MuPAD: Test at the hot single-crystal diffractometer HEiDi at FRM II

The device “MuPAD” for zero-field spherical neutron polarimetry (SNP) has recently been successfully used for experiments in SANS geometry at the very cold neutron beam line MIRA at the research reactor FRM II in Munich and at the cold triple axis spectrometer TASP at Paul Scherrer Institut (PSI). At the hot neutron beam line SR 9 of FRM II, the construction of the polarised neutron single -crystal diffractometer POLI-HEiDi is under way. One of the major applications of this new instrument will be the investigation of complex magnetic structures by means of SNP. As MuPAD has originally been designed for use with thermal and cold neutrons, we carried out a first feasibility experiment with MuPAD at the existing diffractometer HEiDi in order to verify its performance when operated with short wavelength neutrons. In addition, we tested the combination of MuPAD and 3He spin filters that wherewere used as polariser and analyser. Here we present the setup of the experiment and first successful measurements of the complete polarisation tensor of the (3 3 3) Bragg reflection of a Si single-crystal. In addition, we discuss the particularities of using two 3He filter cells simultaneously. A method to perform the corrections of the data, taking into account the time dependence of the polarising efficiency in both cells, is proposed.