High-precision neutron polarization analysis using opaque spin filters

Further progress in testing the structure of the electroweak interaction an d in the precise determination of the weak coupling constants g(A) and g(V) by studying the decay of free polarized neutrons is presently limited by t he accuracy to which the polarization of cold neutron beams can be measured . Current techniques of polarization analysis are based on neutron optical mirror reflection, and are systematically restricted to a precision of grea ter than or equal to 0.25%. In this letter we introduce a new technique whe reby systematic effects are strongly suppressed. The basic idea is to use a n opaque spin filter, i.e., with analysing efficiency close to 100%, in ana logy to an efficient polarizing foil in light optics. The method was succes sfully tested at the Lnstitut Laue-Langevin in Grenoble in a comparison stu dy, in which a well-defined neutron beam was analysed with two very differe nt kinds of spin filters: a solid-state polarized proton target, using spin -dependent neutron scattering, and a spin filter of polarized He-3 gas, whe re spin-dependent neutron absorption is exploited. A difference in the neut ron polarization of only 0.026(55)% was obtained, which demonstrates the po wer of the method. (C) 1999 Elsevier Science B.V. All rights reserved.