Properties of a special double-crystal (DC) monochromator employing bent-perfect crystals of Si in (1, −1) and (n, −m) settings are presented. The first monochromator was the bent Si(111) crystal (4 mm thickness) and the second one was in the form of a sandwich consisting of two bent Si(111) and Si(220) slabs (2 and 1.3 mm thickness, respectively). It has been found that by a simple exchange of diffraction conditions on the second monochromator one can use either Si(111) + Si(111) bent crystals in (1, −1) setting providing good luminosity and worse diffractometer resolution or Si(111) + Si(220) bent crystals in quasi-dispersive (n, −m) setting providing very good diffractometer resolution and correspondingly weaker luminosity. It has been found that besides an excellent focusing and reflectivity properties of the dispersive double-bent-crystal setting the obtained monochromatic neutron current is sufficiently high for diffraction experiments even at the medium-power research reactor.
