Elastically bent silicon monochromator and analyzer on a TAS instrument

The elastically deformed perfect crystals are ideally suited for focusing of neutron beams thanks to the deterministic nature of their ''mosaic'' structure. During several decades it was, however, believed that their integrated reflecting power cannot compete with that of mosaic crystals and their practical application remained reserved to special high resolution instruments. In reality supplementary gains in intensity are achieved thanks to more efficient real space focusing and to avoiding the Seller collimators. In the present paper we demonstrate that a properly optimized horizontally and vertically focusing assembly of elastically bent Si 111 crystals can serve as a highly efficient general purpose monochromator and analyzer on a three-axis spectrometer. Typically, a gain factor of 3 to 5 in intensity is obtained when replacing a horizontally hat PG 002 analyzer with Seller collimators by the Si 111 in open geometry at the same or better energy and momentum resolution. The excellent performance of the silicon crystals is accompanied by their low cost, standard crystalline quality and by the absence of the lambda/2 beam contamination for the odd reflections such as 111.