument uses part time the focused monochromatic beam from the monochromator of the single crystal diffractometer HEiDi at the beam channel 9b. The construction of the separate dedicated for POLI monochromator at the beam channel 9a is on the way and planned be finalized in 2013. As in 2014 POLI will be fully operational on the dedicated beam line SR9a.
The separation between monochromator and polarizer allows the use of the polarized neutrons with different wavelengths and high resolution. This feature of POLI is rather unique especially for hot neutrons.
The incoming beam is polarized along the beam axis by mean of 3He spin filter cell (SFC) placed in the polarizer magnetostatic cavity. The polarization of the incoming beam is determined by the transmission measurement of the SFC using two beam monitors. SNP is implemented on POLI using the zero-field polarimeter Cryopad of the third generation. Nutator and incoming precession coil of the Cryopad precisely turn the polarization vector of the incoming beam along any required direction. The outgoing precession coil and second nutator turn the required component of the polarization along the quantization axis of the analyser (SFC in Decpol). X, Y, Z components of the scattered polarization are measured for each orientation of the incoming polarization and hence a polarization matrix of 9 elements for an individual Bragg reflection is determined. The count rates for the two spin states are corrected for background.
To achieve the best available accuracy optimum strategy is applied for each reflection. The in-situ measurements of the incoming polarization permit easy correction of the measured data regarding the time dependence of the SFCs. Dedicated software for SNP instrument control and data analysis developed at ILL is used. The collected data sets can easily be exported, processed and automatically plotted. Further development of the data refinement software is on the way.
Primary beam (HEiDi)
|crystal||wavelength λ [Å] at 2θM||flux at 2θM = 40°|
|Ge (311)||0.593||1.116||1.443||9·106 n cm-2s-1|
|Cu (220)||0.443||0.870||1.079||4.3·106 n cm-2s-1|
|Cu (420)||0.280||0.552||0.680||2.0·106 n cm-2s-1|
|with Cryopad||without Cryopad|
|-10° < 2θ < 120°||-130° < 2θ < 130°|
|-180° < ω < 180°||-180° < ω < 180°|
|-4° < χ1 < 4°||-5° < χ1 < 5°|
|-4° < χ2 < 4°||-5° < χ2 < 5°|
3He spin filter cell
|Neutron beam polarization with cell P3He (0) = 70 %||initial||after 24 h|
|and T1 = 100 h||> 0.92||> 0.80|
Cryopad (zero-field polarimeter)
Dr. Vladimir Hutanu