Unambiguous determination of structure parameters for soft matter samples made possible with polarization analysis on JCNS SANS using a 3He spin filter

Abstract: Incoherent background can create an intrinsic problem for standard small angle neutron scattering measurements. Biological samples contain hydrogen which is a strong incoherent scatterer thus creating an intrinsic source of background that makes determination of the coherent scattering parameters difficult in special situations. This can especially be true for the Q-range from around 0.1-0.5 \AA^-1 where improper knowledge of the background level can lead to ambiguity in determination of the samples structure parameters. Polarization analysis is a way of removing this ambiguity by allowing one to distinguish the coherent from incoherent scattering, even when the coherent scattering is only a small fraction of the total scattered intensity. ^3He spin filters are ideal for accomplishing this task because they permit the analysis of large area and large divergence scattered neutron beams without adding to detector background or changing the prorogation of the scattered neutron beam. This rapid note describes the application of ^3He neutron spin filters, polarized using the spin-exchange optical pumping method, for polarization analysis on a protein sample to unambiguously extract the coherent scattered intensity.