Bent perfect crystal monochromator at the monochromatic focusing condition

Abstract. In conventional diffractometry with a mosaic monochromator due to a nonnegligi-ble λ−θ correlation of the monochromatic beam impinging the sample, the resolution consid-erably differs in the parallel (+,-) and anti-parallel (+,+) setting. However, a bent perfect crystal (BPC) monochromator at monochromatic focusing condition can provide equal reso-lution properties at both parallel and anti-parallel setting and gives a chance to use both sides of the scattering plane. Three diffraction geometries with the BPC monochromators were tested and the received data of FWHM of the line profiles and the corresponding ∆ d/d prove the possibility of simultaneous measurements in both settings. Introduction Conventional neutron diffractometers employing mosaic monochromators are used only in the parallel (+,-) measurement setting, because the resolution of the anti-parallel (+,+) setting is much worse [1]. However, if we could use bot h settings with the equivalent resolution, the measurement efficiency could be twice of that related to the traditional mode. A strong dif-ference in the resolution properties in the parallel and antiparallel settings is brought about by a nonnegligible λ−θ correlation of the monochromatized beam impinging the sample in the scattering plane. In order to overcome this problem the momentum distribution of the monochromatized neutrons should be symmetric with respect to the axis of the beam imping-ing the sample. Three monochromator arrangements which could provide a rather highly collimated monochromatic neutron beam with a negligible λ−θ correlation and a reasonable ∆λ (keeping the luminosity and the instrument resolution on a feasible level) have been al-ready tested. These are: 1.