The 3He filter on a polarized thermal neutron three-axis spectrometer

The use of Heusler alloy crystals in the thermal-energy range implies severe neutron flux losses as compared to pyrolithic graphite. On TAS instruments using successively two such devices (polarizer and analyzer) this results in overall penalty factors of up to two orders of magnitude in detector count rate as compared to the unpolarized neutron work. First tests of the He-3 filter carried on the IN20 spectrometer confirmed the filter performance followed the theoretical predictions, permitting optimization of the filter transmission versus the neutron beam polarization. In general, higher monochromatic flux for neutron energies above 30 meV can be expected, accompanied by an increase of the solid angles due to the use of PG or silicon focusing crystals of a considerably larger size than possible with magnetic crystals. (C) 1998 Elsevier Science B.V. All rights reserved.