High-precision neutron polarization analysis using opaque spin filters

Further progress in testing the structure of the electroweak interaction and in the precise determination of the weak coupling constants gA and gV by studying the decay of free polarized neutrons is presently limited by the 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 ≥0.25%. In this letter we introduce a new technique whereby systematic effects are strongly suppressed. The basic idea is to use an opaque spin filter, i.e., with analysing efficiency close to 100%, in analogy to an efficient polarizing foil in light optics. The method was successfully tested at the Institut Laue-Langevin in Grenoble in a comparison study, in which a well-defined neutron beam was analysed with two very different kinds of spin filters: a solid-state polarized proton target, using spin-dependent neutron scattering, and a spin filter of polarized 3He gas, where spin-dependent neutron absorption is exploited. A difference in the neutron polarization of only 0.026(55)% was obtained, which demonstrates the power of the method.