ELASTICALLY BENT SILICON MONOCHROMATOR AND ANALYZER ON A TAS INSTRUMENT

The elastically deformed perfect crystals are ideally suited for focus ing of neutron beams thanks to the deterministic nature of their ''mos aic'' 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 intensi ty are achieved thanks to more efficient real space focusing and to av oiding the Seller collimators. In the present paper we demonstrate tha t a properly optimized horizontally and vertically focusing assembly o f elastically bent Si 111 crystals can serve as a highly efficient gen eral purpose monochromator and analyzer on a three-axis spectrometer. Typically, a gain factor of 3 to 5 in intensity is obtained when repla cing a horizontally hat PG 002 analyzer with Seller collimators by the Si 111 in open geometry at the same or better energy and momentum res olution. The excellent performance of the silicon crystals is accompan ied by their low cost, standard crystalline quality and by the absence of the lambda/2 beam contamination for the odd reflections such as 11 1.