THE HE-3 FILTER ON A POLARIZED THERMAL-NEUTRON 3-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 analyze r) this results in overall penalty factors of up to two orders of magn itude in detector count rate as compared to the unpolarized neutron wo rk. First tests of the He-3 filter carried on the IN20 spectrometer co nfirmed the filter performance followed the theoretical predictions, p ermitting optimization of the filter transmission versus the neutron b eam polarization. In general, higher monochromatic flux for neutron en ergies 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 c onsiderably larger size than possible with magnetic crystals. (C) 1998 Elsevier Science B.V. All rights reserved.