A149Sm polarising filter for thermal neutrons

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Abstract
The design and construction of a new filter method for polarising thermal neutron beams are described. The filter contains polarised 149Sm nuclei which preferentially absorb one of the neutron spin states from the incident beam. The filter material is a paramagnetic salt doped with 149Sm atoms which is cooled to dilution refrigerator temperatures in order to generate a usefully high nuclear polarisation to give good spin discrimination. Neutron measurements of the characteristics of the filter at two wavelengths (0.096 and 0.118 nm) are compared with theoretical predictions.