A high performance focusing silicon monochromator

The concept of high resolution neutron powder diffraction (HRNPD) that emerged in 1975 at the Institut Laue-Langevin (ILL) is based on flat mosaic crystal monochromators at large take-off angles (typically 20Θ ≈ 120°) and Soller collimators of narrow angular divergence [l]. Because of the unavoidable heavy losses at high resolution with tight collimation, this concept has mainly been implemented at high flux research reactors [2–31. Its advantage is that the resolution does not depend on the sample size, so that one can gain intensity with large samples. Disadvantages are high cost and complexity. The HRNPD instrument at the Brookhaven National Laboratory (BNL) has been pushed to the the technical limit in every respect [3], but has thus become quite a mammoth. Much work has been done to develop its monochromator: a high stack of flat mosaic plates, each a soldered packet of thin wafers of squashed germanium, with vertical focusing by tilting [4–51.